US3629861A

US3629861A - Control for chain printer

Info

Publication number: US3629861A
Application number: US877354A
Authority: US
Inventors: Lynn W Marsh Jr; Edward M Schneiderhan
Original assignee: Mohawk Data Sciences Corp
Current assignee: DECISION DATA Inc A CORP OF; NationsBank of Texas NA; Mohawk Systems Corp
Priority date: 1969-11-17
Filing date: 1969-11-17
Publication date: 1971-12-21
Anticipated expiration: 1988-12-21
Also published as: GB1311167A; FR2069153A5; JPS504492B1; DE2056274A1; DE2056274C3; CA926784A; DE2056274B2

Abstract

A control system for a horizontal-type carrier, or ''''chain,'''' printer wherein a recirculating shift register memory is employed to store the code representations of characters to be printed on the printline. A tracking or address counter is advanced in synchronism with the movement of data through the shift register and serves to identify each character stored therein with the particular print position in which it is to be printed. Generation of print command signals is accomplished by comparing, in sequence, each character stored in the memory with the output of a character generation counter which is advanced in synchronism with the movement of the type carrier to identify the order in which the types on the carrier move past the print positions. Actuation of the comparator and advancement of the character generation counter are controlled directly by the output of the tracking counter. Beginning-of-font sync pulses are generated by gating a primary index signal derived directly from the type carrier with a secondary index signal derived from a rotating code disc. A plurality of index marks are provided on the code disc in a manner to enable generation of properly timed beginning-of-font signals for any of a variety of different font length type carriers interchangeably employable in the printer.

Description

United States Patent [72] Inventors Lynn W.Marsh,.|r.

Melroae; Edward M. Sehnelderhan, Billet-lea, both of Man. [21] AppLNo. 877,354 [22] Filed Nov. 17,1969 [45] Patented Dec. 21,1971 [7 3] Assignee Mohawk Data Sciences Corporation llerklrner, N.Y.

[54] CONTROL FOR CHAIN PRINTER 8 (llalnis, 8 Drawing Figs.

[52] U.S.Cl 340/1715, 101/93, 101/111 [51] lnt.Cl ..G06k15/08 [501 FleldolSearch 340/1725; 101/93, 111

[56] References Cited UNITED STATES PATENTS 2,831,424 4/1958 MacDonald...,.............. 101/93 2,918,865 12/1959 Wooding 101/93 3,216,348 11/1965 Oldenburgetal. 101/111 X 3,303,775 2/1967 Giannuzzi 101/111 X 3,314,360 4/1967 Foster..... 101/93X 3,430,201 2/1969 Kintner... 340/1725 3,463,081 8/1969 Levine 101/93 Primary Examiner-- Raulfe B. Zache Assistant Examiner-Paul R. Woods Attorneys-Francis J. Thomas, Richard H. Smith. Thomas C. Siekman and Sughrue, Rothwell, Mion, Zinn and Macpeak ABSTRACT: A control system for a horizontal-type carrier. or "chain, printer wherein a recirculating shift register memory is employed to store the code representations of characters to be printed on the printline. A tracking or address counter is advanced in synchronism with the movement of data through the shift register and serves to identify each character stored therein with the particular print position in which it is to be printed. Generation of print command signals is accomplished by comparing, in sequence, each character stored in the memory with the output of a character generation counter which is advanced in synchronism with the movement of the type carrier to identify the order in which the types on the carrier move past the print positions. Actuation of the comparator and advancement of the character generation counter are controlled directly by the output of the tracking counter. Beginning-of-t'ont sync pulses are generated by gating a primary index signal derived directly from the type carrier with a secondary index signal derived from a rotating code disc. A plurality of index marks are provided on the code disc in a manner to enable generation of properly timed beginning-ot font signals for any of a variety of different font length type carriers interchangeably employable in the printer.

24 PRINT HEAD 26 FIRE 01 C16$l 55 CONTROL LOGIC (;p ClROUITS NELS DATA - T? m on EXTERNAL supra:

PATENIEI] IIEc2I IQII 29 1 SHEET 5 or 6 l-fi /3o IA 5 C D I PEFfiWT; H

PHASE COLUMN PRINTED l 5 9 l3 l7 2I-I4I 4 2 2 6 l0 l4 l8 22--|42 3 3 7 ll I5 l9 23---- I43 454 4 8 I2 I6 20 24--l44 F|G.7 FIG.8

COLUMN 050005 ZONE DEGODE IN PUT OUTPUT INPUT OUTPUT A4 A3 A2 AI A8 A7 A6 A5 0 0 o 0 c: o 0 o 0 2| 0 0 o I 02 0 o o l 22 o o I 0 c3 0 o I o 23 o 0 l I 04 0 0 l I 24 o I o 0 c5 0 I 0 0 25 o I o I C6 0 I o I Z6 0 I I 0 c7 0 l l 0 z? 0 I I I 09 0 I l I 28 I o 0 0 09 I 0 o 0 as I 0 0 I (:I0

I o I 0 all I 0 I I cm I l 0 0 (H3 I I o I cm I I I 0 C|5 l I I l CONTROL FOR CIIAIN PRINTER BACKGROUND OF THE INVENTION In the art of on-the-fly, high-speed line printing two basic fonns of print mechanisms are employed. The oldest form is the so-called drum print mechanism. This mechanism employs a bank of individually controllable print hammers. usually one for each of the possible print positions in the printline. in association with a constantly rotating type drum having engraved about its periphery the various characters of the type font arranged in rows and columns. The type characters of each column are aligned to cooperate with one of the hammers and the type characters of each row extending across the width of the drum are identical. The number of rows of type depends upon the number of difierent characters in the type font.

The second form of mechanism is the so-called chain" print mechanism which also employs a bank of individually actuatable hammers but which, instead of a type drum, utilizes a constantly moving type chain or belt which moves the type characters past the print hammers in a direction parallel to the printline. The most frequently cited advantage of the chain mechanism over the drum mechanism is that horizontal registration of the printed characters (alignment of the characters with respect to a horizontal reference line) is much easier to control. That is, the unavoidable slight inaccuracies which occur in the timing of the print hammers cause the printed character images to be misplaced slightly in a horizontal direction rather than in a vertical direction, as is the case with the drum mechanism. The result is that the chain mechanism produces a much more even printline which has a more pleasing aesthetic appearance, is easier to read, and is more suitable for applications requiring a relatively high degree of print format control.

Additional advantages inherent in the chain mechanism include easy replaceability of individual types or type groupings on the carrier, lower cost of carrier manufacture, particularly when it is desired to produce carriers having different length type fonts or fonts with different character arrangements, and ease of removal and replacement of the carrier within the printer. Additionally, ghosting" is more simply avoided with the chain mechanism as it is necessary merely to space the characters on the carrier slightly farther apart than the distance between print positions on the line.

The principal difi'lculty encountered in the implementation of a chain printing system lies in the area of control. The generation of properly timed print hammer actuation signals is more complicated because at any given instant each print hammer is confronted with a different character rather than with the same character as is the case with the drum printer. Thus, character generation, which is the function of keeping track of which character is printable at each of the print hammers during each print subcycle, requires more than the simple code wheel or single counter used for character generation in the drum system. This complexity is further compounded when the type spacing on the carrier is greater than the print position spacing since all the hammers do not become actuatable for printing at the same instant.

OBJECTS AND SUMMARY OF THE INVENTION Accordingly, it is a principal object of the present invention to provide a simplified, low-cost and reliable control system for a chain printer.

More specifically, objects of the invention are to provide an improved and simplified character generation system for a chain printer and to provide a control system operable, without major alteration, with a variety of different-type carriers having different font lengths.

According to a first aspect of the invention, simplified character generation is accomplished for a system where type spacing is greater than print position spacing by tracking the scan of the memory (containing the characters to be printed in a line) with a single tracking or address counter and employing the output of this counter to control the character generation and comparison functions. A subsidiary "phase counter is employed to arrest the advance of the character generator when predetermined counts are exhibited by the tracking counter whereupon the character generation sequence is maintained in proper synchronism. Correlative to this, the comparator is activated during the times that the character generator is arrested.

In accordance with a second aspect of the invention, proper generation of beginning-of-font or index" sync pulses regardless of the length of the type font is accomplished by deriving a sequence of index pulses from a code disc and utilin'ng the moving carrier to gate the proper one of these index pulses to the control circuits in accordance with the length of the font provided on the carrier.

In accordance with a third aspect of the invention, reliability of the control circuits is enhanced and power requirements are reduced by locating the individual hammer driver circuits physically close to the hammers they control and by utilizing a coincident signal selection scheme for selecting hammers to be actuated whereby the number of control lines running from the logic circuits to the hammer mechanisms, located in a portion of the printer chassis remote from the logic circuits, is minimized.

These and other objects, features and advantages will be made apparent by the following detailed description of a preferred embodiment of the invention, the description being supplemented by drawings as follows:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a block diagram illustrating the basic components and the intercomponent flow of data and control signals for the chain printing system of the invention.

FIG. 2 is a schematic diagram showing the mechanical and control circuit portions of the print head component of FIG. I.

FIGS. 3 and 4 taken together constitute a schematic circuit diagram of the control circuit component of FIG. 1.

FIG. 5 is a waveform diagram showing the operation of the system of the invention during a single print cycle.

FIG. 6 is a diagram illustrating the relationship between the spacing of type characters on the type carrier and the spacing of print hammers (print positions) along the printline.

FIGS. 7 and 8 are truth tables showing the relationship between inputs and outputs of the column and none decode circuits shown in FIG. 4.

GENERAL DESCRIPTION OF EMBODIMENT Referring to FIG. I, the line printing system of the invention comprises two basic components, the print head mechanism and the logic circuits. The print head includes a print chain cartridge 18, a motor 22 and associated mechanical components for driving the print chain at a constant velocity, and a bank of print hammers 16 which are individually operable under control of a set of driver circuits [4. The print head further comprises means including a pair of paper feed tractors 20 for feeding a print form [1 in a step-by-step fashion between the hammers l6 and chain 18 and a code disk 24 and set of sensing circuits 26 for generating basic control signals for synchronizing the operation of the control circuits with the movement of the type chain. Also, an ink ribbon (not shown) is provided between the hammers and the paper 11 in a conventional manner.

The control circuits II, which may be contained on a set of logic or circuit panels in a section of the system remote from the print head, receive and store coded signals transmitted from an external information supply device It) representing the characters to be printed in a printline. Supply 10 also provides timing signals T? which are used by the control circuits to supervise the entrance into storage of the data characters. when a full line of characters has been entered, supply 10 transmits a print command signal PRT to the printer whereupon the printer launches into a print cycle to print the stored data. When the printline is completed the control circuits transmit a signal OK back to supply indicating that the print system is prepared to receive the next line of data. OK is terminated by PRT and remains inactive throughout the print cycle to inhibit supply I0 from transmitting any new data before the termination of the print cycle.

To enable proper synchronization of the operation of the control circuits [2 with the movement of the type chain a sequence of character sync signals CP and index sync signals IND are generated by the code disk 24 and sensing circuits 26 and are transmitted to the control circuits. The CP pulses occur at a repetition rate identical to the rate at which the type characters on the type chain pass a fixed reference point. The index signals IND occur at a repetition rate equal to the repetition rate at which a predetermined character in each type font on the chain passes the stationary reference point. The type characters in each font are arranged in identical order. The index pulse is a beginning-of-font" pulse. Since both C? and IND are generated by sensing marks inscribed on the code disk 24 which is connected to the type chain drive mechanism, the unavoidable variations in the velocity of the type chain correspondingly alter the rep rate of CP and lND, enabling the control circuits to operate in proper synchronism with the type chain in a manner described subsequently.

During the print cycle, the control circuits transmit to the print head combinations of print hammer selection signals on lines Cl through C16 and SI through S9. These signals operate, as described subsequently, in accordance with coincident selection principals to prime" or "cock the driver circuits 14 associated with selected ones of the print hammers 16. After this priming operation a FIRE signal is transmitted to the print head to actuate all of the primed driver circuits whereupon the print hammers associated therewith are driven against the ink ribbon to force paper ll against the chain to print the desired characters. This alternating prime and fire sequence continues until all the characters stored in the memory have been printed whereupon OK is initiated to begin a new data loading sequence and the paper feed mechanism is operated to step the paper to the next print line position.

DETAILED DESCRIPTION OF EMBODIMENT DEFINITION OF CIRCUIT SYMBOLS Before proceeding with a detailed description of the preferred embodiment of the invention the meaning of the logic circuit symbols used in FIGS. 2, 3 and 4 is given. It is to be understood that the logic schematics shown operate, as is conventional, on a binary voltage level basis wherein the inputs to the circuits and the outputs therefrom always exist at either of two discreet voltage levels, the upper voltage level (H) of the system or the lower voltage level (L) of the system.

An AND circuit is represented by a D-shaped block containing an & symbol. The input lines are always connected to the straight side of the block and the output line is always connected to the curved side of the block. The function of this circuit is to provide an H output voltage only when all input lines exist at the H level. When a small circle appears at the point where the output line joins the block then the function of the circuit is to provide an L level output only when all inputs are at the H level.

An OR circuit is represented by an arrow-shaped block containing the symbol OR. Input lines are always connected to the concave side of the block and the output line is always connected to the point. The function of this circuit is to provide an H level output only when any one or more of the input lines is at the H level.

A flip-flop circuit is represented by a rectangular block containing the symbol FF. The inputs are labeled set (S) and reset (R) and the outputs are labeled 1 and 0. This circuit is bistable in nature and its outputs are always at opposite voltage levels. When an L to H voltage level transition is presented at the S input the l output goes to H and the 0 output goes to L unless the outputs are already in such a state in which case the output levels do not change. When an L to H transition is presented to the R input the 0 output goes to H and the 1 output goes to L unless the outputs already exist in such a state in which case there is no change in the output levels.

A single-shot multivibrator is represented by a rectangular block containing the symbol SS. The input line to the circuit is always connected to the left or bottom edge of the block and the output line is always connected to the right or top edge of the block. The function of this circuit is to generate an L to H to L square-wave output pulse of fixed duration in response to an L to H transition occurring at the input. When a small circle appears at the point where the input line joins the block then the function of the circuit is to provide the square-wave output pulse in response to an H to L transition at the input.

An inverter circuit is represented by a triangular block containing the symbol l and having a small circle at the point where the output line joins the block. The function of this circuit is to provide an output level which is always opposite to the input level.

A delay circuit is represented by an elongated oval-shaped block with a pair of transverse stripes nearest the input end. The function of this circuit is to generate an output level which follows the input level but which changes state at some fixed period of time after the input changes state.

A gate circuit is a rectangular block containing the symbol G. Inputs into the gate circuit are identified by an arrowhead. The function of this circuit is to transfer the voltage levels on a plurality of input lines to an equal plurality of output lines whenever the gate control input line is at the H level. The latter line is a single input connected to one of the ends of the gate block. A gate circuit is usually made up of a plurality of AND circuits, one for each input line other than the gate control input. Each input into the gate is connected to the input of a different one of the ANDs and each output from the gate is taken from the output of a different one of the AND circuits. The gate control input line is connected to an input of all the ANDs.

A binary counter is represented by a rectangular block containing the symbol CTR. Inputs are supplied at an advance (ADV) input and a reset (RST) input, a pulse at the former operating to advance the value of the binary count exhibited on the output lines by one and a pulse at the latter operating to force the outputs to the all-0 state (unless some other reset count is indicated in lieu of the RST legend PRINT HEAD MECHANISM The pertinent details of the print head mechanism are schematically illustrated in FIGv 2. The type chain cartridge 18 includes a type-carrying band or chain 30 which is entrained for rotation about a pair of

pulleys

34 and 36. The chain carries a plurality of complete type fonts having characters evenly spaced about the full periphery of the chain. An anvil plate 32 backs up the chain in the area of the printline. While not shown in FIG. 2, it is usual to provide teeth on the pulleys to mate with teeth on the chain whereby a positive, nonslip drive is obtained. It is also a usual practice to make the chain cartridge assembly, i.e., the chain 30, anvil 32 and

pulleys

34 and 36, readily removable as an integral unit from the print head mechanism.

The motor 22 drives the chain through connection with the pulley 36. The end of the motor drive shaft is provided with splines or is keyed or made square to enable easy disengagement of the pulley 36 from the drive shaft while at the same time providing a positive driving connection.

The code wheel 24 is also positively connected to the motor drive shaft so that it rotates in synchronism with the pulley 36. The code wheel has inscribed about its periphery a plurality of marks 38 and also is provided with a plurality of radial marks 40 on its upper surface. These marks may be, for example, transparent apertures in an opaque disk or they might be magnetically detectable marks on a ferrous disk. The marks 38 correspond to the type characters on the chain 30 and are sensed by a transducer 46 which generates an output pulse in response to each mark. The marks pass transducer 46 at the same rate that the characters on the chain pass a stationary point. Each such output pulse is amplified and shaped by amplifier 52 and transmitted thereby as a CP pulse. The marks 40 are sensed by a transducer 44 and the outputs generated therefrom are processed by amplifier 50 and transmitted to one of the inputs of an AND circuit 56. In the embodiment herein described, there is one mark 40 for each eight marks 38.

A transducer 42 is positioned to sense beginning-of-font marks inscribed on the chain 30. There is one such mark inscribed at some predetermined location within each of the type fonts on the chain. For ease of manufacture this mark may be placed on a type slug which carries a character or characters common to all different font arrangements usable with the printer. For example, if each of the different fonts in cludes a set of alphabetic characters then a predetermined alphabetic type slug wound be suitable for supporting the beginning-of-font mark. This procedure, of course, simplifies the manufacturing of the type slugs and the assembly of the chains.

Each output pulse generated by transducer 42 is conditioned by an amplifier 48 and is fed to the set input of a flipflop S4. The 1 output from the flip-flop is fed to the other input of AND 56 so that each beginning-of-font mark on the type chain sets the flip-flop and causes AND 56 to become partially conditioned. Concurrently with this or slightly afterwards, a mark 40 is sensed by transducer 44 whereupon AND 56 is activated and the output pulse IND is generated. Because the actual timing of the IND pulse is determined by the mark 40, which is accurately oriented with respect to the marks 38 carried on the same member, the IND and CP pulses bear the precise timing relationship with one another which is necessary for correct operation of the logic circuits.

In the present embodiment, the 8 to l ratio between marks 38 and marks 40 insures correct generation of the IND pulse for any type font having a number of characters evenly divisible by 8. Thus, type chains bearing fonts of characters in any of the standard lengths, e.g., 48 characters, 64 characters, 96 characters, 128 characters, etc., may be used with the printer without alteration of the sensing circuits 26. Of course, as explained subsequently in connection with the control circuits, a slight modification of the character generation circuits must be made to account for the particular number of characters employed in the type font in use.

The print hammers 16 may be of conventional design and are supported in alignment with each of the print positions along the printline. A standard arrangement is to place the hammers on 01-inch centers to enable the printing of characters per inch. In the present embodiment, there are 144 print hammers 16-1 through 16-144. Since each hammer and hammer drive assembly is identical, only the first and last are shown in FIG. 2.

Each hammer is actuated for printing by a solenoid coil which is connected to receive a current pulse from a driver circuit 68. Each such driver circuit is in turn controlled by the output from an AND circuit 64 which has a pair of inputs connected respectively to the l output of a flip-flop 62 and to a line which supplies one of four fire pulses F1, F2, F3 and F4 from the control circuits. Each flip-flop 62 has an AND circuit 60 connected to its set input and a delay circuit 66 connected to its reset input. The latter is connected to a fire pulse input line while the former receives a pair of inputs from the hammer select control lines CI through C16 and 51 through S9. Each AND 60 has one input line connected to one of the C control lines and its other input connected to one of the 8 lines.

ANDs 60-1 through 60-16 associated with the first l6 print hammers are each connected to a different one of the 16 control lines C1 through C16, AND 60-] being connected to Cl, AND 60-2 being connected to C2, etc. The second group of sixteen AND's 60-17 through 60-32 are connected in similar sequence to the lines C1 through C16. This pattern of input connections repeats for each of the remaining seven groups of sixteen AND circuits 60. The S control lines are connected to the ANDs sixteen at a time. That is, line S1 is connected to the second input of each of the first sixteen ANDs 60-] through 60-16, line S2 is connected to the second input of each of the ANDs 60-17 through 60-32, line S3 is connected to the second input of each of the ANDs 60-33 through 60-48, etc.

Thus, when the control circuits of the system determine that a character to be printed is coming into alignment with the proper print position, signals are simultaneously transmitted to the print head on the proper C line and the proper 5 line to select the AND circuit 60 associated with that print position. This sets the corresponding flip-flop 62. Thereafter. at a time determined to be the precise instant in which to begin actuation of the hammer, a fire pulse (Fl-F4) is transmitted to the print head to activate the AND 64 which is connected to the output of the set flip-flop 62, whereupon the hammer is actuated to print. After the firing operation has been initiated the delay circuit 66 associated with the set flip-flop passes the fire pulse to the reset input of the flip-flop to restore it to its initial condition. The fire pulse lines Fl-F4 are connected to the hammer drive circuits in an every-fourth-hammer alternating sequence, i.e., line F1 is connected to control hammers 16-1, 16-5, 16-9, etc., line F2 to hammers 16-2, 16-6, etc., line F3 to hammers 16-3, 16-7, etc. and line F4 to hammers 16-4, 16-8, etc.

Since the print head mechanism of FIG. 2 is located in a portion of the printer chassis which is remote from the control circuits, the reliability of the system is enhanced by the fact that only 29 control lines, i.e., the four fire lines, the C lines and S lines, are coupled between the logic circuits and the print head to control the 144 print hammers. This, coupled with the fact that signals are transmitted on the 5 lines only when it is desired to select a hammer for actual firing, minimizes the number of signals transmitted over the relatively lengthy connecting cable and thus the amount of electrical noise radiated into the system by the cable is kept to a minimum.

Further, power consumption is reduced since the F1-F4, C and S control signals are all low-level logic signals which can be transmitted over the long connecting cable without significant power loss. The circuits which dissipate the most power are the driver circuits 68 which must feed high-current drive pulses to the hammer solenoid coils. In the present system, power losses and noise are minimized by physically mounting the driver circuits 68 as close as possible to their associated hammer coils. This technique is pennitted by the use of compact integrated circuit drivers mounted on printed circuit cards placed directly on the print head frame immediately adjacent the hammer modules.

CONTROL CIRCUITS The control circuits 12 are shown in FIGS. 3 and 4. The basic components are a buffer memory including a shift register 70 and input-output register which function to store the characters to be printed, a master six-position binary counter and a six-position binary slave counter 114 which perform the character generation function, a six-bit comparator for generating hammer select signals, an eight-position binary address or tracking counter 152 which supervises the various control operations, a two-position binary phase counter 166 and two-bit comparator 102 for controlling character generation and print comparison, and a pair of decode circuits 170 and [72 which operate from the address counter outputs to control generation of the hammer select control signals C and S.

Before proceeding with a description of the control circuits, reference is made to FIG. 6 for explanation of one of the basic precepts upon which operation of the system is based. FIG. 6 shows the spatial interrelation between the type characters on the type chain and the print positions in the printline. As noted from FIG. 6 the center-to-center distance between print positions (print hammers) is W while the center-to-center distance Y between the type characters on the chain is 4/3 W. The principal reason for making Y greater than W is to reduce "ghosting." This is a phenomenon which occurs when a print hammer, in pressing the paper against the chain character to be printed, also creates sufficient pressure to generate a slight impression of the edge of the chain character adjacent to the character being printed. Ghosting is a problem with drum printers since the lateral spacing of types on the drum must be equal to the spacing of the print hammers.

By opening up the spacing of types on chain 30, as shown in FIG. 6, ghosting is reduced or eliminated completely. A subsidiary reason for making the distance Y greater than the distance W is to stagger the operation of the print hammers. As noted from FIG. 6, during the time it takes to print chain to move right-to-left through the distance Y (which is defined as one print cycle) all print hammers come into exact registration with one chain character. However, as illustrated in the table of FIG. 6, all of the column hammers do not come into registration with a type character at the same instant. Hammer registrations occur in four groupings or phases, dil, d2, 63 and d 4. This number of phases is dictated by the ratio of Y to W which in the case of the present embodiment is four-thirds (the distance spanned by four print columns equals the distance spanned by three character spaces on the chain).

Thus, during phase l of the print cycle, hammers I, 5, 9, I3, etc., through 141 are only hammers in registration with characters on the chain. During phase 02, hammers 2, 6, I0, I4, etc., through I42 come into registration with chain characters while during phase 4:3, hammers 3, 7, etc., through 143 are in registration and during phase M, hammers 4, 8, I2, etc., through 144 are in registration. This means that even if one desires to print all the letters of the alphabet arranged in sequence across the printline, which will require all hammers to fire during the same print cycle, all the hammers do not fire simultaneously but instead the firings are staggered in four groupings of 36 hammers each.

It can be realized that if such a pattern was printed with a chain having type characters spaced at the distance W, every harruner would be required to fire at the same instant, which causes an excessive drain on the power supply and an unduly high force of impact is imposed on the type chain, ink ribbon and paper with potentially disasterous results. While it is true that the use of a chain having Y equal to W results in slightly faster completion of the print line, the small sacrifice in printing speed where Y is made greater than W is certainly justified.

It is noted that the number of phases X in any given system is equal to the ratio of Y to Y minus W. In the present embodiment X equals 4. In a system where Y equals l.5 W, for example, X would equal 3 and in a system where Y equals 2.0 W, X would equal 2.

Referring back to FIGS. 3 and 4, data is loaded into the buffer memory shift register 70 (FIG. 3) during the data loading cycle by the alternating presentation of T? timing signals and data signals from the external input device. Each TP pulse operates through an OR-circuit 72 to advance the data characters stored in shifi register 70 one position to the right. An input data character is gated through a gate 74 and a set of OR-circuits 78 into the 1/0 register 80. Thereafter, a TP pulse appears to shift the character from register 80 to the first storage position of the register 70. Thus, after 144 data characters representing the characters of one printline have been received from the input, character number I44 resides in register 80 while characters I through I43 reside in register 70, with character number I residing in the storage position at the right-hand end of the register 70 and character I43 resid ing in the left-hand end position.

Following the loading operation, PRT is transmitted by the input device to set a print mode control flip-flop l 30 (FIG. 4) whereupon OK terminates (shifis negative) and OK is initiated (shifts positive). The existence of OK in the low state inhibits the generation of any further T? or data signals by the input device.

The generation of OK initiates the beginning of print cycles by opening gate 76 to permit the circulation and recirflation of the data characters through the buffer memory. OK also provides a conditioning input to an AND-circuit I32 (FIG. 3) whereupon the next CP pulse to occur activates AND I32, causing it to transmit an output through an OR 136 to trigger a single-shot 138. The output ST generated by single-shot I38 is fed to reset flip-flop I58, to reset the address counter 152 to the "256 state wherein all eight output lines AI through A8 are forced to the I (high-voltage) state, and is further trans mitted to set a flip-flop 82 which conditions AND-gate 84. ST is also fed to an input of AND 164 but since on this first print cycle ST is generated concurrently with CP, an inverter circuit 162 deconditions AND I64, preventing ST from being gated through to the advance input of the phase counter I66.

ST is also transmitted through a delay circuit 146 to the set input of a phase control flip-flop I44. When the flip-flop sets, it conditions an AND-gate 148 to begin transmitting the regularly occurring outputs from a clock circuit I50 as the signal CLK to the advance inputs of address counter I52 and shift register 70, the latter being fed CLK through AND 84 and OR 72. The pulse train CLK is also presented through AND 84 to the inputs of a pair of AND'circuits 94 and I00 and to the input of a delay circuit 96.

The first pulse of the CLK signal causes address register I52 to tum over from an all-l output condition to the all-zero output condition (all eight output lines shifting to the 0 or lowlevel state). It also causes the data in the buffer memory to shift one position to the right whereupon character number 1 of the printline is moved to register and character number I44 is shifted to the first storage position in register 70.

The CP pulse which triggered the above sequence of operations is also fed through an AND 165 to reset the phase counter 166 to its 00 output state.

As the print cycle continues, the CLK pulses fed from AND 148 simultaneously advance the address counter I52 and the shift register 70. At the beginning of the I45th count of counter I52 the A5 and A6 output lines thereof simultaneously shift positive, activating AND 156 whereupon flip-flop I58 sets. The resulting positive shift at the 1 output of the flipflop triggers a single-shot I60 to produce a tire time pulse FT. The latter triggers one of the four single-shots I61, I63, I67 or 169 to generate a fire pulse and is also fed to the reset input of flip-flop 82 which causes AND 84 to be deconditioned, terminating the supply of CLK pulses to the shift register 70. This halts the advance of the shift register and since it has been shifted I44 times prior to the halt of CLK pulses, the position of the data stored in the shift register and [/0 register is the same as it was at the beginning of the count sequence i.e., character 144 is stored in register 80 and characters I through 143 are queued from right to left in register 70.

Address counter I52 continues advancing in response to the C LK pulses gated by AND 148 until the beginning of the I6Ist count. At that time the A6 and A8 outputs from the counter simultaneously shift positive, causing AND 154 to generate an output through OR I42 to reset phase control flipflop I44. This shifts the 1 output of the flip-flop negative whereupon AND 148 is deconditioned and the supply of CLK pulses to the address counter terminates. The resetting of flip-flop I44 switches the 0 output thereof positive whereupon AND I34 is activated to retrigger single-shot I38 through OR 136. This action produces another ST pulse from single-shot I38 and initiates a second phase cycle which is executed in exactly the same manner as that just described. It is noted that this second ST pulse, since it does not accompany the generation of a CP pulse, is gated through AND I64 and advances the phase counter to its OI output state.

At the end of the second phase cycle AND I54 again produces its output which resets flip-flop I44 and generates a third ST pulse. The third ST pulse advances phase counter 166 to its 10 state.

At the end of the third phase cycle AND 154 again generates an output which results in the production of a fourth ST pulse which initiates a fourth phase cycle and advances the phase counter to its 11 output state. At this point it is noted that since the output state of the phase counter is ll during the fourth phase cycle, the output of AND-circuit 160 is at a low level whereupon AND 134 is deconditioned. Thus at the end of the fourth phase cycle the resetting of flip-flop 144 cannot trigger single-shot 138 to produce ST. This means that the control circuits will then cease operation until the next CP pulse occurs to activate AND 132 to initiate the next print cycle.

The character generation circuits comprise a pair of six stage binary counters 110 and 114. Counter 110 is termed the master counter since it loads a new count through a gate 112 into counter I14, termed the slave counter, at the beginning of each print cycle. Master counter 110 is advanced through an AND 106 one count by each C? pulse and thus keeps track of the instantaneous position of the type chain in respect to a fixed reference point. As the beginning of each character font on the chain passes the reference point IND is received from the print head mechanism and resets the master counter 110 to the count representative of the first character of the font. decondition AND 106 whereupon the CP pulse occurring concurrently with the IND pulse does not advance the counter. With this arrangement it can be seen that it is important that C? and IND be exactly coincident. This is obtained by deriving both from the single code disk 24 of FIG. 2.

An instant following the generation of ST at the beginning of each phase cycle, delay circuit 104 opens gate 112 to load the output count from the master counter 110 into the slave counter 114. Thereafter, as each phase cycle proceeds, AND 100 gates CLK pulses to advance the slave counter in accordance with the output of an inverter circuit 98. The latter is connected to the output of an AND circuit 94 which acts in response to each CLK pulse to sample the output of the phase comparator I02. Comparator I02 compares the

outputs

1 and 2 of the phase counter with the two lowest order outputs A1 and A2 of the address counter. Whenever the value of these two-digit binary numbers is the same, comparator 102 generates an output which conditions AND 94 which in turn deconditions AND 100. This prevents that particular CLK pulse from advancing the slave counter and the output count thereof does not change. When FT is generated on completion of each memory scan, the slave counter I14 is reset to an all 0 output state.

Hammer selection pulses are generated by the six-bit comparator circuit 120 which compares the code character appearing at any given instant at the output of register 80 with the code character represented at the output of the slave counter. If the outputs match, comparator 120 generates a positive pulse which is fed to an input of an AND-circuit 122. The latter will be activated by the CLK pulse appearing at the output of delay circuit 96 if AND 94 is producing an output indicative of a match between the phase counter output and the AI-A2 output from the address counter 152. The output generated by AND 122 is a true signal TR which is fed to the inputs of a set of nine AND circuits 174-1 through 174-9 and which is also fed back to clear the 110 register 80. The latter action in effect erases the particular character from the memory and replaces it with an all 0 character. This, as described subsequently, indicates that that particular character has been printed.

puts to circuit 170 are in the 0 state an output signal is generated on line C I. All of the other C output lines are at that time inactive.

Zone decode circuit 172 receives at its inputs the high-order outputs A5 through A8 of address counter I52 and provides in response thereto a signal on one of nine output lines ZI through 29 in accordance with the truth table shown in FIG. 8. It can be seen by comparison of FIGS. 7 and 8 that the zone decode circuit 172 is identical to the column decode circuit 170 except that the final seven output lines are not employed for zone decoding.

The nine outputs Z1 through 29 are fed to the inputs of the corresponding nine AND-circuits 174-1 through 174-9. The output signals generated by these AND circuits are the nine hammer select control signals S1 through S9. It can thus be seen that for each different count of address counter 152 the column and zone decode circuits generate a unique pair of output signals, one signal being from the C group and the other being from the Z group. The number of possible different combinations of C and 2 signal combinations is equal to 144, one for each different print hammer (print position) at the print head. If for any of these signal combinations a TR pulse is generated by AND 122, the appropriate AND-circuit 174 is activated to produce an S selection signal which combines with the active C selection signal to prime the corresponding hammer driver circuit at the print head by setting its flip-flop 62 (FIG. 2).

A circuit including single-shot 86 (FIG. 3), flip-flop 88, OR 90 and AND 92 is provided to generate an end-of-print signal EP upon completion of the printline. When FT shifts negative, single-shot 86 produces an output pulse which resets flip-flop 88. If during the following complete memory scan any valid data character is detected in the memory (only characters containing one or more 1 bits are valid characters) the resulting output signal from OR 90 operates to set flip-flop 88. The resulting negative shift at the zero output thereof deconditions AND 92 so that when FT goes positive upon completion of the scan EP cannot be generated. If nothing but all 0 data characters are circulated during a memory scan, flip-flop 8B is not set and when FT goes positive at the end of the scan EP is generated.

EP is fed 2 reset the print control flip-flop (FIG. 4) whereupon 0K shifts negative and 0K shifts positive to signal the external data supply device 10 that the printer is ready to receive the next line of print data.

Each of the four fire pulse single-shots 161, I63, 167 and 169 is gated by a phase-decoding AND circuit at its input whereupon each responds to FT during a different phase of the print cycle to produce its respective fire pulse. Thus, the AND which triggers fiigleslgt 161 to produce Fl during phase I isied by FT, 4n and 952 (4:1 and 4:2 are the inverted forms of 4:1 and d2, respectively). Similarly, single-shot 163 is triggered to produce F2 during phase 2 on FT-Yldii, singleshot I67 produces F3 during phase 3 on FT-dzI-daZ and singleshot 169 produces F4 during phase 4 on FI-1-2.

OPERATION With reference now to the waveform diagram of FIG. 5 and to the apparatus diagrams of FIGS. 2, 3 and 4, a description is hereinafter given of the operation of the system through one complete print cycle.

Assuming that the data loading operation has just terminated, the first CI pulse shown in FIG. 5 is the first such pulse to occur after print control flip-flop 130 has been set by the PRT command. CI thus activates AND 132 to generate ST. CP also resets phase counter 166 to switch it from its 11 output state to its 00 output state which is indicative of phase 1. An instant following the rise of CP and ST, at a time determined by the delay of delay circuit 146, the phase control flipflop 144 switches from its 0 to its l output state whereupon the shift register 70 and the address counter I152 begin advancing. The first advance of the former presents to the input of comparator I20 a code representation of the data character to be printed by hammer number 1 at print position I. At the same time, the advance of address counter 152 causes it to assume its all output state whereupon since the phase counter is also in its all 0 output state comparator I02 activates AND 94 to cause AND 122 to sample the output of comparator 120. If at that time the output of slave counter [14 represents the same character then residing in 1/0 register 80, AND 122 generates TR. The latter signal activates AND l74-1 to produce S1 which, in combination with Cl which is then present at the output of column decode circuit I70, activates AND 60-1 (FIG. 2) which sets flip-flop 62-1. This prepares hammer l to be fired when F1 is generated later in the phase.

On the next advance of the address counter and shift register, the character to be printed by hammer 2 is presented to comparator 120 and the address counter is switched to an output of OOOOOOI. Since the lowest order pair of digits of this number (01 does not match the 00 output from the phase counter 166, comparator 102 does not activate AND 94 and consequently the latter circuit does not provide a conditioning input to AND 122. This inhibits the comparison operation. At the same time, however, AND 100 is activated to advance slave counter I14 one count.

The same procedure ensues for the next two advances of the address counter and shift register since the respective Al-AZ outputs of the address counter for each of those steps l0 and l do not match the 00 output of the phase counter.

However, on the fifth advance of the shift register and address counter the AI-A2 outputs of the latter again assume the 00 state and comparator I02 again inhibits advance of the slave counter and conditions AND I22 to sample the result of the comparison of the slave counter output with the data character to be printed by hammer number 5.

This sequence repeats throughout the first phase of the print cycle whereupon every fourth data character presented by register 80 is compared with the output of the slave counter while the latter is advanced three counts for every four counts of the address counter. When Fl comes up, all hammers selected during the memory scan are fired.

At the beginning of the second phase ST advances the phase counter to an output state of OI whereupon the output of com parator 120 is sampled on the second, sixth, tenth, fourteenth, etc, advances of the address counter while the slave counter is advanced only during the first, third, fourth, fifth, seventh, eighth, ninth, eleventh, etc., advances of the address counter.

During the execution of the third phase of the print cycle, the output state of the phase counter is ID so that AND 122 is conditioned to sample the comparison results on the third, seventh, eleventh, etc. advances of the address counter while the slave counter is advanced on the first, second, fourth, fifth, sixth, eighth, etc. advances of the address counter.

It can thus be seen that during each phase of the print cycle the comparator I02 operates in response to the output counts from the address counter I52 and the phase counter 166 to control both the performance of the print comparisons and the advance of the slave counter. The comparisons effected during each given phase through this control conform to the table of FIG. 6 while the advance of the slave counter conforms with the character presentation sequence dictated by the Y to W spacing ratio also shown in FIG. 6.

Referring again to FIG. 5, it can be seen that at the end of the first print cycle the next CP pulse should be received at the time N if the type chain has not departed from its nominal velocity. However, as shown in FIG. 5, the next CP pulse occurs slightly later than it should, indicating that the velocity of the type chain decreased slightly. However, due to the inhibiting effect of the output of AND 168 (FIG, 4) during phase 4, the resetting of flip-flop 144 does not trigger ST to begin the next print cycle. Instead, the system waits until CP is received whereupon the next print cycle is initiated and the operation of the control circuits is thus resynchronized with the movement of the type chain.

On the other hand, if the next CP pulse had occurred early (to the left of N in FIG. 5) AND (FIG. 4) is activated to prematurely reset flip-flop 144 at a time before the address counter has reached its full phase count of and the next print cycle is begun slightly ahead of the time it would have begun under nominal conditions. This again, resynchronizes the operation of the control circuits to the movement of the type chain.

It can be seen from FIG. 5 that if the velocity of the chain increases sufficiently to cause CF to be initiated prior to the time indicated by X, the address counter I52 would not be allowed to reach the 144 count necessary to set flip-flop I58 and trigger the fire pulse F4. This would cause a printing error since the print hammers which had been primed during the preceding phase four would not be fired at the end of phase 4 as they should be and instead would be fired upon the next generation of a tire pulse during some subsequent phase cycle. To prevent such errors the 1 output from flip-flop 158 is used to condition AND 140 so that if CP occurs before the flip-flop has been set, the CP pulse does not have its usual efiect and the control circuits are allowed to complete the phase 4 operation. After normal completion of phase 4 the circuits wait for the next CP pulse to trigger the next print cycle. For the same reason, AND is provided to gate CF to the reset input of the phase counter I66 only after flip-flop 158 has been set. If AND 165 was not present there would be a possibility that an excessively early occurrence of CI, in switching the phase counter to the 00 output condition prior to completion of the full fourth phase memory scan, could prevent the performance of the proper comparisons required during the final portion of the memory scan and cause erroneous comparisons to be made.

The basic print cycle illustrated in FIG. 5 and just described is repeated by the system until all data in the buffer memory has been printed, as indicated by the generation of B? When this occurs, the print control flip-flop I30 is reset and further operation of the control circuits is inhibited until a new line of data has been received and PRT begins a new series of print cycles. Of Course, CP and IND continue to advance and reset counter 110 so that synchronism with the chain is not lost.

The character generation counters 110 and 114 repetitively cycle through count sequences having a number of count F equal to the number of characters in the type font. The slave counter runs through N/F cycles four times for each single count advance of the master counter, where N equals the number of print positions in the print line. If, as previously mentioned, the type chain is replaced by one carrying a different length font, some provision must be made to allow a corresponding alteration in the basic count cycle (F) of the counters. This may be done at the field maintenance level by a wiring change in the counter circuits or a simple switching network may be provided to permit a supervisory operator to make the appropriate circuit alterations.

It will be appreciated that various changes in the form and details of the above described preferred embodiment may be effected by persons of ordinary skill without departing from the true spirit and scope of the invention.

We claim:

1. In a control system for a line printer having a type carrier moving parallel to the printline and having individual types thereon spaced apart from one another by a distance Y which is greater than the space W between print positions on said line, the combination comprising:

a memory for storing code representations of all characters to be printed on said line;

a memory address counter for controlling access to said memory so as to present all said stored code representations at the output of said memory during a single uninterrupted count sequence;

pulse-generating means for driving said counter through X uninterrupted count sequences during the time said type carrier moves the distance Y, where X equals Y/ Y W a character generation counter operable at the beginning of each said count sequence to generate a code representation of the type character on said carrier next coming into alignment with a stationary reference position;

a phase counter for counting 1 through X;

means for advancing said phase counter once for each said memory address count sequence;

means for advancing said character generation counter upon each advance of said memory address counter;

first means for comparing the count of said phase counter with the count of the lowest order positions of said memory address counter and for generating a signal in response to each equal comparison detected;

means for inhibiting said character generation counter advancing means during the occurrence of said signal from said first comparing means;

print hammer means associated with said print positions for printing characters therein;

selectively actuatable second comparing means for receiving and comparing the code representations at said memory output and the code representations of said character generation counter and for generating a print hammer actuation signal in response to each equal comparison condition detected; and

means for actuating said second comparing means each time the count of said lowest order positions of said memory address counter equals the count of said phase counter.

2. The control system set forth in claim 1 wherein said memory comprises:

a shift register having capacity for storing all characters to be printed on said line; and wherein said memory address counter drives said register so that it shifts one character position for each advance of said memory address counter.

3; The control system set forth in claim 2 further comprising:

a master counter operable to set a count into said character generation counter prior to each count sequence of said memory address counter, means for advancing said master counter one count each time said carrier moves the distance Y, and means for resetting said master counter to a predetermined reference count each time said carrier moves the distance ZY, where Z represents the number of types in the full type font.

4. in a system for generating print hammer actuation control signals in a line printer having a type carrier moving parallel to a print line having N possible print positions. said carrier having individual types thereon spaced apart from one another by a distance Y which is greater than the space W between said print positions, the combination comprising:

a first counter;

means for driving said first counter through X count sequences each time said carrier moves the distance Y, X equaling Y/Y-W, each said count sequence comprising at least N steps;

a second counter operable at the beginning of each said count sequence to generate a code representation of the type character on said carrier coming into alignment with a stationary reference position;

a third counter for counting 1 through X;

first means for advancing said third counter once for each said count sequence of said first counter;

second means for advancing said second counter upon each advance of said first counter;

first means for comparing the count of said third counter with the count of the lowest order positions of such first counter and for generating a signal in response to each equal comparison detected;

means for inhibiting said second advancing means during the occurrence of said signal from said first comparing means; selectively actuatable second means for comparing the code representation of the character to be printed in the print position identified by the count of said first counter with the count of said second counter and for generating a hammer actuation control signal in response to a match condition and;

means for actuating said second comparing means each time the count of the lowest order positions of said first counter equals the count of said third counter.

5. In a system for generating print command signals in a line printer wherein a type carrier moves parallel to an N-position print line and carries individual types spaced apart from one another by a distance Y which is greater than the space between print positions on said line the combinations comprising:

a recirculating memory storing code representations of all characters to be printed on said line;

timing means for driving said memory to circulate all said stored code representations past the output of said memory a plurality of times during the time said type carrier moves the distance Y;

memory address counting means driven by said timing means to advance one count each time a code representation is presented at said memory output, each output count from said counting means identifying a different one of said N print positions;

character generation means operating in sync with the movement of said carrier to generate code representations identifying the order in which the types on said carrier move past said print positions; and

comparing means operable in response to the presentation of predetermined output counts from said counting means for comparing the code representation appearing at said memory output with that generated by said character generation means and for generating a print command signal upon detection ofa match.

6. The system set forth in claim 5 wherein said timing means drives said memory to circulate said stored code representa tions Y/YW times during the time said carrier moves the distance Y, where W represents the spacing between adjacent print positions on said line.

7. The system set forth in claim 6 wherein said comparing means is operable every X advance of said memory address counting means, where X equals Y/Y-W.

8. The system set forth in claim 7 wherein said character generation means comprises:

a character counter operable at the beginning of each circulation cycle of said memory to generate a code representation of the type character on said carrier next coming into alignment with a stationary reference position; and

means for advancing said character counter in response to each advance of said memory address counting means occurring intermediate each said X advance.

l i i l

Claims

1. In a control system for a line printer having a type carrier moving parallel to the printline and having individual types thereon spaced apart from one another by a distance Y which is greater than the space W between print positions on said line, the combination comprising: a memory for storing code representations of all characters to be printed on said line; a memory address counter for controlling access to said memory so as to present all said stored code representations at the output of said memory during a single uninterrupted count sequence; pulse-generating means for driving said counter through X uninterrupted count sequences during the time said type carrier moves the distance Y, where X equals Y/Y-W; a character generation counter operable at the beginning of each said count sequence to generate a code representation of the type character on said carrier next coming into alignment with a stationary reference position; a phase counter for counting 1 through X; means for advancing said phase counter once for each said memory address count sequence; means for advancing said character generation counter upon each advance of said memory address counter; first means for comparing the count of said phase counter with the count of the lowest order positions of said memory address counter and for generating a signal in response to each equal comparison detected; means for inhibiting said character generation counter advancing means during the occurrence of said signal from said first comparing means; print hammer means associated with said print positions for printing characters therein; selectively actuatable second comparing means for receiving and comparing the code representations at said memory output and the code representations of said character generation counter and for generating a print hammer actuation signal in response to each equal comparison condition detected; and means for actuating said second comparing means each time the count of said lowest order positions of said memory address counter equals the count of said phase counter.

2. The control system set forth in claim 1 wherein said memory comprises: a shift register having capacity for storing all characters to be printed on said line; and wherein said memory address counter drives said register so that it shifts one character position for each advance of said memory address counter.

3. The control system set forth in claim 2 further comprising: a master counter operable to set a count into said character generation counter prior to each count sequence of said memory address counter, means for advancing said master counter one count each time said carrier moves the distance Y, and means for resetting said master counter to a predetermined reference count each time said carrier moves the distance ZY, where Z represents the number of types in the full type font.

4. In a system for generating print hammer actuation control signals in a line printer having a type carrier moving parallel to a print line having N possible print positions, said carrier having individual types thereon spaced apart from one another by a distance Y which is greater than the space W between said print positions, the combination comprising: a first counter; means for driving said first counter through X count sequences each time said carrier moves the distance Y, X equalIng Y/Y-W, each said count sequence comprising at least N steps; a second counter operable at the beginning of each said count sequence to generate a code representation of the type character on said carrier coming into alignment with a stationary reference position; a third counter for counting 1 through X; first means for advancing said third counter once for each said count sequence of said first counter; second means for advancing said second counter upon each advance of said first counter; first means for comparing the count of said third counter with the count of the lowest order positions of such first counter and for generating a signal in response to each equal comparison detected; means for inhibiting said second advancing means during the occurrence of said signal from said first comparing means; selectively actuatable second means for comparing the code representation of the character to be printed in the print position identified by the count of said first counter with the count of said second counter and for generating a hammer actuation control signal in response to a match condition and; means for actuating said second comparing means each time the count of the lowest order positions of said first counter equals the count of said third counter.

5. In a system for generating print command signals in a line printer wherein a type carrier moves parallel to an N-position print line and carries individual types spaced apart from one another by a distance Y which is greater than the space between print positions on said line, the combinations comprising: a recirculating memory storing code representations of all characters to be printed on said line; timing means for driving said memory to circulate all said stored code representations past the output of said memory a plurality of times during the time said type carrier moves the distance Y; memory address counting means driven by said timing means to advance one count each time a code representation is presented at said memory output, each output count from said counting means identifying a different one of said N print positions; character generation means operating in sync with the movement of said carrier to generate code representations identifying the order in which the types on said carrier move past said print positions; and comparing means operable in response to the presentation of predetermined output counts from said counting means for comparing the code representation appearing at said memory output with that generated by said character generation means and for generating a print command signal upon detection of a match.

6. The system set forth in claim 5 wherein said timing means drives said memory to circulate said stored code representations Y/Y-W times during the time said carrier moves the distance Y, where W represents the spacing between adjacent print positions on said line.

7. The system set forth in claim 6 wherein said comparing means is operable every Xth advance of said memory address counting means, where X equals Y/Y-W.

8. The system set forth in claim 7 wherein said character generation means comprises: a character counter operable at the beginning of each circulation cycle of said memory to generate a code representation of the type character on said carrier next coming into alignment with a stationary reference position; and means for advancing said character counter in response to each advance of said memory address counting means occurring intermediate each said Xth advance.