CA1112369A - Video processing logic - Google Patents

Abstract

COMPUTER

Abstract of the Disclosure The computer system of this invention has, as the heart of the system, a simple processing unit for providing most data processing by the computer system under control of a read-only memory which contains only instructions and other data for the CPU. The system also includes a random access memory, a keyboard, a video terminal, and a port device in the form of a tape recorder/player. A master clock initiates timing used throughout the system. A multi-line data bus interconnects the CPU and the different memories of the system including the keyboard and the video RAM. Bi-directional communication is possible on the data bus. The addressing of these different memories is by way of an address bus from the CPU, which is a uni-directional bus. Data to be operated upon is basically stored in the random access memory. The keyboard is used for inputting data to the CPU and the video terminal is used for displaying data. Features of the present invention include a special reset scheme for the CPU, a multiplexing scheme for addressing the RAM, a teaching for simply altering the control to provide capabilities of different capacity memories, alternate display of characters to provide, for example, either a 32-character line or a 64-character line, an improved keyboard selection scheme, and improved video processing means.

1.

Description

lR4/712 ~ 3$~

1 Background and Summary of the Invention ~

2 The present invention relates in general to a computer

3 system and pertains more particularly to a system that is con-

4 structed quite inexpensively employing on the order of 80 : 5 integrated circuits and having the capability of communicating 6 with a number of port devices.
7 One object of the present invention is to provide an 8 improved computer system having manual reset means for control-9 ling the central processing unit.
Another object of the present invention is to provide an 11 improved addressing scheme ~or the random access memory of the 12 system.
13 A further object of the present invention is to provide a.
14 computer system that has the capability of a video output that may be either alpha numeric or graphic.
16 Still another object of the present invention is to provide 17 a computer system having a video output for pro~iding different 18 size characters. In accordance with the present invention the 19 output can be controlled so as to display either 32 characters per line or 64 characters per line.
21 Another object of the prescnt invention is to provide an 22 improved computer system for providing a chain cotitrol of the 23 video RAMs or alternatively control directly from the data bus 24 of the CPU.
25 Another object of the present invention is to provide a .
26 computer system having the capability of graphic display. In 27 accordance with the invention, the display field is demarcated 28 into rectangular segme~ts with each segment in turn sectioned , : ~ 2.

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lR4/712 1 into, for example, 6 parts which are individually selectable 2 by data bus information.
3 Still another object of the present invention is to provide 4 an improved computer system having a novel keyboard entry scheme.
A further object of the present invent:ion is to provide an 6 improved computer system having a novel port control s~ecially ~ :
~ useful with a port device such as a tape recorder/player.
8 To accomplish the foregoing and other`objects o~ this inven-9 tion7 there is provided a computer system including a central processing unit, means for storing lnstructions for the central 11 processing unit disclosed as a read-only memory (ROM), random 12 access memory means for storing data, keyboard means for entering 13 data lnto the computer system, and display means disclosed in the 14 form of a conventional CRT television display. Connections from the central processing unit, ~CPU) incIude control lines, a 16 plurality of data lines, forming a data bus and a plurality of 17 address lines, forming an address bus. The data lines are bi-18 directional whereas the address lines are uni-directional. The 19 CPU interrogates other components of the computer system by way of the address bus to indicate where the data it is looking 21 for is located. The data bus is the means of communication for 22 dàta bo~h to and from the CPU. The ROM contains the instructions 23 for the CPU indicating to the CPU what to do, how to carry out 24 the instruction, and where to put the data after the instruction is completed. The CPU essentially looks to the ROM for instruc-26 tions and then follows the instructions of the ROM. In all .
27 communlcations, the CPU applies address locations to both the ~ Z8 ROM, RAM, and keyboard. However, address decoding determines : ~
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1 which of these actual memories the CPU is looking for. In the 2 system of this invention only the CPU communicates with all other 3 sections. ~or ex~mple, data is to be transferred from the ~O~I
, 4 into the RA~I, the trans-fer is accomplished by way of the ~PU.
The keyboard means enables entry of instructions and data to the 6 CPU. The system of this invention also includes a video random 7 access memory (video RAM) which couples to a video processing 8 section which in turn couples to a video output terminal or g monitor such as a television receiver. Data in the video RAM is automatically displayed on the monitor.
11 In accordance with one feature of the present invention, 12 there is provided a reset switch which is operable by the operator 13 of the computer system to reset the system by forcing the CPU to ]4 a known address. This reset switch rese-ts the microprocesser :
when it is lost. At power-up the microproc,esser (CPU) is reset 16 with instructions being initiated from the ROM starting at an 17 initial address. If at a later time the CPU becomes lost for 18 any reason in accordance with this invention there is provided 19 a reset switch for resetting the CPU starting with execution of instructions from a predetermined address in the ROM. In the 21 disclosed embodiment, this predetermined address is ~66. The 22 reset swi~ch is operable at the conventional interrupt input 23 to the microprocesser. The reset switch preferably has an R-C
24 circuit associated therewith w'hich is charged when the reset switch is released to permit the CPU to continue operation.
26 In accordance with another feature of the present invention~
27 there is provided a means for readily selecting different 28 :apacity memories especially with regard to ~he random access .

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1 memory of the computer system. In this regard the system of 2 the present invention employs an address decoder for ROM/RA~I
3 selection. The address decoder is responsive to an address code 4 from the central processing unit for providing separate outpu-ts, some of which at least correspond to different coded inputs 6 representative of different capacity memories. At the output of 7 the address decoder 9 there is a selection means for selecting 8 different outputs frorn the address decoder to provide a memory 9 enable signal. The address decoder in accordance with the presenL
invention preferably decodes the higher order address lines, 11 specifically four such lines, with the output of the decoder 12 providing up to 8 output si.gnals, only one of which at a time is 13 active. The selection means preferably includes a selection shunt 1.4 means having input terminals coupling to the address decoder and with some of its output terminals commonly tied to provide the 16 memory enable signal. One section of the shunt preferably 17 contains 4 shorting bars, commonly tied at their output terminals.
18 For a 4K memory capacity, one bar is shorted, for 8K, two bars 19 are shorted, for 12K, three bars are shorted, and for 16K~ all 2~ four bars are shorted. In the disclosed embodiment this means 21 that the enabling signal for the random access memory is active 22 all the way from address ~000 to address FFFF~
23 In accordance with anDther feature of the present invention 24 the random access memory uses a multiplexing scheme to input two partial addresses into the memory which together define one 26 particular storage address. The internal logic in the RAM
27 interprets two parts of the address code to provide one address 28 typic~lly with a total of 14 bits. One portion of the address ' .

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1 is defined as a row address select while the other portion is 2 defined as a column address select with a multiplexing signal 3 being defined between these two address selections. Preferably 4 there is also provided a selection means associated with the addressing of the RAMs which may be in the form of a shunt for 6 directing different signals to the input enable for the memories.
7 For a smaller capacity memory such as a 1K memory, a memory enable 8 signal is always present, however, for a larger capacity memory 9 such as a 16K memory, the shunt is selected under control of the multiplexing signal to provide different address line signals 11 to the enable input of the memory. In this way the RAMs can 12 -easily be operated at different memory capaci-ties depending upon 13 the capacity desired.
14 In accordance with another feature of the present invention the computer system has the capability of changing the format 16 of characters on the di~splay to, for example, either 64 characters 17 per line or 32 characters per line. In the disclosed embodiment, 18 the display has 16 character lines and thus for a line containing 19 64 characters, there are thus 1024 character locations in the video RAM that are to be accessed. In the alternate format, the 21 characters appear twice as large with 32 characters per line and 2Z thus~there are only 512 video RAM locations that are to be accessed.
23~The system includes a video RAM for the storage of character codes 24 preferably in an ASCII code which may be interpreted as either an Z5 alpha numeric character or a graphic symbol in accordance with . .
26 another feature of the present invention. The video RAM is 27~ addressed to take one code at a time from storage to a latch which 28 in turn couples to a character generator for receiving the ~ .

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1 character code. The character generator decodes the input code ~J .
2 and in accordance with a fan-line count, generates dot signals 3 stored in a shift register to be shifted out, one dot at a 4 time for forming one line o~ a number of lines forming the character. The data is shifted ou-t of the shift regis-ter by 6 means of a clock signal referred to herein as a shift signal.
7 This signal is controlled in at least two difEerent manners for 8 providing different video signals. In the disclosecL embodiment 9 the control is provided so as to give a format of either 32 characters per line or 64 characters per line. In accordance 11 with the invention there is a basic clock signal ~hich generates 12 the shift signal. ~or the 32-character format, the shift signal 13 is at one half the clock frequency whereas for the 64-character 14 format the shift signal is at the cloc~ frequency.
In accordance with another feature of the invention, the 16 computer system provides for two different types of formats~
17 including an alpha numeric format and a graphic format. Although 18 there are two difLerent formats, the same basic data stored in 19 the video memory is used for the generation of both formats.
`~ 20 In this regard, there is thus provided in the system a video 21 code storage means which also includes skorage of preferably one 22 bit of information for determining whether the final format is 23 alpha numerlc or graphic. This system also includes a character 24 generator means for recèiving the video codes, one code at a time, and a graphic generator means which also receives the video codes, 26 one code at a time. Preferably, there ls a common latch circuit 27 which has its output couple in common to both the character 28 generator means and the graphic generator. Also, preferably at :

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1 the output of these generators there are provided shift registers, 2 one for each genera-tor means. The shift registers convert the 3 dot patterns from the generator means into a serial signal. This 4 signal is coupled to the output video mixing circuit. Finally, in accordance with this feature, the system includes a means 6 responsive to the state of the video :Eormat type signal for 7 enabling either the character generator or the graphic generator.
8 This latter means preferably comprises a gate means responsive to 9 the state of certain bits forming each video ~ode.
In accordance with still another feature of the present inven-ll tion, there is provided the capability in accordance with the 12 computer system of this invention of interpreting codes stored 13 in a video memory either as a graphic display or as an alpha 14 numeric display. In accordance with the alpha numeric display, as previously mention, there are 1~24 character locations, rith 16 each location being defined by a 12 x 6 rectangle in accordance 17 with the graphic display of the present invention, this rectangle, 18 rather than being formed into a character is subdivide~ into a l9 plurality of smaller rectangles such as six smaller rectangles to provide a basic graphic cell. This cell is ~he smallest area 21 of graphic information that can be selectively displayed on the 22 screen. Each cell is four scan lines high and three dots wide 23 in the disclosed embodiment. Thus, in accordance with this feature 24 of the invention there is provided a video code storage means for 25 ~storlng a plurality of codes with one code at a time being 26 presented to a graphic cell generating means. A vertical addr~ss 27 is~ provided preferably ln the form of two bits also coupled to Z8 the graphic generatlhg means. The graphic generator is responsive , ~ - .

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lR4/712 1 to both the vertical address and the video code for providing 2 separately forrned cells over the graphic area. This graphic 3 genera-tor is pre-ferably in the form of a selector circuit 4 responsive to both the vertical address and the sta-te of certain 'bits defining the video code for determining the state o~ the 6 cells in a horizontal direction.
7 In accordance wi-th another important -feature of the present 8 invention there is provided an improved keyboard scheme, one that 9 is relatively simple in construction and which is readily adapted to a simplified software scheme. In accordance with this feature, 11 the keyboard comprises a plurality o~ keys arranged in a matrix 12 having input lines depicted as horizontal lines formed in a ~irst 13 gI`OUpg and output lines disclosed as vertical lines in a second 14 group, The address lines coupl~ from the central processsing unit, couple respectively to the input lines of the first group while 16 the output lines tie to the data bus which also'communicates with 17 the central processing unit. The matrix is arranged so that when 18 a switch is closed, there is essentially a connection between a 19 horizontal common line and a vertical common line. l~hen the keyboard enabling signal from the CPU is provided, essentially at 21 the same time the address lines are all brought to a like state, Z2 such as a high state. If the output signal is detected on one 23 of *he data lines, this indicates to the central processing unit 24 that there has been a key pressed on the keyboard. The central processing unit is essentially always in readiness for a keyboard Z6 detection when in the keyboard enable mode. Once the central ' 27 processing unit makes this detection, then under control of the .
Z8 ROM, -the address lines are scanned~ one-by-one until the'proper data ~ , .

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line has been detected. In this way, a first detection on a data line represents one vertical loca-tion on the keyboard matrix while a detection at a later time identifies t.he horizontal position on the matrix, thus ldentifying one and only one key. After identifying the output, the ROM instructs the CPU to generate the ASCII code for that particular key.
Another feature of the present invention is the provision for an output~input port device which is preferably ; in the form of a tape recorder/player. The tape recorder is operable as both an input and output port device. In the output mode there is a recording of data on the tape. In this connection, the signals on the data line al90 control the motor of the recorder. In the input mode data is trans-ferred from the tape recorder to the central processing unit.
In accordance with this feature of the invention the data lines which may comprise si~ separate lines couple to the recorder and may be provided in two groups. In the preferred embodiment, iD~ the input mode data is taken from the recorder onto a single data line. In the output mode in the disclosed ~ embodiment there are four input data lines, one of which pro-vldes the mode select signal, another of which controls the motor and the two remaining ones of which are used to provide : :
signals for recording data on the recorder tape, In accordance with a particular embodiment of the invention, a video processing logic for providing a video serial~slgnal to a receiver ln accordance~with an alternate mode control to provide dîfferent width characters to be displayed by the receiver, comprises: memory means for storing a p1ura11ty of character codes, storage means coupled from the mémory~means for holding a charactex code at a time transferred from the memory means, means defining a character .:: ~ ; ; . :

line code comprising at least a part of a video divider chain including a line counter, character generator means receiving said character code and said character line code for providing line dot signals representative of a portion of the character, parallel-to-serial means coupled from said character generator means receiving the line dot signals including means for pro-viding a serial video signal, and control means for control-ling said parallel-to serial means to provide at least two ; different video rate signals representative of different width ~:
characters to be displayed by the receiver, said control means comprising, means for establishing a mode control signal having .
alternate states for indicating different width characters, cIoek means for providing a first frequency signal, divider means coupled from said elock means for providing a seeond frequency signal lower in frequency than said first frequency signal, and logic eircuit means responsive to said first and second frequency signals and said mode control signal for pro-viding a shift control signal coupling to theparallel-to-serial means and in one state of the mode control signal coupling the : 20 ~flrst frequency signal and in another state of the mode.control sLgnal coupling the second frequency signal, said logic circuit means also having means for providing an input signal to said video divider chain that is at a constant frequency irrespect- --; : ive of said mode control state.
Description of~the Drawlngs ~, ~ Numerous other objects, features and advantages of .
the inventlon should now become~apparent upon a r~ading of ~the followlng detailed descr1ption taken in conjunction with the accompanying drawings, in which: : :

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lR4/712 ~ 3 1 FIG. 1 is a block diagram o:E the computer system of the 2 present invention;
3 . . FIG. 2 shows a memory map in accordance with this invention;
4 FIG. 3 is a table of addresses for identifying the different memory devices of the system;
6 FIG. .4 is a waveform chart associated with circuitry in 7 FIG. 14A;
8 FIG. 5 is a timing diagram associated with the video 9 processing section of the system;
FIG. 6 is a block diagram showing representative frequencies ll in ~e video divider chain;
12 FIG. 7 schematically depicts a graphic cell arrangement from 13 the graphic generator portion of the system;
14 FIG. 8 is a timing diagram associated with the video process ing section showing sync mixing;
16 FIG. 9A is a simplified schematic diag~am of a portion of 17 the video mixing circuit depicted in detail in FIG. 13C;
18 FIG. 9B shows a composite video output signal;
19 FIG. 10 shows a waveform for the tape recorder output signal associated with the logic circuitry shown in FIG. 13B;
21 FIG. 11 shows audio processing waveforms also associated 22 with the circuitry of FIG. 13B;
23 FIG. I2:is a timing diagram also associated with the port 24 device shown in FIG. 13D;
FIGS. 13A-13D show in detail one portion of the system of 26 the present invention;
27 FIGS. 14A and 14B show in de~ail most of the other portion of ; ~: 28 a detailed system of this invention;
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lR4/712 ~ 3~ 9 1 FIG. 15 shows the k~yboard matrix of this system; and 2 FIG. 16 shows a portion of further detail of the switch 3 arrangement of the keyboard of FIG. 15.
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1 Detailed Description 2 FIG. 1 is a system block diagram of the computer of this 3 invention. The overall computer may comprise on the order of 4 80 integrated circuits or separate chips, however, the system is basically broken down into the primary components shown in FI~.
6 1. In FIG. 1 these different sections comprising the computer 7 are disclosed along with their interrelationship therebetween.
8 The heart of the system may be considered as the central process-9 ing unit (CPU) 10. The CPU 10 and other components of the system are shown in more detail hereinafter with regard to a specific 11 p~eferred embodiment thereof. In the block diagram of FIG. 1 12 multiple lines such as data lines and address lines are shown 13 by respective data and address buses.
14 Most of the connections at the CPU are data lines and address lines. The CPU interrogates other sections of the 16 computer by way of the address bus so as to indicate where the 17 data it is looking for is located. The data bus is the vehicle 18 for information transfer to and from the CPU. FIG. l shows the 19 address bus 12 and the data bus 14. The address linesof the address bus 12 only provide for communication from the CPU to 21 other sections of the system. On the other hand, the data lines 22 of the data bus 14 permit bi-directional communication between 23 the CPU and other sections of the system.
24~ The read only memory ~ROM) is in a sense the brain of the~
system. For example, the ROM indicates to the CPU what to do, 26 how to do it, and where to transfer the data after the instruction ;~ 27 is completed~. When power is first applled to the system9 the CPU
~ 28 outputs an address to the ROM 16 so as to locate the first ~: . : -::::

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' 1 1 instruction for the CPU. The ROM trans~ers the ~irst instruction 2 to the CPU thus initiating communication therebetween. The CPIJ, 3 under ROM supervision, performs all the housekeeping ~asks for 4 the system.
S In FIG. 1 in addition to the ROM 16, the system also 6 comprises a random access memory (RAM) 18. The random access 7 memory 18 stores data which is to be operated upon by this CPU
8 and also stores programs for pro~iding certain control of the 9 CPU.
Assuming that the operator of the computer inputs instruc-11 tions to the computer via the keyboard 20, such as to count to 12 the number N, then the CPU 10 stores the instruction in the 13 RAM 18. The intercommunication between the sections is as follows.
14 The CPU tells the ROM an instruction has been entered. The ROM
signals the CPU to interrogate the keyboard to determine the 16 instruction. The CPU then signals the ROM to take over. The ~17 ROM then interrogates the CPU initiating communication with 18 the RAM. The ROM program essentially tells the CPU how to 19` perform the instruction of counting to the number N. After this has been completed, the ROM tells the CPIJ to determine what to 21 do with the data. The CPU informs the ROM that the number N
22 is to go to the display and is to be also stored. The ROM tells 23 the CPU how to puk the data on the display and then also indicates 24 where to store the number N in the RAM. The CPU signals the ROM
when the job is completed. The ROM then indicates to the CPU to 26 maintain a monitoring c~ondltion of the lceyboard.
27 ~ The CPU essentially looks to the ROM for instructions. The 28 CPU then follows the instructions of the ROM and looks to the ~; ' ~ . I
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lR4/7 1 keyboard and thell to the R ~ ~n ~ 1 cases, the CPU applies 2 address locations to the ROM, RA~I and keyboard. Ihe data lines 3 are then checked for input data that corresponds to these 4 address locations. In case of an output from the CPU to the RAM, the CPU selects the address, puts data on the data lines, 6 and then instructs the RAM to store the data that is on the 7 data lines.
8 In the system of this invention, only the CPU communicates 9 with al] other sections. If the CPU is told by the ROM to store something from the ROM into the RAM, the CPU cannot make the 11 RAM receive the ROM data directly. Instead~ the CPU takes the 12 data from the ROM and transmits it to the RAM. The CPU essen-13 tially functions as an intermediary between these two sections 14 of the system. This is because the CPU is the only section tha-t can address locations and pass da-ta to all other sections.
16 The keyboard section 20 is the means for making kno~Yn the 17 instructions to the CPU. The system also includes a video random 18 access memory (video RAM) Z2 which has its output coupled by way 9 of the video processing section 24 to a video terminal or monitor 26. Data in the video ram 22 is automatically displayed 21 on the monitor 26. The video processing section 24 handles this 22 transfer. Data outputted from the video RAM 22 is in ASCII code.
23 The video processer 24 has as its function the conversion of the 24 ASCII code into alphanumeric symbols for display on the monitor 26. The ROM 16 contains all of the dot patterns for formin~ these 26 alphanumeric symbols. The ASCII code from section 2Z identifies 27 the character pattern, and the video processer 24 sends this 2~ pattern to the monitor 26 on, of course, a synchronized basis.
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1 The composite video signal which is coupled to the video 2 monitor 26 is typically a complex signal. In addition to the 3 video signal, this signal also includes horizontal and vertical 4 synchroni~ation. These signals must be quite stable and outputted in the proper sequence. In accordance with the present invention 6 the video divider chain 28 under control of the master clock 30 7 handles this control. The video divider chain 28 generates the 8 sync signals, and addresses the video RA~I in a logical order so 9 that the video processer 24 can handle the video data efficiently.
Associated with the video RAM 22 is a mu].tiplexer (MUX) 32 11 discussed in more detail hereinafter. This multiplexer functions 12 analogously to a multi-pole, multi-position switch. When the 13 video divider chain is in control, the MUX 32 is switched so that 14 only addresses from the divider chain are directed to the video RAMs. The CPU may need to read or write data into the video RAM.
16 If so, the ~nJX is switched so that the CPU has control over the 17 addresses of the video RAM. A~ter the CPU is ~inîshed processing, 18 the addressing task is reassigned to the di~ider chain.
19 FIG. 2 shows a table o~ a memory map in accordance with the 2~ present invention representJing the address ~ as HF.X addresses.
21 For the basic system, ~ mory locationA 000~ to 0FFF. The 22 keyboard is controlled through addresses 3800 to 380F. The video 23 display~is located from address 3C00 to address 3FFF. The RAM
24 addresses commence at address 40p0 and depending upon the capacity of the memory in the system,can extencl all the way down 26 to address 7FFF.
27 As mentloned previously, upon power-up, an address location 28~ is outputted from the CPU requestlng information from the RO~I.

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1 Since the ROM is controlled from the lower addresses, the CYU
2 is outputting addresses in this area. If the CPU requires key-3 board data, it will output addresses 380~ - 38~F and determine 4 if anything is in this "memory" location. If the CPU desires to show the progra~ller something on the display, the CPU adclresses 6 the video display section of the map storing data in these 7 locations. The video display shows exactly wha~ is in memory 8 locations 3C~ - 3FFF. In FIG. 2, although the RAM locations 9 extend from 400~ to 4FFF, part of these locations are used for general housekeeping tasks. Hence, the user accessible RA~I
11 actually starts at address 42~.
12 FIG. 1 also shows a power supply 34 for providin~ certain 13 voltages useable in the system of this invention. This power 14 supply may be operated by way of an AC adapter 36 from a con-ventional AC power line. Also shown in FIG. 1 is the tape inter-16 face 38 and tape recorder/player 40. The tape interface 38 ties 17 into the data bus 14 and will be discussed in more detail herein-18 after. Also shown in FIG. 1 i9 a ROM/RA~I select 42 for receiving 19 data on the address bus 12 to select either the ROM 16 or the RAM
18. Similarly, the system includes a keyboard/video select 44 21 for selecting either keyboard Z0 or video RAM 22. Again, further 22 description is found hereinafter with regard to this portion of 23 the system.
24 FIG. 1 discloses the basic components of the system of this invention and hereinbefore has been a brief description of some 26 typical operation of this system. Now a discussion follows of 27 the theory o operation of a preferred specific example of a 28 computer system of t~s invention. In this connection reference ' ~ .

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lR4/712 ~ 3 1 is made to FIGS. 13-16.
2 ~y~tem Clock 3 The system clock or master clock depic-ted in PIG. 1 as 4 clock 30 is shown in detail in FIG.13A. The system clock 30-comprises a fundamental cut, crystal Yl having a Eundalnental frequency of 10.6445 MHz, and two inverters Z42 which form along 7 with resistors R46 and R52 and capacitor C43~ a series resonant 8 circuit. Feedback between the inverters is supplied by capacitor 9 C43. Resistors R46 and R52 force the inverters used in the oscillator to o ~ c in their linear regionO The waveform 11 at pin 5 oE one~of theJ inverter Z42 is a sign wave at a 12 frequency o-f 10.6445 MHz. The output of the oscillator, however, 13 should not be measured at this point due to the load:ing effect 14 test equipment may have at this node. Ihe rneasurement point is pin 6 of the inverter Z42 which is the output of the oscillator.
16 The output of the clock (note the CLOCK signal) couples to the 17 timing circuit for the CPU, to the video divider chain 28, and 18 to the video processing circuit 24.
19 Central Processing Unit (CPU) __ .
FIG. 14A shows the microprocesser Z40 which is a Z80 21 central processing unit MK3880. This is a conventional device 22 that may be purchased by any ~eof the well known companies 23 making such devices such as Motorola, Fairchild, or Texas 24 Instruments. This microprocesser has the capability of 158 instructions wi.th total software capability~ It contains 22 26 internal registers and has three modes o-E fast interrupt and 27 additionally a non-maskable interrupt. The unit directly inter-28 faces with standard speed, static or dynamic memories with little 18.
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- , interconnecting logic. The processer has a 1.6 mlcro-second instruction executlon speed and operates from a single 5 volt supply with a single phase 5 volt clock. E'IG. 14A clearly indicates the connections to and from the micxoprocesser including the address lines and the data lines. Throughout the description -the address llnes are identifie!d as AO-A15 while the da-ta lines are iden-tified as lines DO-D7.
CPU TIMING
As previously indicated, the microprocesser Z40 requires a single phase clock source for operation. The basic clock frequency of 10.6445 ~Iz is applied, as indicated in FIG. 14A, to a standard ripple counter Z56 at pin 1. The device 56 may be a conventional divide-by-12 counter connected to provide a divide~by-6 count. For example, this ~lay be a device 74LS92 providing at its output pin 8, a signal or Erequency of about 1.774 MHz. Ihis signal is applied to the input o-f inverter Z72A. The output of this inverter is coupled to the microprocesser Z40 identified as its Q input or clock input.
Resistor R64 pu115 Up the output of inverter Z72A and it insures a rapidly increasing rise time for the clock signal.
~lote that the enable input of inverter Z72A is tied to ground.
Inverter Z72A is thus an enablable gate; since the enable input is tied to ground, this gate is always active enabling passage of signals therethroughO ~he clearing of the counter Z56 is at pins 6 and 7. When one or both of these pins are at a low voltage lavel, the counter opera-tes normally, when either pin goes to a high level the counter is cleared or reset. Note the inverter Z~2 coupled to inputs 6 and 7 of counter Z56 is used .
to disable counter Z56 during automatic testing of the system.
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Resistor R67 pulls the input to inverter Z42 to a positive voltage which causes the output of the inverter to stay at a logic low level. Elowe~er, during testing the input oE this inverter may be selectively pulled to a low logic leve]. thus disabling and clearing the counter Z56. With regard to the CPU
timiny, the device Z72 may be a 74LS367 while the device Z42 may be a 74 LS04. Usually a plurality of such inverters are provided per device. In this connection the designation, such as inverter Z42, may pertain to other inverters shown in the description, but each inverter can be specifically identified by its input and output pin numbers or by a designation system such as Z42A, Z42B, etc.
Power-U~-Clear and System Reset As mentioned previously in connection wi-th the description of FIG. 1, upon power-up the CPU accesses known address in the ROM for instructions. The circuitry which causes the starting address output is shown in FIG. 14A as including gate Z53A, and inverter Z52A. Gate Z53A may be a two input NA~D gate 74LS132 drawn, however, as an inverted input OR gate~ ~hen power is :~
first applied to the sys-tem, capacitor C42 is discharged. Upon application of power, capacitor C42 is charged through resistor .:~
R47 at a predetermined rate. During the initial charging of ~
capacitor of C42, the output of gate Z53A is high. This high ~ :
signal is inverted by gate Z52A to provide at the output thereof a low signal which i~ applied to pin 26 o~ the microprocesser Z40. A low at lnput terminal 26 to the microprocesser forces the microprocesser to output the starting address:0~ on its 16 address line ZA00-2A15. When capacitor C42 charges past about 1.4 volts, the gate Z53A has a , : :
~, ' .

i,23~ :
low level on its output which causes the output from gate Z52A
to revert to its high level. The CPU i5 now out of its reset state and will start executing instructions from the ROM, starting at address ~p. ~hus, the pin 26 to the microprocesser is low for only a few milliseconds after power is applied. Once capacitor C42 charges past: its threshold level, this reset input to the microprocesser stays at a high level until capacitor C42 is again discharged when power is removed. It is noted that the gate Z53A, although implemented as a NAND gate is functionally shown as an OR gate having inverted inputs. The "not" circles at the input indicates that :
the gate is looking for a signal that is low to cause an output that is high. E~ad the gate been drawn as a NAND gate, it would ~;
not have been as obvious that the output is active when high~
~: -This functional type of logic symbolization is used throughoutthe schematics.
In FIG. 14A, above the reset circuit, there i5 shown a -somewhat similar circuit including switch S2, gate Z53B, and ; gate Z37A. Switch S2 is a reset switch. The circuitry associated with the reset switch~includes capacitor C57 and ~ : . . :
~ ; reslstor R65. It is noted that capacitor C57 has a smaller .
value than the capacitor C42 shown therebelowO Ilence, ; ~-capacitor C57 charges up faster than capacitor C42 assuming that capacitor C57 has charged in that the pin to input gate ~ . .:
Z53B is high, the output from this gate will be in its low state~and the output of gate Z37A will be at its high state.
W~lth~the~input at pin 17 to the microprocesser held high, the - ~ -~
microprocesser~is~operable. If switch S2 is pressed, capacitor -~
C57~d~lscharges through the switch. The resulting low level -~-s~ignal is~applied to a second input~of :: ~:,: ~ ::
~ ~ . .:: . .-gate Z53B and the ouput of gate Z53B goes to its high level.
Gate Z37A, at its output is then forced low. A low at pin 17 of the CPU forces the microprocesser to restart at a~dress ~66. When switch S2 is released, resistor R65 begins -to charge capacitor C57 until a :Logic high level is applied to pin 1 of gate Z53B. At this time pin 17 of the CPU goes back to its high level and the CPU starts executing instructions from address ~66 in the ~OM.
Switch S2 is used to essentially reset the microprocesser when it is "lost"~ The operation o-f this switch forces the CPU
to a known address to enable it to get on the right trackO An example of a CPU that would be "lost" might occur during a bad cassette load attempt. If a cassette is loading and suddenly there is information missing on the tape caused possibly by dirt or age of the tape, the recorder may never stop, Switch S2 may then be pressed, which directs the CPU out of the cassette load routine and back into its ready mode.
Associated with the reset circuit including switch S2 is the HALT output at pin 18 of the microprocesser to the second input of ~ate Z53B. For most application in the basic system, the output from pin 18 is always high. It does to its low level only when a software HALT instruction is generated by the microprocesser Z40. Usually, such an instruction is not included in the read only memory. However, there is a possibility that pin 18 of the microprocesser could go to it5 . ` .
low state due to some malfunction. In such a case, switch S~
is not effective to reset the CPU and about all that can be done is to shut down the computer and restart power. -The output from gate Z53B, also couples to pin 3 of gate ~

' ' - . , ,;: : : . : . . ~ . ~ .

Z37B. The ouput from gate Z37B is referred to as a system reset (SYSRES). This signal is normally high and onl~ goes to its low level during power-up or when switch S2 is pressed.
When power is interrup-ted or turned off to the system because of a "lost" CPU, the operator should wait at least 10 seconds before power is reapplied. If this period is not waited, capacitor C42 may not discharge suEficiently and thus the CPU may not revert to address ~ during a restart. By waiting, capacitor C42 discharges sufficiently and thus upon power-up, the system will start at the correct ~OM location.
CPU Functions Wait, Int, Test The microprocesser Z~0 has three inputs identified as WAIT
(wait), INT (interrupt) and BUSRQ (bus request). All three of these inputs are pulled up by respective resis-tors R51, R50 and R58. These inputs are active on a low input signal and thus when there is no input signal the resistors maintain these inputs inactive.
The WAIT input, pin 24 of microprocesser Z~0, slows t~e CPU
down i~ there are slow memories that it is accessing. If this line goes low, the CPV goes into a ~IT status untiI it goes back to its high level. Once this signal is high, the CPU
continues with the operation. For example, assuming that there is a memory system that takes 100 microseconds before address data can be guaranteed to be present at the output, when the memory logic sees that the CPU wants data, it will force the WAIT Iine low. At the end of the 100 microsecond interval, the logic will make the WAIT input high, and the CPU will input the data.
The INT (interrupt request) signal is provided at pln 16 of .. , . - . . , . . ~ :.
- . ~ .

3$~D
~ lR4/ 2 ~3 1 the microprocesser Z40. This input, when low, forces the CPU into 2 an interrupt request sec-tion of the memory. It then performs 3 an instruction associated with the in-terrupt. An example of this 4 is as follows. Assuming that a door on the back of the computer should always be closed, there is a switch provided connected to 6 the door, such that when the door is opened, the switch contacts 7 are shorted. The s-witch connects between gr~und and pin 16 of 8 th~microprocesser. If the door is opened, the computer is inter-9 rupted and there is printed on the screen the indication of the open door. The CP[I is interrupted until the door is properly 11 closed.
12 The TEST input is useful in trouble-shooting. Pin 25 of 13 the microprocesser is labeled BUSRQ ~bus request). When pin 25 14 is brought low, it forces the data, the address and the control lines into a disabled or floating state. Although this function 16 may not be used in normal oparation, it is quite use:Eul when some-17 one desires to shut down the CPU to test other portions of the 18 system-19 CPU Address Bus __ FIG. ~shows the address bus 12 comprised of address lines 21 AO-A15. Because these lines couple to all other components of 22 the system such as the keyboard and the random access memory, 23 these lines are buffered for at least two reasons. First, the 24 buffers must be able to supply the address bus with proper logical levels. The microprocesser cannot supply the current necessary 26 to drive all the sections connected to the address bus, and 27 bufEers are needed for current gain. Secondly, it may be 18 necessary to switch off the ~ddress bus. For example~ if an .

, .

2~.

. . .

6~ ;t`~
expansion interface is connected to the bus, it may be necessary to address the RA~I in the ~lain unit Eor a data transfer~ Therefore, there must be some method to take the CPU
off the da-ta bus. The buffers are tri-state devices such as the conventional 74L$367~ This essentially means that they will either act as a buf-fer or as an open switch. Gate arrays Z38, Z39 and part of Z2~ and Z55 form the address line buffers. It is noted in the gates Z38 and Z39 there are essentially two sections of buffersO The first section contains four buffers and the second section contains only two buffers. Each section is controlled by a single pin. The first is controlled by pin 1 and the second by pin 15. When these control pins are at a logic low level, the buffers are enabled and will operate normally. When the control pin is a-t a high logic level, the buffers are disabled and wilL show a high impedance from input to output. The signal that controls the address buffers is defined as the ENAB1E signal and has it source at gate Z52B. The input of this ga-te is tied to the TEST line. The resistor R58 keeps this line high under normal operation. Hence, the control line for the address buffers is usually at a logic low level permitting operation oE the buffers. If the test line is shorter to ground, the address buffers are disabled. This feature is useful in trouble-shooting.
CPU Data Bus The data bus 14 is buffered similarly to the address bus 12. There are only 8 data lines at -the CPU identified as lines DO-D7~ However, there are 16 buffers because ~he CPU must -~
receive data as well as send data. The address lines on the other hand .. . .

are strictly outputs from the CPU. mere are therefore two sets of buffers for the data lines, one set for handling output data from the CPU while the other set handles input data to the CPU .
The output data buffers comprise all of the gate arrray Z75 and one section of the array Z76. The input buffers on the other hand comprise one se~tion of the gate array Z55 and the other section of the gate array Z76 (three gate). The input and output buffers are connected "head-to-toe". ~his c~uld cause a problem if both were active at the same time, however, the control inputs to the buffers are controlled so that this does not occur. The control inputs to the output buffers are all connected together on the line labeled DBOUT* and are in turn tied to gate Z53 pin 6. The input buffer control line is ~-identified by the signal DBIM*. This line connects from the output of gate Z53C. The signals DBIN and DBOUT are essentially mutually exclusive.
:
The output from gate Z53, pin 6 provides the major ~ -cvntrol. If this~output is high, the signal DBOUT* is high~and the signal DBI~* is low. Therefore, the input buffers are enabled and the output buffers are disabled. If the gate Z53, pln 6;is low, then the signal DBOUT* lS low and the signa~
~DBIN* is h1gh. In this case, the output buffers are enabled and the input buffers are disabled.
Pln 4 of gate Z53 which is a NA~D gate is tied to the TEST*
slgna1. ~If~the~signal TEST* is low, the address buf~fers are d~lsabl~ed~and~also~the output pin 6 of gate Z53 goes high.
Hence~ the~data output buffers are disabled, robbing the CPU's con~trol~over the data lines. Because the signal DBIN* is now . .
h~eld low, the~input data buffers are active, but, this does not ~- ;
cause any ~ ~ 3~
~ lR~ 12 .' ~, r~ 1 problem since the address bus from the CPU has been disab]ed.
~ 2 ~hen the signal TEST* is left alone, in its high state and t ~ 3 if pin 21 of the CPU (the memory read output) is high, pin 6 of :3 ~ 4 gate Z53 goes to its low state. This low signal causes the signal DBOIIT* to be low and the signal DBIN* to be high. There-6 fore, the CPU is outputting data and the buffers are switched 7 accordingly. When pin 21 of the microprocesser goes to its 8 low state, pin 6 of gate Z53 is high. This is almost the same condition as if the signal TEST* went 1ow. The signal VBOU~*
- 10 is high and the signal DBIN* is low but the address buffers are 11 still enabled. The data buffers are now ready for the CPU to 12 accept the data. Thus, it is the read output RD* that primarily 13 controls the inputting and outputting of data on the data bus.
14 CPU Control Gr~
Having now identified the address lines and the data lines 16 associated with the CPU, we can now consider the CPU control 17 group. The data bus is used to gather data into the CPU or to i 18 pass data out of the CPU. The control group functions determine 19 how the CPU stores data in a memory or how it tells ROM or RAM
that it is ready to receive data. The control group functions 21 include signals RD, WR, OUT, and IN.
22 RD (Read) 23 FIG. 14Ashows the con~rol signals generated from the CPU
24 including the read signal RD*. This signal, when activated, will tell other~sections of the system that the CPU is ready to 26 accept data. The RD* signal is generated at gate array Z23, : Z7 pin 6. Pin 5 o the same gate is connected to pin 21 of the 28 microprocesser which is the RD* (read) output of the microprocesser.

__ 27.
, .

~ lR4/i ~.1r~3~

!
1 Pin 4 of gate Z23 is tied to pin 19 or the memory request output 2 of the CPIJ. Therefore, when both signals on lines 19 and 21 3 from the microprocesser go to their low level an RD* signal is 4 provided at the output pin 6 of the gate array Z23. Again, this

- 5 array is shown as an AN~ type ga-te and actually a straight OR

6 gate is used and it is drawn like an AND gate with inversions

7 at all terminals rather than a straight O~ gate, the both being

8 equivalent, to indicate that when the memory request and the read

9 signals are present from ~he microprocesser, then and only then will the read signal appear. Thus, a low input on both pins ~
11 and 5 of gate Z23 provides a low output on pin 6, the low indicat-12 ing a read.
13 WR (~rite) 14 The signal l~ denotes a write contro]. Th:is signal, when activated, indicates to other sections that the CPU is ready 16 to transmit data into one of the memory locations. The WR*
17 signal is generated at gate array Z23 pin 11. Pin 12 of this 18 gate is connected to the memory request of the microprocesser 19 while pin 13 of the same gate is tied to the write output signal ¦ 20 from the processer. Again, when there is a low on pin 19 of the 21 processe~ indicating a memory request and when there is a low Z2 on pin 21 of the microprocesser indicating a write portion of the i 23 signal, then and only then is there a low output on signal line ;~ 24 WR* thus indicating a write portion of the memory cycle.
~ OUT (Output) 26 The signal out is for output controlO This signal, when 27 activated, enables circuitry to perform the cassette save 2B functions. It may also be used to control data movement fromthe -~lR4/i 1 basic computer system to an expansion interface. This signal 2 is generated at gate Z23, pin 3. Pin 1 of gate Z23 is tiecl to 3 the write output of the CPU while pin 2 of this gate is tied to 4 the IORQ (input/output request~ output which is pin 20 from the CPU. When there is a low on line 22 from -the CPU indicating a 6 write signal and a low on pin 20 from the CPU indicating an 7 input/output request, then and only then is there a low signal 8 on pin 3 from gate Z23 generating this signal OUT*.
g IN ~Input) The IN signal is for input control. This signal, whe 11 activated, enables circuitry to perform the cassette load 12 function discussed in detail hereinafter. It may also be used 13 to control data movement from an expansion interface to the 14 basic computer system. The IN signal is generated at gate array Z23, pin 8. Pin 10 of this same gate is connected to the pin 20 16 of the CPU while pin 9 of the gate is tied to pin 21 of the CPU.
17 Again, when there is a low at pin 21 of the CPU because of a 18 read portion of the cycle, and when there is also a low signal 19 at pin 20 of the CPU, then and only then is there a signal at the ~utput pin 8 from the gate array Z23 which is the IN* signal.
21 _ontrol Group_Bus 22 The control group signals just previously discussed are 23 buffered for use by the different sections of the system. As 24 part of this buffering system, the control group bus may also need to be switched off at some time. Therefore, there is 26 provided a part of the gate array Z2% which may be a 74LS367 27 including four tri-state devices. It is this array Z22 that is 28 used to buffer the control group signal. Tri-state control is .

~ .

29.

:

provided at line Ll. This control is tied to the address bus control. The ENABT,~* signal effects the stat~s of the address and also the control group bus in the sa~e manner.
Address Decoder As mentioned previously in connection with the diagra~ of FIG. 2, the computer system is memory mapped. Therefore, the address ~lAC (in HEX) is in -the ROM portion of the map.
Address 38~A i6 in the keyboard area and address 3CAA accesses the video display RAMs. Please also refer to the chart of FIG.
3. Since the data and address buses are connected in parallel to all the sections, there must be some way to determine which section is being accessed. A decoding network monitors the higher order address bits and selects which "memory" the CPU
wants to use. FIG. 14B shows the address decoder. In this connection please also refer to the table of FIG. 3. In FIG. 3 it is noted that one could use the two most significant digits of the EIEX code in the decoding scheme and handle the selection of all memories. In the binary colu~ns, one can see that instead o-f using two HEX digits, which is 8 binary lines, two bits can be ignored and thus only 6 binary lines are used. A
dotted line separates the two unused bits from the 6 that are used.
The address decoder depicted in FIG. 14B uses 6 bits, ~;
namely A10-A15, and, in addition, the signals RD* and RAS* (row address select). The address A15 is the most significant bit of the address bus. The 6 higher order bits can have two bits added thereto so that we have two HEX digits A12-AI5 forming the most significant HEX character and A8 All ~orming -the next most significant ~EX character. Addresses A8 and A9 are the two bits ~ ' ~

". J

added to complete the last HEX character. FIG. 3 shows the breakdown of the memory map into HEX and binary.
With reference to FIG. 14B the address decoder comprises device Z21 which may be a device type 74LS156 decoder/demultiplexer. This device is arranged to multiplex two inputs to provide eight different output signals. In addition to the decoder Z21, the address decoder also comprises other decode gates discusssed in detail hereinafter. Address bits A12, A13, and A14 are connected to decoder Z21. The Cl and C2 inputs pins 1 and 15 are connected in such a way to make the decoder Z21 into a three input to eight line decoder. The Gl and G2 inputs pins 2 and 14 to decoder Z21 are chip enablesO When these Gl and G2 inputs are at a logical low level, the decoder Z21 is active. ~hen these inputs are high, the decoder Z21 is disabled and none of its eight output lines are low. This enabling input to the decoder Z21 is controlled by gate Z73A, pins 4 5 and 6. Pin 4 is tied to address line A15, the most significant bit of the address bus. I~ is noted in the memory map breakdown that address line A15 is always low when addressing the various memories. The pin 5 of gate Z73A
is tied to signal RAS*. This signal is generated from the memory request output of the CPU at pin 19 passed by way of gate Z72B shown in FIG. 14A. This buffer source generates RAS*
and it is the same signal essentially as MREQ*. When the signals A15 and RAS* are both low at the same time, a low signal will be outputted from pin 6 to the decoder Z~l. This low signal enables the decoder. When the decoder Z21 is in its ~enabled state, one of its outputs will go low depending upon the code on the address lines A12, A13 and A14. For e~ample, if these -31- ~

:

3~ D

three inputs are at logical zero, pin 9 of the device Z21 is at a low state and all other output pins are at a high state.
mus, it is a low level at the output that is the decoded signal. If all three inputs at A12, A13, and A14 are high, then pin 4 at the output is low. One can consider the code at A12, A13 and A14 as supplying an octal address to the decoder 221. Since there are eight states in an octal code, then there would be one of eight lines selected. Thus, the decoder Z21 decodes the most significant digit of the HEX address. Then, ~
with the use of the last two address bits A10 and All one can ~ -define any one of the four memories available to the CPU.
These four memories include the RAM, ROM, keyboard, and video.
Associated with the decoder 21 is a programmer in the form of a ~imple device X3 referred to as a DIP shunt. This device is like a shorting bar array with some of the bars being shorted and other left open. In this manner the address decoder is programmed to reflect the amount of RAM or ROM the CPU has available for use. In FI~. 14B device X3 is shown with six open shorting bars and 2 closed shorting bars. This -~
configuration will be used in the following discussion.
ROM Decoding When the CPU needs an instruction in order to perform a certain task, the CP~ accesses the read only memory 16. This accessing of the ROM again involves address decoding and the decoder Z21 as shown in FIG. 14B. The ROM decoding is performed as follows. ~le CPU requires a memory and thus the signal ~AS* goes to a low level. From FIG. 3 the address for . ~:. .... . .
the ROM starts with HEX p with address lines A12, A13, A14 and A15 all at a low level~ The decoder Z21 is activated at its inputs G1 and G2 pins 1 and 14 by ~ .
~'~ .

the proper signals A15 and RAS*. The decoder Z21 with all inputs low provides a low output on pin 9. This output couples through the device X3 pins 10 and 7, past the pull-up resistor R61 and out to a terminal identified as ROMA*. This signal couples to ROM A. In par-ticular, this signal couples to pin 20 of ROM A. Pin 20 ~C52) is the chip select input and is active ~.
on a low signal (as the inverted circle on pin 20 shows).
Thus, the ROMA* signal turns ROM Z33 "on" which means that its output becomes active. In this connection note that the ~.
outputs from both ROMs couple to tri-state buffers Z67 and Z68. When the input chip select signal on pin 20 goes low, the outputs from the ROM switch from a high impedance cr of~ state to an on state. When the ROM is thus enabled, the outputs go low or high depending upon the data in the ROM at the address .
that has been selected. In this connection, also note the address inputs to the ROM including addresses AO-A12.
With one of the read only memories being selacted, such as ROM A, there is now a need to insure a data path so that data can pass from the ROM to the CPU. In FIG. I4B the signal ROMA*
is also coupled to one input of NAND gate Z74 (gate Z74 is shown as the equivalent OR gate with inverted inputs). A low .
: at the input of this gate causes a resulting high on its output. mis signal is coupled to pin 9 of gate Z73B whieh is an OP gate. The output at pin 8 at Z73~ passes a high level signal to pin 5 of the NAND gate Z74B. The other input to this gate is tied to the RD* signal now part of th CPU control : group by way of the inverter Z52, pins 12 and 13O Because the CPU is trying to read data from the ROMs RD* is low but the : : othe.r input to gate Z74B is high because of the ,~

: -33-:
: ,'.' ~' -,.'~ ~ ,' - .. ' .. . . '. : ~ . -' : . . : ~: . : .

I - , lR4 12~F~3 ~i1 1 inversion by inverter Z52. Thus, the output from gate Z741 at 2 is a low level signal identified as signal MEM*. As 3 indicated in FIG.14B this signal controls the ROM/RAM buffers 4 ~gate arrays Z67 and Z68). The outputs of these buffers are tied to the data bus 14. The enabling of these buffers by the 6 signal MEM* permits data to be coupled from the RO~Is to the data _ 7 bus. This data flows to the CPU because the CPU is signalling 8 a read portion of the cycle. Because this is to, the signal 9 DBIN* is low and the signal DBOUT* is high. The low signal DBIN*
i 10 enables the input data buffers -to the CPU making data from the ll ROM available at the CPU.
12 Keyboard Decodlng 13 The keyboard 20 i.s identified address-wise frorn address 14 38~ to address 38~F ~see FIGS. 2 and 3). rn the system of this invention the keyboard is considered as a memory device and thus 16 when there is a memory request from the central processing unit, 17 the signal RAS* is low. The keyboard decoding is also associated 18 with the decoder Z21 previously discussed and shown in FIG.14B.
19 In the decoding scheme for the keyboard, the signal AlS is low because we are generating address codes under 8~0~. With 21 reference to FIG. 3 it can be determined that address line A14 22 is low whi:le address lines A12 and A13 are high. With this input 23 combinat.ion and with decoder Z21 being activated at its terminals 24 Gl and G2, there will be a low decoded output at pin 12 (output 3). This output is coupled to gate Z36 pin 4. The gate Z36 is 26 also looking for a low output at its pin 5. Again, re~erring to 27 ~IG. 3 the address line All is high and this signal is inverted 28 by the gate ~ thus providing a low level signal on pin 5 3~.

" .,, :,: .. : ' ' ' ' ' 3~
of the gate Z36. Under this condition the outp~t pin 6 fro~
gate Z36 is also low thus essen-tially enabling both gate Z36A
and gate Z36B. However, only one of these gates will be active depending upon the state of address line A10. Again, referring to the table of FIG. 3 for keyboard decoding, the address line A10 is lowO ~lence, both inputs of gate Z36~ are low and there is provided a low signal on its output identified as -the KYBD*
signal.
The signal KYBD* is shown in FIG. 14B coupling to the enable inputs of the data buffers Z3 and Z4 for the lceyboard.
The lower order address lines AO-A7 are coupled by way of buffer arrays Zl and Z2 to one side of the keyboard matrix, while the other side of the matrix is tied by way of buffer arrays Z3 and Z4 to the data bus including data lines DO-D7.
If a ~sey is pressed, an address line is "shorted" to a data line. A further discussion is found hereinafter on the keyboard operation. In transmitting the data from the keyboard, the signals DBOUT~ and DBIN* shown in FIG. 14A are switched the same way as previously discussed with regard -to ROM selection. Therefore, keyboard data is coupled to the CPU
via the data bus for processing by the CPU.
Video Display RAM Select From the tahle of FIG. 3 it is noted that the binary output for the video RAM address is almost -the sa~e as for the keyboard with the exception of the state of address line A10.
As with the keyboard decoding, the output of decoder Z21 has its low level on pin 12 which couples to pin 4 of gate Z36.
The address line All is high and thus the output fro~ gate Z36 ~ -at pin 6 enables both gates Z36A and Z36B. ~lowever, now rather ~.
"

.. . ~
~ ~' ..~

~ 3~h~
than the output of gate Z36A going low, the output of gate Z36B
will go low because the address line A10 is now high and this high level signal is now inverted by the inverter Z52 to thus provide -two low level signals on the inputs of gate Z36B.
Hence, the output of gate Z36B is low generating the signal VID*. This activates the video RAM. There is a further discussion hereinafter with regard to this video RAM selection.
4K RAM Decoder In the system of this invention there i9 also communica-tion between the CPU and the random access memories. As indicated in FIGS. 2 and 3, the address which selects the RAM extends from HEX 4~ to 4FFF for a 4K memory. The binary breakdown shown in FIG. 3 lists the state of A15 as binary zero.
Furthermore, address line A14 is high while address lines A12 and A13 are low. Memory is still being accessed and thus the signal RAS* is low. Herlce, the decoder Z21 is activated and because of the input address there will be a low output on output pin 7 (output 4). The shunt X3 passes this low through pins 2 and 15 to gate Z74, pin 10. This signal is also outputted directly as signal RAM*. This signal is a signal ~;
that provides a chip enable for the RAMs.
During a read operation the buffering provided by gate arrays Z67 and Z68 shown in FIG. 14B is controlled so as to couple data to the CPU. ThUSJ in that portion of the memory cycle the signal MEM* is low because RD* is low~ However, during a transfer of data from the CPU to the RAM, the signal ~ .:
MEM* does not select the data buffers. Instead, the write signal is active rather than the read signal and these ROM/RAM
buffers ' .' ,~ , ~'~ . .
.. . . . .. .

~
lR4/712 1 Z67, Z68 are not used because the RAM data inputs are on the 2 opposite side oE these buffers. Thus, only during a RO~I/RAM
3 read operation is the signal MEM* n~Jecessary.
4 ~Yith regard to the shunt X3~ the output of the decoder Z21, -this shunt can be adj~sted to program the system for 8K
6 of RAM rather than 4K. This is accomplished by providing a 7 short between both pins 2 and 15 and 3 and 14. In this way, not 8 only would a 4~ address cause signal RAM*, but also a 5000 9 would also enable the signal RAM*. For 12K of RAM~ we could leave also pins 4 and 13 shorted together and for 16K of memorv 11 we can, in addition short pins 5 and 12. For the 16K memory 12 thus, the signal `RAM* would be active from addresses 4~00 to 13 7FFF.
14 With regard to the discussion of the shunt X39 it is noted lS that certain outputs of the decoder Z21 are shorted together.
16 In many applications, shorting output nodes is not good practice.
17 However, in accordance with the present invention using TTL
18 logic9 open collector types are used. These types of gates do no-t 19 have an active pull-up on the output. Instead, the output 2~ transistors have open collectors. It is the responsibility of 21 external circuitry to pull them up. The open collectors are 22 capable of being tied together for a wired OR function. Since 23 decoder Z21 is an open collector decoder, the outputs may be 24 safety tled together. In this connection, note resistors R48, R61, R62 and R68. These are pull-up resistors for the decoder 26 Z21.
27 System RAM
28 Wi th reference to the block diagram of FIG. 19 the system : , .

37.

IR~/712 ~ 3~ ~

1 RAM 18 i5 essentially tied in parallel with the cla-ta bus 14 ancl 2 the address bus 12. sîmilarly to the ROM 16 and the keyboard 20.
3 The data input and output for the RAMs 18 shown in F~G.l~B -are 4 controlled by the signal MErl* which couples to the gate arrays Z67 and Z68. ~ith regard to the addressing scheme for the RA~ls 6 for 4K addresses, one would expect to find 10 address inpu-ts.
7 However, in accordance l~ith one importa;nt feature of the present 8 invention, there are provided only 7 address inputs used in 9 combination with a multiple~ing scheme. In this regard, the address from the CPU is multiplexed into the RAM in two 7 bit 11 parts. The internal logic in the RAM interprets the two parts~
12 and essentially ties them together to form one address schème 13 with a total o~ 14 bits. One part of the addressing is called 14 RAS* (RAM address select); the other part being called CAS*
(column address select). Another signal identi:Eied in FIG.
16 as the MUX (multiplexer) signal controls the switching function.
17 All three of these signals are generated from the logic shown in 18 FIG.l~A.
19 MUX CAS*_R~S*
The logic for developing the signals for controlling the 21 addressing at tlle system RAM incl-lde the seri ~ o~ lip-flops 22 shown in FIG.l~Aincluding two flip-flops Z69~jand one flip-flop 23 Z70. The basic inputs -to this logic include the clock signal 24 and signal MREQ at the output pin 3 ~rom gate Z74. The two OUtpllts at pins 21 and 22 from the microprocesser Z~0 connect 26 to the input pins of the gate Z7~. If there is a low 27 on either of these pins from the CPUJ there isahigh output at 28 pin 3 from the gate Z74. Thus, there is essentially a memory .

.

38.

~ h~
request and there is either a read or a write signal from the microprocesser. The signal MRE.Q is tied to the clear inpu-ts of the flip-flops Z69A and Z69B and part of Z70. I'hese flip-flops are D type flip-flops and ~he signals MVX and CAS* are generated from these flip-flops. E'IG. 4 shows a waveform chart or timiny diagram for this circuit. Line A shows the master clock input to the flip-flops. Line B shows the signal MREQ*
and line C depicts the WR* output from the CE'U, assuming the CPU wants to write data into the ~M. As depicted in FIG. 4 the signal from pin 19 of the CPU goes low first. A short time later, the write signal from the CPU goes low. Line D shows the output pin 3 from the gate Z74 which goes high at the same time that the write signal went low. The flip-flops now have a logical high applied to the clear inputs permitting these flip-flops to operate controlled by the clock waveform. On the next rising edge of the clock, the flip-flop Z69A of pin 5 will assume the logic level that was present at its data input pin ;
at the time that the clock occurred. Since the data input was high when pin 3 went high, then pin 5 will also go to its high state. This high signal is shown on line E of FIG. 4. ~liS ~ . .
signal is also coupled to pin 12 (data input) of the second flip-flop ~69B which is also now high; so that on the next rising edye of the clock its output pin 9 will go to its high state. This is shown on line 5 of FIG. 4. ~he last flip~flop Z70 is now ready to toggle. On the next rising edye of the clock Z70, pin G will go to a low state. This signal is shown on line H of PIG. 4. Now, all three flip-flops have changed state since the write signal from the CPV went lowO The flip-flops will stay in this state so long as the write signal from the CPU stays low. When ,~,, ' - : :
..

lR4/712 1 the signal from the CPU goes high, the fli~-flops will have a 2 low applied -to their clear input; and they will reset back to 3 the clear condition. Line I is the RAS* output. This output 4 is a direct function of memory request output atIpin 19 from the CPU as buffered by the gate Z721~ . The 6 gate Z72~is enabled at pin 1 by the ENABLE* signal. Line J
7 in FIG. 4 shows the MVX signal which has its origin at the 8 output pin 9 of one of the flip-flops Z69fcoupled through 9 buffer Z72~ . Line K in FIG. 4 is the CAS* signal C
and this is buffered by the gate Z721, coupled 11 from the output pin 6 of flip-flop Z70.
12 In summary, the following sequence of events occurs with 13 regard to the diagram of FIG. 4. RAS* goes low ~irst, ~X then 14 changes state. CAS* then changes state one clock cycle later.
Thus, we first get a R~M or row address select, then the multi-16 plexing signal MUX~ followed by the column address select. Hence, 17 the first part of the address will be the row address followed 18 by a switching or multiplexing to the column address.
19 RAM Addressing Data selector/multiplexers Z35 and Z51 shown in FIG. l~B
21 control the addressing to the RAMs. Both of these devices may 22 be 74LS157 devices. These devices have two groups of inputs 23 of either 3 or 4 lines and an output of either 3 or 4 lines 24 depending upon how they are connected. With regard to device Z35 two groups, each of four lines and one group of four is labeled 26 "0" and the other labeled "1". The device Z51 is configured 27 similarly except that there are only three lines per group.
2~ The'0" tells us that when the select pin, which is pin 1 in e~ch 40 .

- .': ....... ' ~ ' "

~ Jj~3 :

dev.ice is low, the multiplexer will be outputting data associated with these i.nput lines. On the other hancl the "1"
tells us the multiplexer will be outputti~g data associated with the other group of lines. The device Z35, therefore, operates like a four pole double throw switch where the select input at pin 1 is performing -the switching. The enable input to these selectors or multiplexers is pin 15. Since pin 15 is permanently grounded, these devices are always enabled.
Reading From RAM
Assume that the CPU requires RAM data. The following discusses the addressing and data paths employed with a 4K .
random access memory~ The cycle commences by the CPU
outputting the signals on its lines 19 and 21. The address decoder outputs the signals MEM* and R~M*. The signal MEM*
activates the R~M/ROM data buffers and the signal RAM* enables the chip select input for the RAMs on pin 13. At the same time the multiplexer loads the address into the RAMs. The signal RA5* ~oes low as depicted in FIG. 4. The MUX signal is low at ;
this time, so the inputs AO-A5 on the RAM receive the lower order address by way of pins 2, 5, 11 and 14 of the device Z35. The signal RAS* is buffered by gate Z68A, and is applied to pin 4 of all of the RAMs depicted in FIG. 14B. The negative going signal at this input pin 4 loads the lower order address in the row selection of each RAM. A short time later the MUX
signal changes state going high as depicted in FIG. 4. The multiplexer comprising devices Z35 and Z51 now switches and the ~ .
higher order addresses are applied to the RAMs. The signal CAS* goes low. CAS* is applied to the buffer gate Z67A. The output of this gate , .' .

,i :-. . .
' lR4/712 ~ 3~ ~

1 passes the signal CAS* to pin 15 of all eight RAMs. On the 2 negative transition of signal CAS*, the high or(ler addresses 3 (A6-All) are loaded in the columned section of each RAM. Four 4 of these addresses are coupled by way o device Z35 and two are coupled by way of device Z51. The RAMs now have the en-tire 6 address from the CPU. The RAM now outputs this addressed data 7 through the associated buffers to the CPU.
8 Writing to RAM
9 During a data write cycle, the CPU sends data to the R~ls.
Hence, the ROM/RAM buffers are not employed and it is not necessary 11 for the signal MEM* to go low. Instead of the CPU issuing a 12 read command, it issues a write instruction. Thus, the signal 13 WR* is tied to all 8 RAMs Oll pin 3. ~hen this pin is low, data 14 is stored in each RAM at the specified address. When this pin is high, the RAMs are in a read cycle.
16 Refreshing The RAMs 17 The computer system of this invention uses a dynamic type 18 RAM. A dynamic RAM differs slightly from a static RAM in data 19 retention. A static RAM retains data stored in it so long as power is applied to the system. A dynamic RAM on the other hand 21 requires periodic addressing to insure that it retains the data 22 loaded into it. This periodic addressing is called refreshing.
Z3 The refreshing of the RAMs is accomplished by the RAS* signal.
24 When this signal goes low, all of the individual RAMs in the system will refresh themselves even though they may not be in 26 use at the time. As mentioned previously, the signal RAS* is 27 generated by the CPU at pin 19. When pin 19 goes low, RAS* goes Z8 low and the RAMs will load the lower order address into the row ' . ~ .
.

42.

.
.

- "
lR4/712 ~ 3~ ~

1 section. The CPU may be looking at system ROM when pin 19 goes 2 low, but the RAM will still receive the signal RAS* and hence 3 be refreshed. In a system of this invention the RAM should be 4 refreshed once every two milliseconds.
RAhl Programming 6 Associated with the RAMs is a shunt X71 shown in FI~ B.
7 This is used to program the size of the memory in the system.
8 Pin 13 on the RAMs is a chip enable and this couples to one sec-9 tion of the shunt X71. In a 4K system, pins 4 and 13 of shunt X71 are shunted. The signal RAM* is on pin 4 so this signal is 11 used to select the RAMs. However, in a 16K system, pins 4 and 12 13 are opened and pins 3 and 4 of the shunt X71 are shorted.
13 Thus, instead of the signal RAM~ there is the address lines A6 14 or A12 depending upon themultiplexer condition.
Video Divider Chain 16 The video divider chain shown in FIG.13Asupplies the video I7 RAMs 22 with addresses in a logical order for video processing.
18 This chain also supplies the horizontal and vertical sync timing 19 pulses so that the video processer can build the composite wave-form for the display. Video RAM addresses, horizontal and 21 vertical sync, and video processing timing are all direct 22 functions of the master clock. Also included in the divider chain 23 is the hardware necessary to generate 32 character line lengths.
24 ~ivider Chain - Input Conditioning In accordance with one important feature of the present Z6 invention, the computer sys~em has two formats for character 27 length. In one forma~t, the display has 16 character lines, each 2~ consis~ting of 64 characters. This means there are 1024 .

I 43.

character locations in video RAM that the divider chain must access. In the other foîmat, the characters appear twice as large. The display has 16 character lines of 32 characters rather than 64 characters. In this case the divider chain accesses only 512 video RP~I locationsO Switching from one format to the other is the task of the input conditioning logic shown in FIG. 13A.
In FIG. 13A the master oscillator circuit couples -to a flip--flop Z70 at pin 11 and also to a mul-tiplexer Z43 which ~ay be a 74LS157 device. The conditioning circuitry also includes a divider 258 which may be a divide by 12 divider. This divider may be a 74LS92 device. The D flip-flop Z70 is wired to perform a divide-by 2 function. The multiplexer is wired so that one can route the master clock frequency or one-half of the master clock frequency fxom the flip-flop Z70 to the divider Z58. Since there are two character length formats, there are two reference frequencies, one that is half as slow as the other. The master oscillator supplies the divide-by 12 counter Z58 as a reference frequency in a 64 character format.
The D flip-flop supplies the counter with the reference ~requency in a 32 character format.
The multiplexer Z43 is of the sa~e type previously discussed as devices Z35. The multiplexer Z43 is controlled by the signal MODE SEL. When the mode select signal is low, the multiplexer Z43 is switched to its 32 character position. ~hen this signal is high, the multiplexer is switched to its 64 character position. First, the 64 character mode is analyzed.
For ~his mode of operation, with the mode select signal high, pin 3 of the multiplexer couples to the output pin 4.
Similarly, pins 6 and 7 are ., :: . : ....

~ "

lR4/712 ~ $ ~

1 intercoupled and pins 10 and 9. FIG. 5 is a waveform chart form 2 this circuit. At line A in FIG. 5, the master clock is shown 3 at the output of its buffer gate Z42, pin 6. Line B shows the 4 action of the D flip-flop Z70 with its divide-by 2 output. The buffered clock is applied to pin 3 of the multiplexer Z43. Since 6 the multiplexer is switched to its '1" state, the counter Z58 7 receives the basic clock frequency at its pin 14. It is noted 8 that the output of flip-flop at pin 9 is tied to pin 2 of 9 the multiplexer. However, this is not performing any function at this time since the multiplexer is not switched to its "0"
11 state.
12 The outputs from the counter Z58Ashown at lines C, D, E and 13 P in FIG. 5. In FIG. 5 the arrows indicate the place where all 14 outputs are Q. It is noted in FIG. 5 that the lines C-F do not count directly up to 11 and then back to 0 using straight binary 16 counting. Instead, the output count from counter Z58 goes from 17 ~-S and then on the next clock it goes from binary 5 to binary 8.
18 From 8 it counts normally to blnary 13 and then in the next 19 cycle it goes back to binary 0.
The inputs at pins 6 and 7 o counter Z58 control the 21 clearing of the counter to 0. The signal CTR to these input 22 pins is generated in FIG.14Aat the output pin 8 of the inverter 23 Z42. Normally, the signal CTR is low. Only during automatic 24 testing is CTR allowed to go high and clear the device Z58.
The ou~put pin 12 from the counter Z58 is identified in 26 FIG.L~ as DOT 1. Pin 9 of the counter is labeled DOT 2. These 27 two slgnals are NANDed by the gate Z24 at pins 1 and 2 to 28 provide an output at pin 3 shown in line G of FIG. 5. This ;

: .

45.

lR4/712 ~ 3~9 1 signal is called the LATCH signal and is used in the video 2 processing circuitry.
3 The input pins 6 and 10 of device Z43 are tied together and 4 connect to the outpu-~ pin 8 of counter Z58. The resulting out-put is at pins 7 and 9 of the multiplexer Z43. The signal at 6 pin 9 is referred to as the CHAIN signal and is the main source 7 for the divider chain comprising devices Z65, Z50, Z12, and Z3Z.
8 The output pin 7 of multiplexer Z43 is labeled "Cl" and is tied 9 to pin 10 of device Z64 which is one of the video RAM multiplexers.
The signal Cl is used to address the video RAM's least significant 11 bit.
12 For the 32 character format, the pin 1 of the multiplexer 13 Z43 is in its low state and therefore pins 2, 5, and ll are tied 14 to the respective outputs at pins 4, 7, and 9. Thus, the half ~70 15~ clock frequency from pin 9 of the flip-flop~ ~ couples to the 16 o~utput pin 4 of the multip].exer Z43. Pin 1 of the device Z43 17 is~held low all the time andthe output CHAIN signal is now one 18 coupled directly from the output pin 9 of the counter Z58 shoiYn 19 in line E of FIG. 5. For the 32 character format,~FIG. 5 shows :,:
the outputs from the counter Z58 at lines H through K. It is 21 noted that the waveform at line B in FIG. 5 is used as the input Z2 to~the dlvider:co~nter Z58 and thusAcounter is used as a divide-23 by 6 counter.~ The output at pin 9 of the counter will be the 24~CHAIN s~igna~l~ instead of at pin 8 ln the 64 character format.
25~ However,~the fre~quency of the CHAIN signal has not changed.~ In ;Z6~the 64 charac~er~mode, the mastel clock was divided by 12 to 27 provlde the~chaln f~equency. That is 10.6445 MHz was divided by Z8 1~Z to~provlde~a~frequency~of 887~041 KHz.~ In the 32 character ; 46.
,. .: ~
. . . .. .

lR4/712 1 mode, one-half of the master clock was used, divided by 6 to 2 provide the same end frequency of 887.0~1 Kllz. However, t~o 3 signals did change. In the 6~ character format, the latch pulse 4 was only one clock cycle wide having a period of 6 clock cycles.
In the 32 character mode, the pulse wid~h has dou~led to two 6 clock cycles and its period is now 12 c:Lock cycles. The other 7 signal that changed was Cl. It was a square wave at the same 8 rate as the chain signal for the 64 character format, but in 9 the 32 character mode, it is held low at all times. The signal LATCII is used to delay a character between the RAM and the 11 character generator. The signal Cl determines if the v:ideo 12 RAM has 1024 or 512 useable addresses.
13 Divider Chain 14 The divider chain circuit comprises 4 bit ripple counters lS Z65, Z50, Z12 and Z32 shown in FIG.13A. FIG. 6 shows a simpli~ied 16 block diagram for the divider chain to enable an easier under-17 standing of the counter chain.
18 Each of the counters comprising the chain may be a 4 bit 19 counter having two different clock inputs coupling to respeative successive St~ges of the counter. In input atpin~ clocks all 21 stages of the counter whereas an input at pin 1 counts only the 22 last three stages of the counter.
23 As depicted in FI~. 6 the counter Z65 may be considered as .
24 being separated into two different parts. The chain input from the conditioning logic is applied to pin 1 of the counter. The 26 outputs B and C fro~ this counter couple to the multiplexer Z~4 27 and are used for addressing the video RAM. The output at pin 8 28 from counter Z65 couples to the next counter Z50 in the chain.

.
: ~ .
47.
~ .

3~`~
lR4/712`

1 This portion of counter Z65 divides the chain frequency by 4 2 as indicated in FIG. 6. Since the chain frequency is 887.0461 3 KHz, the output of counter Z65 at pin 8 is then 221.760 K~lz.
4. The next counter in the chain is counter Z50. The input of this counter is on pin 14 from counter Z65 and the di~ider 6 frequency is at pin 11. This device is externally modified to 7 divide the input frequency by 14. The counter Z50 counts up 8 normally to a binary value of 13. The following shows the counter 9 outputs at that count:
Pin 12 Output A = l 11 Pin 9 Output B - 0 12 Pin 8 Output C = 1 13 Pin ll Output D = 1 14 Upon the next negative transition of the clock pulse the outputs are as follows:
16 Pin 12 Output ~ - 0 17 Pin 9 Output B = 1 18 Pin 8 Output C = 1 19 . Pin 11 Outpu-t D = 1 This provides a 14 count. The AND gate Z661 accep-ts the B, C and 21 D outputs from counter Z50. The output of the gate Z66lat pin 6 22 goes high under the second condition listed above and thus clears 23 the counter Z50 back to 0. This clear pulse is quite rapid on 24 the order of about 50 NANO seconds. The time that counter Z50 is actually reading binary 14 is so short that it can essentially :
26 be ignored. Therefore, counter Z50 counts from 0-13 and is then 27 reset back to 0. Since the frequency of 221.760 KHz is inputted 28 to the counter Z50, the output at pin 11 is 15.840 KHz. This -48.
- . : .

frequency is used by the sync generator circuits to produce horizontal sync as illustrated in the block diagram of FIG. 6.
The next divider or counter in the chain is counter Z12.
I`his counter is wired to provide a division by 12. In this connection, not~ the gate Z66B. The counter Z12 counts up normally until the outputs enable AND gate Z66B. This happens at the 12th falling edge o~ the clock. Gate ~66B, pin 8 will then go high and clear the counter Z12 back to O. Once again, this clear pulse is essentially ignored and hence we can consider the counter Z12 as a divide~by 12 counter. With a fxequency o-f 15.840 KHz being applied at its input, the output at pin 11 of the counter is thus 1.32 KHz.
~he ne~t counter in accordance with the illustration of FIG. 6 is the other part of the counter Z65. Thus, note the output pin 11 from the counter Z12 coupling back to the input `
pin 14 of counter Z65. The output from this counter with regard to its second part is taken at pin 12 which couples back down to counter Z32 at its input pin 14. This portion of counter Z65 divides the 1.32 KHz signal by 2 and therefore, the frequency at pin 14 at the input of counter Z32 is a frequency ,:
of 660 Hz.
The counter Z32 is the last counter in the chain. It divides the 660 Hz input by 11 producing a 60 Hz signal. When the output from the oounter Z32 equal binary 11~ the gate Z66C
outputs a clear pul.se to reset the counter Z32 back to O. The 60 Hz output at pin lI is used by the 5ync generator circuits ~ ;
to p~oduce the~vertical sync ~VDRV) for the video monitor.
~: .-': , :

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.

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1 Video RAM _ l ressing 2 The video RAM 22 depicted in FI~. 1 is addressed for 3 different purposes. First, the CPU addresses the video RA~ls to 4 read data from or write data into specific locations of memory.
The divider chain also addresses the video RAM so that data 6 contained in memory can be processed and displayed on the screen.
7 The video RAMs are ei-ther addressed by the CPU or by the divider 8 chain through the use of the three multiplexers Z64, Z49, and Z31 9 depicted in FIG.13A. These three multiplexers are used for video RAM addressing. From the divider chain previously discussed there~
11 are 10 address lines that are used to address the video RAMs Z45-- 12 Z48 and Z61-Z63 as shown i.n FIG. 13B. These addresses are iden-13 tified as V0-V9 coupling in groups from the multiplexers Z64, Z49 14 and Z31. The chain condltioning logic supplies from multiplexer Z43 the signal Cl. Counter Z65 supplies three addresses i~entified 16 in FIG.13Aas Rl, C2 and C4. The counter Z50 supplies three 17 addresses - C8 ? C16 and C32. The counter Z32 supplies the 18 remaining addresses R2, R4 and R8.
19 Assume an array of rectangles; 16 rectangles vertical and 6~ rectangles horizontal. This would represent a total of 1024 21 rectangles. One could specify any one rectangle by starting at 22 the top left hand corner going down a predetermined number of rows 23 and moving to the right a predetermined number of columns. The 24 16 rows are assigned a binary number from ~ to 15. The 64 columns are assigned a binary number from ~ to 63. Thus, rectangle 0-0 26 15 the one in~the upper left hand corner of the array. Similarly, 27 rectangle 15-63 is in the lower right hand corner. Thus, 4 ~ 28~ bits of binary information specify any one of the 16 rows and 6 : ' : , 50.

- - \
lR4/71~ 3~

1 bits oE binary inEormation specify any one of t]le 64 columrls.
2 This is exactly the addressing format used by the coun-ter chain.
3 Signals Cl, C2, C4, C8j C16 and C32 specify any column and signals 4 Rl, R2, R4 and R8 specify a row. The row/column addressin~
: 5 format is very useful in trouble-shooting video problems in 6 the system.
7 The column and row address ou~puts from the divider 8 chain are applied to the "1" inputs o~ the multiplexers. Part 9 of the address bus from the CPU is tied to the "~" input of the multiplexers. The outputs from the multiplexers are tied to the 11 video RA~Is or to other control logic associated therewith. As 12 far as control is concerned in FIG.13A there is shown the signal 13 VID* that is generated in the address decoding section. This 14 signal selects the ~ideo RAMs. Pin 1 oE the three multiplexers Z64, Z49 and Z31 receive this signal VID*. When the CPU wants 16 control over the video RAM, the address decoder recognizes the 17 vi~eo RAM address and causes the signal VID* to go low. When this 18 occurs the multiplexers each switch from the "1" position to the lg "0" position. The counter chain addresses are switched out of the circuit and the CPU has control over the addressing to the 21 video RAMs. When the signal VID* goes back to its high state 22 under control from thé CPU, the CPU is switched out a~lthe 23 counter chain takes over. Most of the time the counter chain is 24 in control of the video RAMs. The CPU takes charge only when it needs to ~odify data. ~' 26 In addition to the chain and CPU address, there are inputs 27 to the multiplexers not yet mentioned. The first of these inputs 28 is the resistor R49 coupled to pins 6 and 13 of multiplexer Z49.
:: :

,~ ~

51.
-.
-,.

IR4/712 ~ ~

1 These two inputs, which are not needed in the counter chain 2 control over the video RAM, are pulled up to 5 volts by t}liS
3 resistor. The output pins 12 and 7 of multiplexer Z49 correspond 4 to the inputs at pins 13 and 6. When -the chain has control over the video RAhl, pins 12 and 7 output a steady high s~ate. Output 6 pin 12 goes to the read/write control fc)r all RAMs thus not 7 permitting any reading or writing associated with the RAMs. The 8 counter chain does not store data in the RAM at the address it 9 specifies and thus pin 12 should be high when the chain is in control. The output pin 7 from multiplexer Z49 couples to the ~,~6, /3~7 11 video RAM data buffers Z44 and Z60~ l~hen the chain is in control, 12 ~the RAM data bus is to be disabled and there is no reading from 13 or writing into the video RAMs. A high of the signal VRD*
14 (video read) guarantees that the bus is o-Ef. These data buffers look for a low level signal for the enabling thereof.
16 In FIG.13A it is also noted that the signals WR* and RD* tie 17 to pins 14 and 5 o the multiplexers Z49. When the CPU takes 18 charge of the video RAMs, the multiplexer output at pin 12 becomes 19 VWR* (video write). The CPU can store data into the video RAMs by causing the signal V~rR* to go low. If the CPU wants to read 21 data from the video RAMs, the signal RD* can pass through the 22 multiplexer Z49 and generates the signal VRD*. A low on this line 23 will open the data bufers Z60 and Z44. Addressed video RAM
24 data is then placed on the data bus 14. The CPU processes this Z5 data as any other data.
26 Alpha-Numeric Format 27 The video terminal 26 shown in FIG. 1 is a cathode ray tube 28 ~CRT) which is scanned twie per second. The electronic beam in .

- 52.
- , - ~ .

lR4/ 7 ~ 3~

1 the CRT travels from top to bottom of the screen and from left 2 to right. Each screen or Erame consists of 264 scan lines. 192 3 scan lines are used in the "picture". The remaining 72 scan 4 lines are used during the vertical interval and as upper and lower boundaries. Nothing is ever wr:itten or visible within 6 these 72 lines. There are 1024 character locations per screen 7 or 512, depending upon the state of the signal MODE SEL. Each 8 character line comprises 64 or 32 characters depending upon ~lle 9 state of this signal. There are 16 character lines. Each character line consists of 12 scan lines. An alpha numeric 11 character uses 7 scan lines and thus there are 5 blank scan 12 lines disposed between character lines. A discussion of the 13 graphic Eormat follows.
14 Some of the output from counters Z65 and Z50 shown in FIG.13A specify the column address. The outputs from counter 16 Z32 specify the row or character lines. The counter ZlZ speci-17 fies the scan line in any character line. The outputs fro~ this 18 counter are labeled Ll, L2, L4 and L8. These four lines are 19 not used in video RA~I addressing because we have already stated and row and column address which will specify any one of 1024 21 rectangles in our rectan~le array. The outputs from the counter 22 Z12 are used in the video processing for enabling the character 23 generator to output correct data for any charac~er because it 24 knows where the CRT's electron beam is scanning. The si~nal L8 is used by the video processer to BLANK (turn-off) the 5 lines 26 between character lines. Associated with the multiplexers is a 27 NOR gate Z30, pins 8-lOo The inputs to these pins couple from 28 multiplexers Z50 and Z32 and provide a signal BLAN]~*~ This . . - ~ - -,, .

~ 5.,iL"~L~3,~
lR4/712 l signal is used by the video processer to give the 72 scan line 2 blanking for the upper and lower boundaries. It also defines 3 the boundaries onthe left and right of the screen.
4 Video RAMs The video RA~I comprises 7 separate memories 9 one for each 6 of the data bits D0-D6. ~ach o these RAMs is a conventional 7 device such as the 2102AN4L device. These RAMs are static RA~!s 8 and hence do not require refreshing. The data bus 14 is wired 9 in the same way as wi~h regard to the system RA~Ss 18. However, there is a different enabling signal which in the case of the ll video RAMs is the signal VRD*. It is noted that there are 7 12 RAMs. 6 of these are used for storage of the ASCII code and the 13 7th is used as a graphic/alpha numeric definition bit. There are 14 thus 8 data lines depicted in FIG.13B including lines D0-D7. Note the line labeled bit 6 which has its source a~ the output pin 13 16 of gate Z30 which is a NOR gate shown as an AND gate with inverted 17 inputs. This gate senses bits 5 and 7 and if both are low, then 18 bit 6 is high.
19 In addition to coupling to the data bus 14, the video RA~ls 22 also couple to the video processing circuitry for the 21 generation of alpha numeric and graphic symbols. This section 22 of the system is discussed now.
~3 Video Processing 24 The video processer 24 depicted in FIG. l may be considered as comprising 5 sub-sections shown in FIG.13Bincluding the data 26 latch, ZZ$9 the character generator Z29, the graphic generator Z8, 27 associated shift registers Zl0 and Zl15 a sync generator, and a 28 video mixing/outpu~ driver. The data latch temporarily stores an ., \
, ~ 5~.
, 3~
lR4/712 1 ASCII or graphic word from the video RAM. The latch retains 2 the byte for processing so tl~at the RAM is free to search out 3 the next byte. The character generator is a read-only memory 4 that is addressed by the data latch and the scan line signals.
This memory contains the alpha numeric format that makes up 6 all characters. The graphic generator is not a memory but is 7 a four line to one line data multiplexer. It operates somewhat 8 like a bit steering circuit. It steers an ASCII word in~o a 9 graphic symbol. The shift registers accept data from the character generator or ~rom the graphic generator and convert 11 parallel dot data into serial dot data. Meanwhile the sync 12 generator circuits are accepting timing signals from the timing 13 chain. l'he sync clrcults shape up the horizontal and vertical 14 pulses, serrate the vertical interval and send signals out to video mixing circuitry in serial format. In the video mixing 16 section, the serial dot video and the serial sync are brought 17 together. The resulting composite video signal is then "fine-18 tuned" in amplitude and dot-to-sync ratio~ and then bu-~fered for 19 a 75 ohm output cable. The signal leaves the basic computer system and is applied to the display. In the display, the signal 21 is separated into its separate components to provide a readable 22 image on the screen. The display may be a conventional CRT having 23 built therein conventional raster circuitry.
24 Data Latch The data latch comprises two separate circuits including the 26 latch Z28 ~or the ASCII code and the latch Z27 for the graphic 27 bit and blanking signals. The device Z27 may be a 74LS175 and 28 the device Z28 a 7~LS17~. The latch Z27 is a quad ~ flip-flop 55.
.
.. . . . - , .
,. . .

lR4//l2 ~ 3~-~

`; 1 device while the device Z28 is a HEX D flip-flop device. The 2 lnputs to latch Z28 are from the 6 video RAMs bits 0-5. The 3 outputs from the latch Z28 couple to the character generator 4 Z29 and also to the graphic generator Z8. The inputs which ;`' 5 control latch Z28 are on pin 9 which is the signal LATCH and 6 on pin 1 which is the signal VCLR*. The latch signal at pin 9 7 is a pulse train previously discussed and depictecL in FIG. 5 and 8 developed by the divider chain input conditioning logic. This 9 signal goes Iow every 6 dot cycles. On the rising edge of the 1~ latch signal the ASCII data in the ~AMs Z45-Z48 and Z61-Z63 is 11 transferred to the outputs and temporarily stored by the latch 12 Z28. Once this has occurred the RAM data at the input to the 13 latch may IIOW change, and the RAM has time to search for the 14 next ASCII character. At the same time that the latch Z28 stores I5 the code, the divider chain changes video RAM addresses. The 16 video RAM is now looking for the next ASCII word. It has exactly 17 6 dot times (about 560 NANO seconds in a 64 character format) to ¦ 18 define the next word before the latch is commanded to store the 19 next word.
2b The device Z27 is a smaller latch storage-wise that operates 21 substantially the same as latch Z28. However, instead of storing 22 an ASCII code, it handles the graphic bit and blanking data.
23 The input pin 4 of device Z27 is tied to the output bit 7 from 24 RAM Z63, the graphic RAM by way of the inverter Z42t, pins 12 and 25 ~1~. Pin 5 of the latch Z27 receives the signal BLANK*. The : ~ ~ 26 signal line L8 ties to the input pin 12 and the signal bit 6 is 27 tied to pin 13 of latch Z27. All of these signals are latched ` 28 into the device Z27 at the same time that the ASCII word is :::
. ~ ' .
- , .
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-lR4/712 ~ 3~

1 latched into the corresponding latch Z28.
2 The inputs to devlce Z27 are essentially independent whereas 3 the inputs to the device Z28 represent a coded word. The graphic 4 bit which is bit 7 coupled from memory Z63 couples by way of 5 inverter Z42 ~ to pin 4 of the latch Z27. This 6 bit determines if the ASCII word contained in the latch Z28 is 7 an alpha numeric character or a graphic word. The input to pin 8 5 of the latch Z27 is the sig ~ BLA~K*. This signal comes 9 from the NOR gate Z30 at pin ~ and controls the upper, lowerj ;

10 left and right boundaries of the video display. When the signal

11 BLANK* is high, the electron beam of the CRT is allowed to draw

12 on the screen. When the signal BLANK* goes low, the beam is in

13 a boundary area so it prevents the beam from drawing anything.

14 The signal L8 is connected to the input pin 12 of the latch Z27.

15 This signal also acts somewhat as a blanking signal. This signal

16 specifies where the electron beam is located in any character line.

17 When the signal L8 is low, this allows the beam to output alpha

18 numeric dot data. When the signal L8 goes high, this shuts off

19 ~the beam because it is now scanning one of the five scan lines

20 between character lines. The last bit of data is coupled to pin

21 13 o.f the latch Z27. This ~nput couples from the output pin 13

22 of gate Z30. This is the signal that is derived from the signals

23 bit 5 and bit;~7. This is the only bit stored in the latch Z27

24 that could ~e considered part of the ASCII word in that this

25 determines whet~her the word represents an alpha numeric character Z6 or~a~graphlc word. The output corresponding to ~he input pin 13 27 ~at pin 15 is applied to the input pin 1 of the character generator 28 Z29 dlscussed in detail hereinafter.

, ~ 57.

., . ~ ,~. .. . . . . .

lR4/712 ~ 3~- ~

1 The input pin 1 of both latches Z27 and Z28, when low, 2 forces the latches to their clear state providing all zeroes 3 at the output terminals. This signal is s]lown as signal VCLR*
(video clear) and is coupled from the output pin 6 of the D
flip-flop Z7. This flip-flop disables the data latches during 6 a CPU interruption of the video RAMs. In this regard the 7 input pin 4 of the flip-flop Z7 is tied to the signal VID*.
8 When this signal goes low, the output pin 6 from the flip-flop 9 will also go low. This low signal clears the data latches Z27 and Z28. When the CPU has finished with communication with the 11 video RAMs, the input pin 4 to the flip-flop Z7 goes back to 12 a high state because VID* goes high. The next time clata is to 13 be latched into devices Z27 and Z28, the flip-flop Z7 will toggle 14 back to its normal reset state and allow the data latches to operate. This clocking occurs a-t the input pin 3 of the flip-16 flop Z7 and is the latch signal. If the device Z7 were not used, 17 the display may be i~proper. For example, if it is assumed 18 that the CRT was drawing a character when the CPU too~ command 19 of the video RAM, after the CPU finished, the video processing circuit sees the ASCII code that was in the latch at the time 21 the CPU suddenly took control. The video circuit would try to 22 redraw the character on the screen. The character may then be 23 seen twice, or half of it would be in one place on the display 24 and half in another place. Clearing out the data latch insures that the video processer does no~ process incorrect data.

26 Character Generator Z7 As part of the video processor 24 shown in FIG. 1, there 28 is a character generator depicted in FIG.13Bas device Z29.

~' .

58.

lR~/712 ~$~3~

1 On the CRT display, each character comprises a dot matrix. As 2 previously ment;oned, the ma-trix is 5 dots wide by 7 dots high.
3 The system also provides ~or one dot between any two adjacent 4 characters that are not ~urned on. This provides a period ~or the end of the sentence, for example. There are 5 dots, a space, 6 5 more dots~ space, etc. Vertical spacing between adjacent data 7 is determined by the fre~uency of the dot clock. In the system 8 of this invelltion the dot clock signal is identified as signal ~3 9 SHIFT. The SHIFT signal couples from the multiplexer ~A~-and relates to the basic clock frequency. The dot clock is equal 11 to the oscillator fre~uency in the 64 character format and is 12 equal to one-half the oscillator frequency in the 32 character 13 format. The horizontal spacing between adjacent dots is a 1~ function of scan frequency. Thus, each row of dots is aligned albng the electron beam's path across the CRT, there being 7 16 rows of character dots and 5 rows of blan~s.
17 Since each character comprises a pattern of dotsj there is 18 to be a way to determine which dot should be on and which dot 19 should be off to ~orm any one character. Ihe character generator controls the dot patterns on the screen.
21 The device Z29 in FIG.l~ is the character generator and 22 may be a device MCM6670P typically made by Motorola. The 7 bit ,. ~
23 ASCII word, stored in the data latch~28, is applied to the inputs 24 o~ device Z29 at pins 1-7. This input address selects a certain area in the character generator. These inputs may be considered 26 as the higher 7 bits of an address. The lower part o~ the

27 addTess is ~putted at pins 8, 10 and 11. This 3~bit input selects

28 the row posi~ion o~ the addressed dot pattern. These are signals 59.

lR4/712 ~ 3~

1 Ll, L2 and L4. The charac-ter generator Z29 OUtpllts 5 dots at 2 one time on output lines 12-16. Since each character consists of 3 7 rows of 5 dots, the character generator actually outputs 7 4 separate times to build one character.
The following is a description of the manner in which a 6 typical character line is written. Assuming -that an ASCII worcl 7 is in the latch Z28, and that the electron beam is on the -first 8 scan line of the character, hence, pins 8, 10 and 11 have a 9 binary 0 applied to them. The generator Z29 outputs the first dot pattern for that particular ASCII code. The next ASCII
11 character is applied to the device Z29. It outputs the first 12 5 dots for that character. This process continues until the 13 beam has scann~d the entire width of the screen. If one could 14 cease action at this time, all one would have would be a line of dots. On the second scan line, the data at pins 8, 10 and 11 16 is incremented to read ~inary 1. The RAM is now prepared to 17 read the second row of dots. The first ASCII character is applied 18 and it will output the second row of dots ~or that character.
19 The first ASCII character is the same character as previously in the latch at the beginning of the first line. The second ASCII
21 word then is put into the latch Z28 and a second row o-f dots is 22 generated, etc. This process continues until all 64 characters 23 have had the second row outputted under the ~irst row of dots.
24 The line counter increments and we apply the first ASCII word once more. Essentially, we paint a row of dots, increment the 26 line counter-and then paint another row. Any character in a 27 line is accessed then at least 7 times. Once the line counter ~8 has gone past the 7th dot, all dots should then ~orm the character , 60.

. ' .~

lR4~ 2 1 ard this dot pattern should be discernible as a character.
2 A~ter the 7 dot scans are outputted, the electron beam is turned 3 off; and 5 rows of blank dots are outputted. The system ls 4 then ready to output the first row of dot patterns for the second character line. The counter Z~2~is the counter that increments 6 in binary to provide the proper binary signals Ll, L2 and 14.
7 This counter is re~erred to as the line counter.
8 Graphics Generator 9 Previously, with regard to character generation, mention , 10 was made of the rectangular array used to de~ine each character.
11 As stated earlier, there are 1024 character locations in the video 12 RAM. I~ the large rectangle Or 12 x 6 is sub-divided into 6 13 smaller rectangles, then there is provided the basic graphic 14 cell as depicted in FIG. 7. This cell is the smallest piece of graphic in~ormation that is to be displayed on the screen.
16 Each cell is 4 scan lines high and 3 dots wide.
17 Adjacent to the character generator Z29 is the graphics 18 generator Z8 which may be a device 74LS153. Actually, the 19 device Z8 is not a generator but rather it steers the ASCII
a~dresses round to simulate a graphics generator. The inPut :
21 to device Z8 ls the ASCII code from the latch Z28 along with the 22 inputs at A and B which are the higher order address liries 23 from counter Z12, namely, signals L4 and L8. These signals 24 can r~epresent any four numbers from binary 0 to binary 3.
Please note FIG. 7. However, since counter Z12 does not go to Z6 ;binary 12? we will only be looking for a binary number from 0 to 27 3. Address lines L8 and I,4 are used to specify the vertical 28~ address of the 6 graphic cells. There are three vertical :

.
.

IR4/712 ~ ~

1 addresses: 00 defines the uppermost pair o cells, Ql defines Z the middle pair of cells and 10 defines the lower pair.
3 The ASCII word, labeled LB0-LB5 determines if the graphic 4 cell is on or off. The position of one of these inputs to device Z8 determines which side of the center line the cell is 6 located with reference to the diagram of FIG. 7. An input at 7 pin 6 of device Z8 specifies a left hand graphic cell while an 8 input at pin 10 specifies a right hand graphic cell. Similarly, 9 pin 5 denotes left, while pin 11 denotes right and pin 4 notes left while pin 12 denotes right. For example, assuming that 11 the signal LB2 is high and all other inputs to device Z8 are 12 low, this high input at pin 5 is associated with a graphic cell 13 location on the left of the character position. Therefore, 14 depending on the status of signal L8 and L4, LB2 will turn on one of the graphic cells on the left of the center line of FIG.
16 7. If L8 and L4 are at logical 00, the upper left cell is turned 17 on. If signal L8 and L4 are 01, the middle left cell will be 18 activated.
19 In summary, the function of device Z8 is to steer the ASCII
word essentially around the character rectangle. The vertical 21 position of the graphics in the cell is determined by the status 22 of addresses L8 and L4. The two outputs from the device are ~",..
23 labeled left and right. This dot information is applied to the 24 graphics shift register Zll. It is in shift register logic that data from the memory Z63 determines if graphis or alpha numerics 26 are to be written in any one character rectangle.
27 Alpha Numeric/Graphic Shift Register 2B The device Z10 shown in FIG.~B is a shift register and is :
:: :
62.
.
.: .

termed herein the alpha numeric shift register. The device Zll is the graphic shift register. Both of these devices receive parallel data from their respective generators. The parallel dot data is loaded into the registers and the dot clock (labeled SHIFT~ shifts the dots out, one behind the other, to the video mixer comprising transistors Ql and Q2.
~ here are some restrictions as to when the alpha numeric shift register may send its serial data. First, the data must be alpha numeric and not graphic. Second, the electron beam is to be on one of the 7 scan lines that are r~served for dot data ~ `
and not on one of the 5 lines that are blanked between . .
character l1nes~ Third, the electron beam is to be on one of the 192 scan lines that define the video portion of the screen. Once all three restrictions are met, the dot data is parallel loaded into the register. NAND gate Z26A at its Input pins insures that all conditions are met before data is stored in the shi~t register Zl0. The delay bit 7* is coupled from ;the~output pin 2 of~latch Z27 and couples to one input gate Z26A. When this input is high, data in the memory Z63 is low thus defining an alpha numeric character rather than a graph~ic. The delaye~d signal L8 has lts source at pin ll of the lat~ch Z27~which couples to another input of the gate Z26A.
When this input is h1gh, the beam is scanning in a character line and not between the character lines. The delayed Bl,ANK
signal has~its sou~rce at pin 7 of the latch 227 and is tied to a~further input~of the yate Z26A. When this input is high, the electron beam~is 1n~the video~portion of the screen and is not ;lo~ated naar~a sync~pulse or in som~e boundary region. All three restrictions have then ~3 ~?
been met. me input at another pin of the gate Z26A is tied to the inverted signal LATCH. I~en this input goes high, the dot load process is activated by a :low at the outpu-t of gate Z26A.
Upon the next clock pulse by the SHIFT signa:l at pin 7 of the register Z10, the dot data is loaded into the shift register.
After the signal LATCH goes back high (one dot time after going low), the shift register starts clocking dot data out at its pin 13 in a serial stream. When the signal I.ATCH goes high, gate Z26A has its output pin go high. Thus, each time that the signal LATCH goes high, it forces ASCII and conditional data to be stored in devices 227 and Z28. During this time this shift register Z10 is not shifting dots out. This register only shifts data out when the input pin 15 is high. When this is low, this forces the shift register ZlO to load data from the character generator.
It is noted that there are unused inputs to the shift register ZlO. Pin 9 is a clear input which is pulled up by way of resistor R40. When this pin is low, the screen is blanked.
Pins 14, 3, 2, 1 and 6 are tied to ground. Pin 14 gives the ~-blank dot between adjacent characters. Pins 2 and 3 are not used by are ragister inputs like pins 4 and 5. Pin 1 is for serial data input and pin 6 inhibits the clock input. They are not necessary in the particular application shown in the dr awi ng .
~ The operation of the graphics shift xegister Z10 is similar ; ~to the operation of the shift register Zll except for the : : , condi~tions that must be met for its use. First, the memory Z63 must specify a graphics character instead of an alpha numeric .

character. Second, the electron beam is to be in the video ~region of the ..
~ 64-; ~ ;' ':

screen. Further~ore, since a character rectangle ends where another starts, there is no inter-character line blanking. If one tuxns on all of the graphic cells, there will be a full large square with no holes and boundaries surrounding the square. Once all of the restrictions are met, graphic dot data may be loaded into register Zll for shifting to the video mixer. Ihe other NAND gate Z26B having its output at pin 6 is used as the graphics load enable this gate senses all of the proper restrictions for graphics. It is noted that the inverse of delay bit 7* is used coupled from pin 3 of the device Z27.
This signal is applied to one input A of gate Z26B~ When high, ....... .
this input tells gate 226B that the memory Z63 contains a "1"
: .
which defines a graphic code rather than an alpha numeric code. m e signal delay BL~NK i5 tied to two inputs of the gate Z26B. When this signal is high, this tells the gate that the electron beam i 5 indeed in the video portion o~ the screen.
Once all conditions are met and the signal LATCH goes low, the gate Z26B has its output go low. This signal will load dot data into the shift register Zll and when the input pin 15 goes back to its high level, the shift process will start. The 6 :; ` ` ~ :
gr~a ~ic~dots are shifted out on the output pin 13. ~hese 6 ~ dots r~epresent dots in a series along a single scan line within `~ the rectangle of FIG. 7~ It is also noted that pin 9 of the register ZI1 is pulled up by resistor R40. Likewise, pins 3, ... ..
2, l and 6~are tied to ground but pin 14 is used. In graphics ;
there is;not a blank space between character rectangles.
Sy~c Generator The sync generator circuit accepts timing signals from the divider~chain to develop horizontal and vertical sync pulses for : ~
~ 65-~ , ~
:
:

; ~ . . . . :
,:
, the display. These pulses are used by the display to control the electron beam of the CRT. The sync generator receives hori~ontal and vertical drive signals and provides a single composite output signal referred to as the SYNC signal.
For the sync generator refer to FIG. 13C. The inverters Z6A, Z6B, Z6C, Z6D are used to generate the ho.rlzontal pulse while inverter gates Z57A, Z57B, Z57C, ZS7D gen~rate the verti.cal pulse. Signal HDRV (horizontal drive) is taken from :
the divider chain at counter 50, pin 11. This signal is buffered by gates Z6 and applied to potentiometer R20. Ihis potentiometer controls where the vertical pulse starts in . .
reference to the signal HDRV. When the wiper of potentiometer .:.
R20 is close to pin 2 of gate Z6B, the horiæontal pulse will start almost at the same time as the signal HDRV goes high. .. .... .
When the wiper is moved in the opposite direction, there is a delay between the time the signal HDRV goes high and the time the horizontal pulse starts. This phase shift if performed by potentiometer R20 in combination with capacitor C20 and the two other inverters Z6C, Z6D. This circuit arrangement provides the complete shift netwoxk.
In operation, when the signal HDRV goes high, this causes the outpat at pin 2 of gate Z6B to go high to say 5 volts. A
current flows through potentiometer R20 charging capacitor C20. While this capacitor charges, the voltage at the input of gate Z6C slowly increases from 0 as the current through ~
potentiometer increases. After a length of time, the voltage -.
to gate Z~C is sufficiently positive. When that occurs, the output~pln of the gate Z6D goes high. This causes capacitor .
C20 to charge rapidly.
, ~", ' . ~.

~ ~ -66- .

.:, .

.
, me logic stays in this mode until the signal HDRV goes to its low state. At that time -the capacitor C20 starts to clischarge at the sa~e rate i-t charged. ~nen the voltage at the input of gate Z6C decreases to a logic O level the out:put of gate Z6D
goes low. Capacitor C20 then rapidl~ discharges. The process cycle is now comple-ted until the next HDRV signal goes high.
The time and voltage level at the input of gcLte Z6C stays above the minimum logic 1 level determines the amount of shi-ft -from the signal HDRV. The effect of potentiometer R20's position which adjusts the time delay, on the screen is a horizontal shift of video display.
After the horizontal signal is phase shifted, the hori~ontal pulse is shaped. The circuit including capacitor C21 and resistor R43 form a differentiation network which creates a smaller pulse of no width -from the shifted ~DRV
signal. ~en the output of gate Z6D goes high, capacitor C21 and resistor R43 differentiate the rising edge. A narxow pulse is passed to gate Z6E inverted twice to provide a like output pulse at the output of gate Z6E. Ihis provides a pulse Df about 4 microseconds duration referred to as a horizontal sync pulseO , The vertical sync phase shift operates in the same manner as the horizontal phase shift~ Instead of the gates Z6, there are provided a series of gates Z57A - Z57F with potentiometer R21 and capacitor C26 forming the delay network. The differential network comprises capacitor C27 and resistor R44.
The only basic difference between the horizontal and vertical circuits is the value of the two capacitive devices.
Horiz_ntal and V~rtical_Mixing Once the two sync pulses are phase-shifted and pulse-shaped, -67- ,--~f :
- - . .

!
3~'~
lR4/712 1 they are coupled to the gate arrangement including gate array 2 Z5 comprised of four separate NAND gates~ This gate arrangement 3 is used to mix the two signals together and serrate the vertlcal 4 interval. In this connection, reference is made to FIG. 8 which shows idealized waveorms at different locations of the gate array 6 Z5. FIG. 8 identifies each of the pin connections at gate array 7 Z5. Line A shows the horizontal pulses. Line B shows the vertical 8 pulses, pins l and 5 of gate Z5 are tied to the waveform shol~n g at line A. The resulting output pin 3 of ga~e Z5 is shown in line C of FIG. 8. The waveform at line C is now used as a source to 11 NAND the horizontal and vertical syncs once more. Line D shows 12 the result of combining lines A and C. Line E depicts the result 13 o~ providing a NAND operation between lines B and C. Line F
14 shows the resulting waveform which is a mixed 5yllc wave shape created by combining waveforms of lines D and E at gate Z5, pins 16 9 and 10. It is noted in FIG. 8 at line F that this output is 17 a "false" composite sync. In other words it is inverted away 18~ from true form. Secondly, the gate array Z5 may be evaluated 19 down using Boolean algebra into a two input exclusive OR gate.
The output at line C may be expressed as VH-~HV, where V is 21 vertical sync at line B and H is the horizontal sync at the line ,- 22 A in FIG. 8.
- ~ 23 Video Mi_ing 24 The video mixing circuitry is shown in FIG.~C and generates the composite video signal for the display. As previously 26 mentioned, the display may be a conventional CRT having raster 27 scan circuitry. Video mixer accepts both alpha numeric or 28 graphic dot data from the shit register, level-shifts it, and .
:.
68.

~ ., . .

lR4/71 ~ 3~~~
1 places it atop the composite syncs. The composite waveform is 2 then bu~fered to drive a 75 ohm impedance and is sent, via cable, 3 to the video display to our video terminal 26 as depicted in 4 FIG. 1.
Dot data from either the shift -register Z10 or the shift 6 register Zll is applied to the inputs of the NOR gate Z30( ~-7 . Signals will not be present at both of these inputs 8 at the same time. ~hile the register Z10, for example, is output-9 ting alpha numeric data, the register Zll at pin 13 should be continuously at a low level. Conversely, if the shift register 11 Zll is outputting graphic data, then the output pin 13 from the 12 shift register Z10 is at a low level. The net result at the 13 output pin 1 of gate Z30 is a single wave-shape of video dot data.
14 This data is applied to device Z41 at the input pins 6 and 7.
The composite sync data is coupled from the output pin ~ of 16 gate array Z5 and is applied to the base of transistor Q2. Each 17 time the base of transistor Q2 goes about 0.6 volts below a 5 18 volt level, the transistor turns on thus applying 5 volts to 19 resistor R28 but actually, the voltage applied to resistor R28 is slightly less than 5 volts due to the voltage drop at satura-21 tion across the collector and emitter o transistor Q2.
22 The dot data from gate Z30 at pin 1 is inverted by the 23 device Z41 and a resulting output at pin 5 is a normally low 24 signal which goes high only when the shift registers output a dot.
The device Z41 lS a high current driver. The output at pin 5 ~; 26 is the collector of the output buffer transîstor as illustrated.
27 Thus, the video and sync are going to two transistors. These 28 transistors function as switches controlling current flow through ' . . . .

.

lR~/712 ~ 3~ ~

1 the resistor network of resistors R28~ R27 and ~23. FIG. 9A shows 2 a simplified drawing of this circuit. In FIG. 9A the transistor 3 Q2 and the device Z41 are represented as mechanical swi-tches.
4 When transistor Q2 is opened, there is no voltage applied -to resistor R28 and the output node is at ground level. ~hen 6 transistor Q2 closes and with device Z4:L a]so held closed, ~he 7 output voltage goes up to about 1.23 vo:Lts. This voltage is 8 referred to as the black level voltage. A voltage be~ow this level 9 is referred to as a sync level. A voltage above 1.23 volts may be called a white level. Normally, the black level stays at 1.23 11 volts until the sync occurs at pin 8 of gate Z5 with this 12 output going high turning off transistor Q2 and forcing the output 13 at the node to go to ground. When dot data causes switch Z41 to 14 open, the voltage at the output node increases to about 2~75 volts. Thus, we now have a signal at the output node referred 16 to as the output in FIG. 9A which contains both video dots and 17 sync information. This signal is almost ready for display. All 18 that is necessary is some level shifting and output buffering.
19 In the video mixing circuit the transistor Ql is used as a common emitter amplifier. The composite video is applied to the 21 base of transistor Ql and the emitter outputs the waveform shown 22 in ~IG. gB. rhis final signal is used by the video terminal 26 23 for operation thereof. Capacitors C7 and C2, together with 24 resistor R30 form a filter network for the collector of transistor Ql~ The capacitors insure the DC bias level on the collecto-r is 26 video free and helps in reducing power dissipation in ~ransistor 27 Ql.

~' 70.

1 Keybo _d 2 The keyboard 20 of FIG. 1 ls described in detail in FIG. 15 3 ancl comprises 53 single pole, single throw, normally open keys 4 molded in a plastic base. The base is mounted toge~her wit}-L~ integrated circuits and associated resistors to a keyboard 6 printed circuit board. This keyboard is not of a conventional 7 type that outputs an ASCII code. Rather, each key represents a 8 switch across a matrix node. When the switch is closed, the 9 switch will short out a horizontal line to a vertical line.
Software in the ROM 16 detects the node short and generates an 11 ASCII word equivalent for the particular key. The keyboard is 12 accessed by decoder signal KYBD*. When this signal is low, it 13 enables tri-state buffers Z3 and Z4. The inputs to these buffers 14 are normally held high by the pull-up resistors Rl-R8 at the top o the keyboard. All of the horizontal address lines A0-A7 go 16 high at the same time that the signal KYBD* goes low. If the 17 CPU detects a logical "1" on one of the data lines D0-D7, this 18 indicates to the CPU that there has been a key pressed on the 19 keyboard. Thus, the CPIJ continuously is in readiness for SUCil a keyboard detection. Once the;CPU detects this, the RO~ will 21 then scan the address lines, one-by-one until it finds the 22 "l" output on the data bus. ~fter locating the output, the ROM
23 instructs the CPU to generate the ASCTI code for that particular 24 key. At that time the CPU also checks the status of the two shift keys. If one of these keys is not pressed~ the ASCII code 26 is not modified. If a shift key is pressed, the ASCII code is Z7 modified accordingly. This modification is for providing upper 28 and lower case outputs.

3~
Ihe inverters on the address lines in the keyboard are open collector types. With no key pressed there is no voltage applied to the lines KRO-KR7. T~hen a key i9 pressed, the associated p~ll-up resistor supplies a voltage. Then there will be activity on the KR line.
Input and Output Port The computer system of this invention, as previously mentioned, is memory mapped. However, it may be provided with input/output ports. In memory mapping, the C'PU knows where the data is~ ~lowever, with regard to a port, thP CPU does not know where the data is located. If the port is some kind of memory device the CPU will output that data to the port and it is up to the port circuitry to process and store the data. In the -input condition, the CPU accesses the input port and it is up to the port to find data and feed this data to the data bus which couples to the CPU. The CPU can access up to 256 output/input ports. However, in the system of this invention as described, only one is used and this is the cassette recorder 40 as depicted in FIG. 1 and shown in more detail in FIG. 13D. Its address in HEX is E'F. Ports are accessed using only the lower 8 address lines.
Port Addressing Because in the basic system only one output/input port is used, there is only provided one port decoder. The NAND gate Z54 (~IG. 13D) monitors the address bits Al-A7 while the gate Z52 monitors the address line AO. When the code HEX FF is outputted~on address lines AO~A8 the outputs from these gates at Z54 and Z52 couple to gate Z36 providing a low output ~ -:
72~

:.' ~,:

'' .

~;,r J

signal at the output pin 3 of gate Z36. This signal is combined in ga-tes Z25A and Z25B with -the signals IN* and OUT*.
If there is a low at signal OUT* because the CPU wants to access an output port, there is a singal from the output of gate Z25B identified as the signal OUT SIG*. If on the other hand, the signal IN* is low because the CPU wants to access an input port there will be an output fro~ gate Z25A generating signal IM 5IG*~ The signals IN* and OUT* will never be active at the same time and hence the corresponding signals IN SIG*
and OUT* will not be low at the same time.
OUT SIG*
This signal line is used to control two cassette functions and one video function. It is used to generate the audio signal for the cassette recorder under a CSAVE condition. It is used to control the motor of the recorder also. Its video function is to control the signal MODE SEL ~mode select). This signal will change between 64 and 32 character formats. The signal OUT SIG* is also for controllin~ a latch ~ade up of the NAND gates Z24 discussed later and depicted in FIG. 13D as OR
gates having inverted inputs.
The device Z59 is a data latch having its clock input at pin 9 controlled by the signal OUT SIG*. This latch accepts Zata fro~ the data lines DO-D3. The data lines DO and D1 are tied to pins 4 and 5 of this device. I~ese two inputs are used to input data that is recorded on tape during a C5AVE
function. m e input data line D2 is connected to pins 12 oF
the latch. This input controls the statu~ of the ~otor of the recorder. The last lnput at pin 13 is connected to provide the signal MODE SEL*.
' : ' -:
,, .... ...

lR4/, 1 The inputs to the latch Z59 are stored and transferred to 2 theoutput each time that the signal OUT SIG* goes high (rising 3 edge triggered). For example, i input D2 is high when the 4 clock signal occurs, the output pin 10 goes to a high state and stays in that state. This signal turns the recorder's motor 6 on. On the other hand if the input D2 is low when the clock 7 signal occurs, the outpu~ pin 10 is low and the recorder's motor ¦ 8 will be turned of.
¦ 9 Cassette Motor Control At the start o a CSAVE unction, the cassette recorder 11 motor is to be turned on. Thus, the CPU will cause the signal 12 OUT SIG* to go low and apply a logic high level to data line D2.
; 13 When the signal OUT SIG* goes high, the high signal on line D2 14 is transferred and held at pin 10 which is one of the output pins of device Z59. This output is connected to relay driver Z41 16 at pins 1 and 2. The output pin 3 at the collector of the 17 transistor in Z41 goes low causing current to 10w through relay ; 18 coil Kl. The contacts~lassociated with relay Kl close shorting 19 out pins 1 and 3 of the connector J3. These two pins are associated with the remote jack at the recorder. This action turns on the 21 motor of therecorder.
22 A diode CR3 is coupled across the relay coil Kl. This diode 23 is a standard silicone diode used for an anti-chatter func~ion.
24 When power is applied to or removed from the coil Kl, a counter EMF is generated. This voltage could be high eno~gh to damage 26 the output transistor of device Z41 or could cau ~ 1 to click .
27 off and on a few times producing undue wear to the switch contacts.
2C The diode CR3 shunts the counter EMF voltage around coil Kl and .

74.

IR4/71~ ~ ~h~3~

1 prevents transistor damage or relay chatter. The zener diodes 2 CR9 and CRl0 which are connected in series are used in somewhat 3 the same way. These diodes protect the switching contact 4 associated with relay coil Kl. When the recorder is turned on, a high voltage spike may be produced. The contacts operated 6 by the coil Kl could be welded together but the diodes CR9 and 7 CR10 prevent possible damage by shunting any voltage spikes above 8 a certain level.
9 Cassette Audio Output After the motor is turned on, the CPU may output data for 11 storage on the cassette tape. All data timing ~or this output 12 function is software control. The decoder Z59 is used to store 13 data from the CPU and it constructs the output waveEorm using 14 CPU data. CPU data, under software control, is applied to the latch Z59 on pins 4 and 5 as da~a inputs D0 and Dl, respectively.
16 Output pins 2 and 6 from the device Z59 are connected to a 17 resistor network comprising resistors R53-R56. As the signal 18 OUT SIG* is clocking data into the device Z59, the resulting 19 output on the line labeled CASSOUT, resembles a sign wave constructed of square waves. In this connection, reference is 21 made to FIG.l~ which is an illustration of one bit time of 2 22 milliseconds.
23 In FIG. 10, the voltage output is a function of the levels 24 on the output pins 2 and 6 from ~e device Z59. In the period labeled Tl, the output is shown at 0.46 volts. ~The time Tl is 26 the period in which the output pin 2 is ~ and the output pin 6 27 is high. The voltage during period T2 is outputted when pin 2 is 28 hlgh and p~n 6 is also hlgh. This voltage may be 0.85 volts.

75.

~J' -``
lR~/ Z ~ 3 1 The voltage during period T3 is outputted when pin 2 is low 2 and pin 6 is low thus represented by a 0 volt level. From the 3 start of 1 bit time to the start of the next bit time is 2 4 milliseconds. A 1 or 0 is dependent upon the presence or absence of a pulse between the start of two bit times. For 6 example~ when a the CPU outputs a 1 bit, it will generate a ~ 7 start pulse. 1 millisecond later another pulse will be generated.
, 8 1 millisecond thereafter a start pulse o a new bit is generated.
9 If this bit is to be a 0, then there will be a 2 millisecond delay before another pulse is generated and this pulse starts 11 the third bit time. Now the pulses are outputted to the 12 cassette recorder at pin 5 of jack J3. This pin is tied to the 13 auxiliary ~AUX) input of the recorder. The CPU outputs all of 14 the instructions inthe system RAM to tape during this CSAVE
function. When the function is complete, audio to the recorder 16 is disabled and a low is outputted at line D2, shutting off the 17 motor of the recorder.
18 Data is written on the tape in the following manner. I~hen 19 a CSAVE function is to be executed, the CPU via data lines D0-D3 orces the device Z5~ to output 128 zero bits. The CPU
21 then outputs HÆX AS used by the CPU during CLOAD for synchroniza-22 tion. A 2 byte starting address and a 2 byte ending address is 23 next added. Then the data follows for however long it is. After 24 the data, the last portion to be stored on the tape is the check sum. This one byte number is the sum of all data added together.
26 It is used by the CPU to insure what it CLOADed-in is what it 27 was CSAVEd-out., If the check sums don't match up, then there was 28 a load error.

76.

: ' ' , ~ly ~ ''P~
Cassette Audio _n~
If the recorder could faithfully give back what was sent to it, one could eli~linate a quad operational amplifier and a handful of associated components. However, -t:his cannot be assured and thus there is the need for the operational amplifiers Z4 (Four such amplifiers)~ Actually, the recorder even adds extraneous information to the signal such as motor noise and 60 cycle hum so as to complicate signal processing.
Upon a CLOAD instruction ~cassette load) from the CPU, the recorder motor turns on and cassette audio is applied to pin 4 of jack J3. This signal is referred to as signal CASSIN. This audio signal is coupled to capacitor C24 and resis-tor R67 at the input of the audio processer section. Amplifier Z4 at pins 1 and 6 and output pin 5 form an active filter. This part of the circuit is used to filter out undesired noise and hum present in the signal CASSIN. mis is a high-pass filter with about a 2KHz roll off.
The CASSIN input signal has data pulses riding atop a 60 cycle hum signal. After passing through the high-pass filter, the resulting waveform has the 60 cycle removed and only the data pulses are left. The signals are swinging above and below a base line of about 2.0 volts. FIG. 11 shows some idealized cassette signals. The signal at line A in FIG. 11 is the type that can be expected at the output of the active filter at pin 5 of one of the amplifiers Z4.
Once the filtering has occurrad, the next section of operational ampll~ier is used as an aFtive rectifier. Note the diodes CR4 and CRS, together with biasing resistors such as ,- ~ - . , - , , ~: ~

lR4/712 1 resistors R24, R34 and R35. This arrangement provides a full 2 wave rectifier to the data pulses. A typical output on the 3 cathode side of diode CR4 is shown at line B of FIG. 11.
4 After rectification, the signal is inverted and amplified.
The amplifier Z4, pins 8, 13 and 9 are wired to form an 6 inverting amplifier circuit. The ratio of resis-tor R41 to 7 resistor R42 provides a gain of about 2 for the amplifier. Line 8 C in FIG. 11 shows a typical output at amplifier Z4, pin 9.
9 The last stage of the operational ampllfiers is used as a level detector. In this last stage, the diodes CR6 and CR7 11 together with capacitor C39 form a power supply of sorts. The 12 amplified audio signal from arnplifier Z4 at pin 9 is applied 13 to the anode of diode CR6. Diodes CR6 and CR7 decrease the 1~ vo]tage level of the incoming slgnal by about .8 to 1.1 volts.
Capacitor C39 filters the resulting voltage and creates a DC
16 signal such as the one shown on line D of PIG. 11. If the 17 signal output from amplifier Z4 at pin ~ drops below the 18 reference voltage level at capacitor C39 of the amplifier Z4 19 at pin 10 will go low. It will stay at this low state as long as the voltage on pin 12 stays below the reference voltage.
21 ~ine E in FIG. 11 shows the resulting output from amplifier 22 Z4 at pin 10. It is noted that a couple of pulses oE audio 23 have been lost because the signal did not swing toward ground 24 enough to trigger the amplifier Z4 at pin 10. The negative transition at pin 10 is used to set flip-flop Z2~ comprised of 26 two cross-coupled gates. Cassette data is converted into 27 program data by the software in the ROM and the CPU. The data 2- frDm the flip-flop of cross-coupled gates Z24 is coupled by ' 78.
.

means of buffers Z44, pins ll, 12 and 13, 14 to the data lines D6 and D7.
IN SIG*
The transfer of data from the casse-tte to the CPU involves the generation o-~ the signal IN SI&*. l`he gate Z2S receives the signal IN* from the CPU. This is a control group signal from the CPU. Ihis signal goes low when the CPU wants to input data from a port. Port addressing has already been discussed.
A low at gate Z25A and a low at the output of gate Z36, pin 3 causes a low at the output pin 6 at the gate Z25A. This signal is the IN SIG* signal. This signal controls the buffer gates Z44. The gate Z44 at pin 12 i 5 coupled from the pin 8 output of gate Z24. The two gates Z24 are wired to form a set reset latch.
If the input to gate Z24 from pin 10 of device Z4 goes low, pin 8 at its output will go high. Pin 8 is cross-tied to the other gate Z24. If pin 13 i 6 high with pin 12 also being high, the output pin ll is low. With a high at pin 8 and a low at pin ll, the flip-flop is considered as being in its set state.
If pin 8 is low and pin ll is high, flip-flop is considered as being reset. The flip flop is set by cassette data and reset ,. . ~, .. . ..
by the signal OUT SIG*. The gates Z44 monitor the status of the flip-flop Z24 under command of the signal IN SIG*. When a cassette load signal is entered via the keyboard (CLOAD), the signal OUT SIG* goes low starting the motor of the recorder and resetting flip-flop Z24 by pulsing the input pin 13 low. The first time the input pin 9 of Z24 goes low, this sta~ts the first bit time. miS is shown in FIG. 12 a-t line A. Line D, the output of the latch .:
'-. ~ .

lR4/71~

1 at pin 8 of Z24 goes high as soon as pin 9 goes low. Next, the 2 signal OUT SIG* goes low after a short time delay as indicated 3 on line C of FIG. 12. This signal resets the flip-flop as indicated ~ in line D of FIG. 12. A short time after the signal OIJT SIG*
goes back to its high state~ the CPU tests the device Z2~ at its 6 pin 8 to determine the status by enabling the buffers Z44. Line 7 D is low at this time. The CPU recognizes a logical 0 during 8 bit time 1 as shown by the 0 under line D. The next time line A
9 goes low is the start of bit time 2. The low on device Z24 a-t pin 9 sets the flip-flop. The signal OUT SIG* resets the flip-11 flop a short time later. The signal IN SIG* then enables -the 12 buffers Z44 and checks the s-tatus of the flip-flop. The CPU
13 sees a 0 again, so bit time 2 is a 0 bit. The next low on line 14 A starts bit time 3. Again, its sets the flip-flop and a short time later a signal OUT SIG* resets the flip-flop.
16 Before the signal IN SIG* can test the status, another low lY comes fromthe audio processing level detector and sets the 18 flip-flop. Now the signal IN SIG* goes low checking the status.
19 It finds the output pin 8 from the device Z24 is high. The CPU labels bit time 3 a 1 then rather than a 0. Now the CPU
21 resets the ~lip-~lop be~ore bit time 4 starts. Line C shows 22 the added signal OiJT SIG* pulse to reset the device Z2~. The 2~ flip-flop is reset and stays reset until the next low on line A
24 sets i* again. The CPU finds bit time 4 to contain a 0. This Z5 set~reset process continues until the CPU has read every bit time 26 of the program that was stored in the cassette. It is the 27 responsibility of the CPU to assemble the bit times into data 28 word-; the wo~ds into text; and store the text in the random 80.

lR4/712 1 access memory.
2 The basic language of the computer is stored in the ROM
3 and in accordance with the system of the present in~ention it is 4 quite easy to expand the mathematical and symbolic capabilities.
In a first generation system there is employed a 4K ROM whereas 6 a higher generation system employs a 12K RO~I. The basic hardware 7 of the system does not change. The only basic different being 8 in the machine language contained in the ROM. In the higher g generation machine, there may be contained three 4K ROMs instead of the two 2K ROMs depicted in the drawings. This ROM connects 11 to the CPV at addresses All, A12 and A13 and also receive the 12 signal ROM*.
13 In FIGS. 13 and 14 showing the majority of the details of 14 a specific embodiment of the invention, each of the devices has an identi~ication number. For example, de~ices Z35 and Z51 .
16 shown in FIG. 14B are multiplexer devices identified by a well-17 known part number 74LS157. These types of devices can be made by 18 any one of the well-known integrated circuit manufacturers such as 19 National Semiconductor, Texas Instruments, or Motorola. Below is a table setting forth each of the components including 21 resistors and capacitors along with the specific value or type 22 of component that is used;

24 Cl 220 u~, 16V, Electrolytic, Axial 25; C2 10 ~F~ 16V, Electrolytic, Radial 26 C3 0.01 ~F, 10% 25V, Disc 27 C~ 10~Fj 16V, Electrolytic, Radial :
z~ C5 10~F9 16V, Electrolytic, Radial : ' ~ , .
81.

lR4/712 ~3~

1 C6 lOO~F, 16V, Electrolytic, Radial 2 C7 O.Ol~F, 10%, 25V, I)isc 3 C8 2,220~F, 35V, Electrolytic, Axial 4 C9 lO,OOO~Fj 16V, Electrolytic, Axial S C10 lO~F, 16V~ Electrolytic, Radial 6 Cll lO~F, 16V, Electrolytic, Radial 7 C12 470pF, 50V, Disc 8 C13 470pF, 50V, Disc 9 C14 O.Ol~F, 10% 25V, Disc C15 O.Ol~F, 10%, 25V, Disc 11 C16 O.l~F, 10% 12V, Disc 12 C17 O.l~F, 10%, 12V, Disc 13 C18 O.l~F, 10%, 12V, Disc 14 Cl9 O.l~F, 10% 12V, Dlsc C20 330pF, 10% 50V, Disc 16 C21 750pF, 10%, 50V, Disc 17 C22 O.l~F, 10%, 12V, Disc 18 C23 O.l~F, I0%3 12V, Disc 19 C24 220pF, 10%~ 50V, Disc C25 220pF, 10% 50V, Disc 21 C26 0.047~F, lOOV, Polyester Film 22 C27 0.022 F, lOOV, Polyester Film 23 C2~ O.l~F, 10%, 50V, Disc 24 C29 O.l~F, 10%, 12V, Disc C30 O.l~F, 10%, 50V, Disc 26 C31 O.l~F, 10%, 12V, Disc 27 ~32 O.l~F, 10%, 50V, Disc 28 C33 O.l~F~ 10%, 12V, Disc .
.

~2.
... .. . ., _ .

lR4/713 ~ 9'3~ 9 1 C34 0.1~F, 10%, 50V, Disc 2 C35 0.1~F, 10%, 12V, Disc 3 C36 0.1~F, 10%, 12V, Disc 4 C37 0.1~F, 10%, 12V, Disc C38 0.1~lF, 10%, 12V, Disc 6 C39 0.1~lF, 10%, 12V, Disc 7 C40 0.1~F, 10%, 12V, Disc 8 C41 0.1~F, 10%, 12V, Disc 9 C42 22~F, 16V, Electrolytic, Radial C43 47pF, 10%, 50V, Disc 11 - C44 0.1uF, 10%, 12V, Disc 12 C45 0.1~F, 10%, 12V, Disc 13 C46 0.l~F, 10%, 12V, Disc 14 C47 0.1~P, 10%, 12V, Disc C48 0.1~F, 10%, 12V, Disc :: : :
16 ~ C49 0.1~F,~10%, 12V, Disc 17 ~~ CS0 0.1~F, 10%~ 2V9 Disc 18 G51 ~ 0.1~F, 10%, 12V, Disc . ~
l9~ CS2 ~ 0.1~F, 10%, 12V, Disc 20 ~CS3 0.1~F,.10%, 12V, Disc 21 C54 ~ 0.1~F, lQ%, 12V, Disc 22 CS5 0.1uF~ 10%, 12V, Dlsc . : ~ .
23 ~~ C56 ~ 0.1~F, 10%, lZV, Disc 2~4 ~CS7 ~ ~;10~F, 16V, Electrolytic, Radial 25 ;~ DIODES
26~`CRl ~ lN4735, 10%, 6.2V, Zener 27~CR2~ ~lNS231, 5%9 S.lV, Zener 28~CR3~ lN4148,~75V

83.

. .

IR4/712 ~ 3~

1 CR4 lN4148, 75~' 2 CR5 lN4148, 75V
3 CR6 lN4148, 75V
: 4 CR7 lN4148, 75V
CR8 Bridge Rectifier, MDA202, 2A, 202V
6 CR9 lN9829 75V, Zener 7 CR10 lN982, 75V, Zener 9 Jl Conncctor, Socket, DlN, 5Pin J2 Connector, Socket, DlN, 5Pin 11 J3 Connector, Socket, DlN, 5Pin 13 K:l 5V Relay Ql 2N3904, NPN
16 Q2 MPS3906, PNP
17 Q3 TIP29, Driver 18 Q4 2N6594, Power 19 Q5 MPS3906, PNP
Q6 MJE34, Power .. 22 Rl 68 ohm, 1/2W, 5%
23 R2 2.7 K, 1/4W, 5%
24 R3 750 ohm9 1~4W, 5 R4 0.33 ohm, 2W, 5%
26 R5 lK Trim Pot, 30%
27 R6 1.2 K, l/~W, 5%
26 R7 1.2K, 1/4W, 5%

. ~

. 84.

lR4/712 ~ 3~ ~

1 R8 100 K, 1/4W, 5%
2 R9 3.3K, 1/4W, 5%
3 R10 lK, Trim Pot, 30%
4 Rll 3.3K, 1/4W, 5%
R12 3.3K, 1/4W, 5%
6 R13 2.2K, 1/4W, 5%
7 R14 12K, 1/4W, 5%
8 R15 1.5K, 1/4W, 5%
9 R16 1.2K, 1/4W, 5%
lQ R17 2K, 1/4W, 5%
11 R18 5.6 ohm,- 3W, 5%
12 Rl9 220 ohm, 1/2W, 5%
13 R20 100 K, Trlm Pot, 20%
14 R21 100 K, Trim Pot, 20%
R22 75 ohm, 1/4W, 5%
16 R23 120 ohm, 1/4W, 5%
17 R24 680 K, 1/4W, 5%
18 R25 1.6 Megohm, 1/4W, 5%
19 R26 1 Megohm, 1/4W, 5%
R27 330 ohm, 1/4W, 5%
21 R28 270 ohm, 1/4W, 5%
22 R29 1.8K, 1/4W, 5%
23 R30 47 ohm, 1/4W, 5%
24 R31 10 ohm, l/4W9 5%
R32 lOK, 1/4W, 5%
26 R33 360 K, 1/4W, 5%
27 R34 470 K, 1/4W, 5%
28 R35 470 K, 1/4W, 5%

85.

.

lR4/712 ~ 3 1 R36 360 K, 1/4W, 5%
2 R37 560 K, 1/4W, 5%
3 R38 270 K, 1/4l~, 5%
4 R39 4.7 K, 1/4W, 5%
R40 4.7 K, 14/W, 5%
6 R41 470 K, 1/4W, 5%
7 R42 1.0 Megohm, 1/4W, 5%
8 R43 10 K, 1/4W, 5%
- 9 R44 10 K, 1/4W, 5%
R45 470 K, 1/4W, 5%
11 R46 910 ohm, 1/4W, 5%
12 R47 10 K, 1/4W, 5%
13 R48 4.7 K, 1j4W, 5%
14 R49 4.7 K, 1/4W, 5%
R50 4.7 K, 1/4W, 5%
16 R51 4.7 K, 1/4W, 5%
17 R52 910 Ohm, 1/4W, 5%
18 R53 1.2K, 1/4W, 5%
19 R54 7.5K, 1/4W, S%
R55 7.5K, 1/4W, 5%
21 R56 220 K, 1/4W, 5%
22 R57 4.7 K, l/4W9 5%
~, Z3 R58 4.7 K, 1/4W, 5%
24 R49 4.7 K, 1/4W, 5%
R60 4.7 K, 1/4W, 5%
26 R61 4.7 K, 1/4W, 5%
27 R62 4.7 K, 1/4W, 5%
Z8 R63 4.7K, 1/4W, 5%

86.

: -lR4/712 ~ 3~9 1 R64 330 ohm, 1/4W, 5%
2 R65 10K, 1/4W, 5 3 R66 4.7 K, 1/4W, 5%
4 R67 100 ohm, 1/4~, 5~
SWITCIIES
6 S2 DPDT Push 7 SOCKEI`S
8 X3 16 Pin I.C. Socket 9 X13 16 Pin I.C. Socket X14 16 Pin I.C. Socket X15 16 Pin I.C. Socket 12 X]6 16 Pin I.C. Socket 13 X17 16 Pin I.C. Socket 14 X18 16 Pin I.C. Socket X19 16 Pin I.C. Socket 16~ : X20 16 Pin I.C. Socket ~:; ~ :: . - :
17~ X32::~ 24 Pin I.C. Socket :
18~ X33 ~: ~24 Pin I.C. Socket 19~ :X~3~9 40 Pin I.C.:Socket 20 ::~:: X71 16 Pin I.C. Socket 21 : ~ CRYSTALS
2Z Yl ~ 10.6445 MHz9 0.004%, Series Res.
23 ~ INTEGRATED_CIRC~ITS
24 ~ Z~ 723, DIP, Voltage Regulator 25~ Z:2~ 723, DIP, Voltag~e Regulator 26;~ Z4~ : :LM3900, Dual Input Norton Amp. ~.
27~ Z5~ 74COO CMOS, Quad 2-lnput NAND Gate 2~8~ ;Z;6 ~ 7~4CO4 CMOS,~Hex Inverter 7.

IR4/712 ~ w ~

1 Z7 741.S74, Dual D Positive-Edge Triggered ~lip-Flop 2 with Preset and Clear 3 Z8 Z4L,S153, Dual 4-Line to l-Iine Data Selector/
4 hlultiplexer Z9 74LS04, Hex Inverter 6 Z10 74LS166J 8-Bit Parallel In/Serial Out Shift Register 7 Zll 74LS166, ~-Bit Parallel In/Serial Out Shif-t Register 8 Z12 74LS939 Divide by 8 Binary Counter Selector/
9 Multiplexer Z21 74LSl569 Dual 2-Line to 4-Line Decoder/Demulti.plexer 11 Z22 74LS367, TRI-STATE ~lex Buffer 12 Z23 74LS32, Quad 2-Input OR Gate 13 Z24 74LS132, Quad 2-Input NAND Gate 14 Z25 74LS32, Quad 2-Input OR Gate Z26 74LS20, Dual? 4-Input NAND Gate 16 . Z27 74LS175, Quad D Flip-Flop wi~h CIear 17 Z28 74LS174, Hex D Flip-Flop with Clear ~:~ 18 Z29 MCM6670, Character Generator 19 Z30 74LS02, Quad, 2-Input NOR Gate Z31 74LS157, Quad 2-Line t0 l-Line Data Selector/
21 Multiplexer 22 Z32 ~ 74LS93, Divide ~y 8 Binary Coun~er Selector/
23 : Multiplexer : :~ 24 Z33 : 2K x 8 ROM A, 450 ns. 2 Patterns :~ .
; 25 Z34 2 K x 8 ROM B, 450 ns, 2 Patterns :~ 26 Z35 74LS157,:Quad 2-Line to l-Line Data Selector/
27~ ~ Multiplexer :28 Z36 74LS32, Quad 2-Input OR Gate ~ : -~8.

lR4/712 ~ 3~

l Z37 74LS02, Quad 2-lnput NOR Gat:e 2 Z38 74LS367, TRI-STATE l-lex Buffer 3 Z39 74LS367~ TRI-STATE }lex Buffer 4 Z40 Z80 ~ficroprocessor Circuit, Plastic Z41 75452, Relay Driver 6 Z42 74LS04, Hex Inverter 7 Z43 74LS157~ Quad 2-Line to l-Line Data Selector/
8 Multiplexer 9 Z44 74LS367, TRI-STATE Hex Buffer Z45 2102, AN-4L, lK Static RAM
11 Z46 2102, AN-4L, lK Static RAM
12 Z47 2102, AN-4I., lK Static RAM
13 Z48 2102, AN-4L, lK Static RAM
14 Z49 74LS157, Quad 2-Line to l-Line Data Selector/
Multiplexer 16 Z50 74LS93, Divide by 8 Binary Counter Selector/
17 Multiplexer 18 Z51 74LS93, Dlvide by 8 Binary Counter Selector/
19 Multiplexer Z5Z 741.S04, Hex Inver~er 21 Z53 74LS132, Quad 2-Input NAND Gate 22 Z54 74LS30, Triple 3-Input NOR Gate 23 Z55 74LS367, TRI-STATE, Hex Buffer 24 Z56 74LS92, Divide by 6 Binary Counter Selector/
Multiplexer 26 Z57 74C04 C~OS, Hex Inverter 27 Z58 74LS92, Divide by 6 Binary Counter Selector/
28 Multiplexer .

8 g .
~ . - , :

lR4/712 ~ 3~9 1 Z59 74LS175, Quad D ~lip-Flop Wit]l Clear 2 Z60 74LS367, TRI-STATE ~lex Buffer 3 Z61 2102, AN-4L, lK Static RAM
4 Z62 2102, AN-4L, lK Static RAM
Z63 2102, AN-4L, lK Static RAM
6 Z64 74IJS1573 Quad 2-Line to l-Line Data Selector/
7 Multiplexer 8 Z65 74LS93, Divide by 8 Binary Coun~er Selector/
9 Multiplexer Z66 74LSll, Triple 3-Input AND Gate 11 Z67 74LS367, TRI-STATE Hex Buffer 12 Z68 74LS367, TRI-STATE Hex Buffer 13 Z69 74LS74 9 Dual D Positive-Edge Triggered Flip-Flop 14 with Preset and Clear Z70 74LS74, Dual D Positive-Edge-Triggered Flip-Flop 16 with Preset and Clear 17 Z71 Not used 18 Z72 74LS367~ TRI-STATE Hex Buffer 19 Z73 74LS32, Quad 2-Input OR Gate Z74 74LS00, Quad 2-Input NAND Gate 21 Z75 74LS367, TRI-STATE Hex Buffer 22 Z76 74LS367, TRI-STATE Hex Buffer ~.~.

24 A3 DIP Shunt A71 DIP Shunt 26 Z13 4096 bit, Dynamic RAM, 450 ns 27 Zl4 4096 bit, Dynamic RAM, 450 ns 28 Z15 4096 bit, Dynamic RAM, 450 ns ' . ' ' : 90.

., , ~ . . . . . ~ - - ... -lR4/712 1~3~

1 Z16 4096 bit, Dynamic RAM 450 ns 2 Z17 4096 bit, Dynamic RAM, 450 ns 3 Z18 2096 bit, Dynamic RAM, 450 ns 4 Zl9 2096 bit, Dynami.c RAM~ 450 ns Z20 4096 bit, Dynamic RAM, 450 rls 7 A3 DIP Shunt 8 A71 DIP Shunt 9 Z13 16384 bit, Dyrlamic RAM, 450 ns Z14 16384 bit3 Dynamic RAM, 450 ns 11 Z15 16384 bi~, Dynamic RAM, 450 ns lZ Z16 16384 bit~ Dynamic RAM, 450 ns 13 Z17 16384 bit, Dynamic RAM, 450 ns 14 Z18 16384 bit, Dynamic RAM, 450 ns Z19 16384 bit, Dynamic RAM, 450 ns 16 Z20 16384 bit9 Dynamic RAM, 450 ns 17 _YBOARD
18 Capacitors 19 Cl O.l~F, 10%, 12V, Disc C2 O.luF, 10%, 12V, Disc 21 Diodes 22 CRl LED, HP5082-4850, Red 23 Keyboard 24 KBl DS5300, 53 Key, 2-Shot Key caps Resistors 26 Rl 4.7 K, 1/4W, 5%
27 R2 4.7 K, 1/4W, 5%
28 R3 4.7 K, 1/4W~ S%

:

.

91 .
, lR4/712 ~3~

1 R4 4.7 K, 1/4W, 5%
2 R5 4.7 K, 1/4W, 5 3 R6 4~7 K, 1/4W, 5%
4 R7 4.7 K, 1/4W, 5%
R8 4.7 K, 1/4W, 5%
6 R9 330 ohm, 1/4W, 5%
7 Integrated Circ its 8 Zl 74LS05, Hex Buffer with open collector High 9 Voltage outputs Z2 74BS05, Hex Buffer with open collector High 11 Voltage outputs 12 Z3 74LS368, TRI-STATE Hex Buffer 13 Z4 74LS368, TRI-STATE Hex Buffer 14 Wire . Wl Stranded, Prebonded, LED, Red, 10"
16 ~ W2 Stranded, Prebonded, LED, Black, 10"

18 Jl Socket, I.C., 24 Pin 19 Rl Resistor, 4.7K, 1/4W, 5%
Zl I.C., 4K x 8 ROM, 450ns, ROM A
21 ~2 I.C., 4K x 8 ROM, 450ns, ROM B
22 Z3 I.C., 4K x 8 ROM, 450ns, ROM C
. ~ j .
23 Z4 I.C., 74LS42, BCD to Decimal Decoder : 24 : 25 : 27 .,.

_ q2.
- : ' ~

~l~4/71Z

1 By the foregoing we have described a preferred embodlrnent 2 of the presen-t system. However, it is understood that numerous 3 modifications can be made in this system without departing from 4 the scope of the invention. For example 9 in the video generation section there has been disclosed a scheme for generating either 6 32 characters or 64 characters per line. In this arrangement 7 the cloc~ is controlled to provide characters o-f two different 8 widths. However, in accordance with ano-tiler embodiment of the 9 invention the input address lines Ll, L2, L4 and L8 could possibly be multiplexed to also provide for an expansion of 11 the number of character lines per page. In the disclosed 12 embodiment there is mention made of the use of 16 character lines.
13 However, in an alternate embodiment these input addresses to the 14 character generator could be controlled so as to provide 16 character lines for the usual 64 character format or alternatively 16 only 8 character lines for the larger style 32 character per line 17 format.

~1 ~3 ~..

93, , ,: '

Claims (28)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. Video processing logic for providing a video serial signal to a receiver in accordance with an alternate mode control to provide different width characters to be displayed by the receiver, comprising;
memory means for storing a plurality of character codes;
storage means coupled from the memory means for holding a character code at a time transferred from the memory means, means defining a character line code comprising at least a part of a video divider chain including a line counter, character generator means receiving said character code and said character line code for providing line dot signals representative of a portion of the character, parallel-to-serial means coupled from said character generator means receiving the line dot signals including means for providing a serial video signal, and control means for controlling said parallel-to-serial means to provide at least two different video rate signals representative of different width characters to be displayed by the receiver, said control means comprising, means for establishing a mode control signal having alternate states for indicating different width characters, clock means for providing a first frequency signal, divider means coupled from said clock means for providing a second frequency signal lower in frequency than said first frequency signal, and logic circuit means responsive to said first and second frequency signals and said mode control signal for providing a shift control signal coupling to the parallel-to-serial means and in one state of the mode control signal coupling the first frequency signal and in another state of the mode control signal coupling the second frequency signal, said logic circuit means also having means for providing an input signal to said video divider chain that is at a constant frequency irrespective of said mode control state.

2, Video processing logic as set forth in claim 1 wherein said memory means comprises a random access memory.

3. Video processing logic as set forth in claim 2 wherein said storage means comprises a data latch or data register.

4. Video processing logic as set forth in claim 3 wherein said video divider chain comprises a plurality of intercoupled counters for providing, in addition to the line counter, column and row count data.

5. Video processing logic as set forth in claim 4 wherein said parallel-to-serial means comprises a shift register receiving the dot signals in parallel and having a clock input for controlling the output serial shifting of data.

6. Video processing logic as set forth in claim 5 wherein said second frequency is one half of the first frequency.

7. Video processing logic as set forth in claim 6 wherein said logic circuit means includes a frequency divider intercoupled with a multiplexer.

8. Video processing logic as set forth in claim 7 including gate means responsive to said frequency divider for generating a latch signal to control transfer from said memory means to data latch.

9. Video processing logic as set forth in claim 1 wherein said logic circuit means includes a frequency divider and a multiplexer, said multiplexer receiving said mode control signal and said first and second frequency signals.

10. Video processing logic as set forth in claim 9 wherein the frequency divider and multiplexer are intercoupled to pass one of the frequency signals via the multiplexer to the frequency divider.

11. Video processing logic as set forth in claim 1 wherein said logic circuit means comprises a frequency divider and a multiplexer, means coupling the output of the divider means to one input of the multiplexer, means coupling the output of the clock means to a second input of the multi-plexer, said multiplexer controlled to couple alternatively one or the second inputs to a first output of the multiplexer under control of said mode control signal, means coupling a first output from the frequency divider to a third input of the multi-plexer, means coupling a second output different in frequency than the first output, from the frequency divider to a fourth input of the multiplexer, said multiplexer controlled to couple alternately the third or fourth inputs to a second output of the multiplexer under control of said mode control signal, whereby the second output signal from said multiplexer is the input signal to the video divider that is maintained at the same constant frequency during both mode control states.

12. Video processing logic as set forth in claim 11 including gate means having at least two inputs and having an output defining the latch signal for the parallel-to-serial shift means, means coupling one of said first and second outputs from the frequency divider means to a first input of the gate means, and means coupling a third out-put from the frequency divider means to the second input of the gate means.

13. Video processing logic as set forth in claim 12 wherein said gate means comprises an AND gate.

14. Video processing logic as set forth in claim 13 wherein said third output from the frequency divider means is a higher frequency output than the first or second output therefrom.

15. Video processing logic as set forth in claim 14 including means coupling a constant value signal to a fifth input of the multiplexer, and means coupling one of said first and second outputs from the frequency divider to a sixth input of the multiplexer, said multiplexer having a third output and controlled by said mode select signal to couple alternatively the fifth or sixth inputs to the third output of the multiplexer, whereby the third signal from said multiplexer is coupled also to said video divider chain to define video addresses.

16. Video processing logic for providing a video serial signal to a receiver in accordance with an alternate mode control to provide different width characters to be displayed by the receiver, comprising;
memory means for storing a plurality of character codes, storage means coupled from the memory means for holding a character code at a time transferred from the memory means, a video divider chain comprising a plurality of successively coupled counters and including means defining a character line code, character generator means receiving said character code and said character line code for providing line dot signals representative of a portion of the character, parallel-to-serial shift means coupled from said character generator means receiving the line dot signals in parallel and including a clock input for providing a serial video signal, and control means for controlling said parallel-to-serial shift means to provide at least two different video rate signals representative of different width characters to be displayed by the receiver, said control means comprising, means for establishing a mode control signal having alternate states for indicating different width characters, clock means for providing a first frequency signal, divider means coupled from said clock means for providing a second frequency signal lower in frequency than said first frequency signal, and logic circuit means responsive to said first and second frequency signals and said mode control signal for providing a shift control signal coupling to the parallel-to-serial means and in one state of the mode control signal coupling the first frequency signal and in another state of the mode control signal coupling the second frequency signal, said logic circuit means also having means for providing an input signal to said video divider chain that is maintained at the same constant frequency during either of the states established by said means for establishing a mode control signal having alternate states.

17. Video processing logic as set forth in claim 16 wherein said memory means comprises a random access memory.

18. Video processing logic as set forth in claim 17 wherein said storage means comprises a data latch or data register.

19. Video processing logic as set forth in claim 18 wherein said second frequency is one-half of the first frequency.

20. Video processing logic as set forth in claim 19 wherein said logic circuit means includes a frequency divider intercoupled with a mixer.

21. Video processing logic as set forth in claim 20 including gate means responsive to said frequency divider for generating a latch signal to control transfer from said memory means to data latch.

22. Video processing logic as set forth in claim 16 wherein said logic circuit means includes a frequency div-ider and a multiplexer, said multiplexer receiving said mode control signal and said first and second frequency signals.

23. Video processing logic as set forth in claim 22 wherein the frequency divider and multiplexer are inter-coupled to pass one of the frequency signals via the multi-plexer to the frequency divider.

24. Video processing logic as set forth in claim 16 wherein said logic circuit means comprises a frequency divider and a multiplexer, means coupling the output of the divider means to one input of the multiplexer, means coupling the output of the clock means to a second input of the multi-plexer, said multiplexer controlled to couple alternatively one or the second inputs to a first output of the multi-plexer under control of said mode control signal, means coupling a first output from the frequency divider to a third input of the multiplexer, means coupling a second out-put different in frequency than the first output, from the frequency divider to a fourth input of the multiplexer, said multiplexer controlled to couple alternately the third or fourth inputs to a second output of the multiplexer under control of said mode control signal, whereby the second output signal from said multiplexer is the input signal to the video divider that is maintained at the same constant frequency during both mode control states.

25. Video processing logic as set forth in claim 24 including gate means having at least two inputs and having an output defining the latch signal for the parallel-to-serial shift means, means coupling one of said first and second outputs from the frequency divider means to a first input of the gate means, and means coupling a third output from the frequency divider means to the second input of the gate means.

26. Video processing logic as set forth in claim 25 wherein said gate means comprises an AND gate.

27. Video processing logic as set forth in claim 26 wherein said third output from the frequency divider means is a higher frequency output than the first or second output therefrom.

28. Video processing logic as set forth in claim 27 including means coupling a constant value signal to a fifth input of the multiplexer, and means coupling one of said first and second outputs from the frequency divider to a sixth input of the multiplexer, said multiplexer having a third output and controlled by said mode select signal to couple alternatively the fifth or sixth inputs to the third output of the multiplexer, whereby the third signal from said multiplexer is coupled also to said video divider chain to define video addresses.