CA1167121A - High speed ribbon cable bus - Google Patents
High speed ribbon cable busInfo
- Publication number
- CA1167121A CA1167121A CA000374567A CA374567A CA1167121A CA 1167121 A CA1167121 A CA 1167121A CA 000374567 A CA000374567 A CA 000374567A CA 374567 A CA374567 A CA 374567A CA 1167121 A CA1167121 A CA 1167121A
- Authority
- CA
- Canada
- Prior art keywords
- cable
- coupled
- receivers
- conductor
- transistor
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01R—ELECTRICALLY-CONDUCTIVE CONNECTIONS; STRUCTURAL ASSOCIATIONS OF A PLURALITY OF MUTUALLY-INSULATED ELECTRICAL CONNECTING ELEMENTS; COUPLING DEVICES; CURRENT COLLECTORS
- H01R25/00—Coupling parts adapted for simultaneous co-operation with two or more identical counterparts, e.g. for distributing energy to two or more circuits
- H01R25/14—Rails or bus-bars constructed so that the counterparts can be connected thereto at any point along their length
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- Details Of Connecting Devices For Male And Female Coupling (AREA)
- Small-Scale Networks (AREA)
- Cable Transmission Systems, Equalization Of Radio And Reduction Of Echo (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE:
A high speed bus structure is described which employs an ordinary flat ribbon cable. ECL receivers are coupled to conductor pairs of the cable through resistors which change the capacitive load of the ECL receivers to a resistive load.
The receivers are coupled to the cable through a plurality of connectors having spaced-apart pins. The pins in each connector engage less than all of the conductor pairs, thus a plurality of connectors are required to completely couple receivers to all the conductors in the cable. This connector arrangement substantially reduces the parasitic capacitance loading on the cable. The invented high speed bus is able to effectively function at 100 MHz with 80 feet of cable and with 16 receivers coupled to each conductor pair.
A high speed bus structure is described which employs an ordinary flat ribbon cable. ECL receivers are coupled to conductor pairs of the cable through resistors which change the capacitive load of the ECL receivers to a resistive load.
The receivers are coupled to the cable through a plurality of connectors having spaced-apart pins. The pins in each connector engage less than all of the conductor pairs, thus a plurality of connectors are required to completely couple receivers to all the conductors in the cable. This connector arrangement substantially reduces the parasitic capacitance loading on the cable. The invented high speed bus is able to effectively function at 100 MHz with 80 feet of cable and with 16 receivers coupled to each conductor pair.
Description
BACKGROUND OF THE INVENTION:
1. Field of the Invention.
The invention relates to the field of high speed buses, particularly those using flat ribbon cable.
1. Field of the Invention.
The invention relates to the field of high speed buses, particularly those using flat ribbon cable.
2. Prior Art~
There is an ever increasing need in the electronics industry for long, high speed buses, particularly for communicating digital information. Current technology provides numerous such buses~ However~in many instances, these buses employ relatively expensive cables, such as co-axial cables and expensive bus driving circuitry and receiving circuitry.
Flat ribbon cables are often used for buses. These cables are relatively inexpensive, moreover, there are numerous cable accessories, such as connectors which are readily available.
However, the bandwidth of these cables, particularly as they become longer, is limited.
As will be described, the present invention employs ribbon cables in a high speed bus. Through use of a combination of unique geometries, isolation and circuitry, a greatly increased bandwidth is obtained. By way of example, a bandwidth of lOOMHz is obtainable with cable lengths up to 80 feet and with 16 receiving stations on the bus.
7~
The high speed bus of the pxesent invention may be used in coun-tless applications. For example, the bus may interconnect computers housed in separa-te cabinets, inter-cabinet connections or intracablnet connec-tions may be made between a memory and a central processing unit, as well as to a host of other equipment.
The present invention resulted from a development program for improving a bus structure in a computerized private branch exchange (PBX). Specifically, the bus was designed for both intershelf connections and intercabinet connections.
For this reason, and also to provide more insight into the invented bus, a specific prior art bus structure shall be discussed, along with the improved bus structure in a PBX
environment. It will be apparent to one skilled in the art that the invented bus may nonetheless be used in a plurality of other applications.
In one prior art PBX, time division multiplexed signals are transferred along a multi-conductor bus in parallel during each time slot. For this structure, it is assumed that the data is "in phase" along -the entire bus. That is, a data bit originating at any station is expected to be propagated to any receiving station within each of the time slots. This restricts the maximum bit rate of the system to a function of the total cable length. As will be seen in the present invention, to eliminate this particular problem, the data is -transmit-ted serially (burst mode transmission). Each - time slot then may be shor-ter -than the propagation time along the cable. To prsvent bus arbitra-tion problems, each station or unit is allocated its own "private"
transmitting line.
In the prior art structure, receiving circuits were separated from the bus (ribbon cable) by relatively long traces on circuit boards. Also, the connector arrangement added to the total effective parallel admittance as seen from the bus, further degrading performance. The present invention provides unique configurations and circuits which substantially improve this aspect of overall bus performance.
SUMMARY OF Tl-IE INVENTION:
.. . . _ A bus structure which employs a ribbon cable for interconnecting a plurality (n) of elec-trical units or sta-tions is disclosed. A plurali-ty of driver circuits, one for each of the units, is used for transmit-ting signals from its respective unit to a pair of conductors wi-thin the cable. At leas-t n-l receivers are associated with each of the units in order that each unit is able to sense the signals transmitted by a:Ll of the other uni-ts (broadcast transmission mode). Connector means, one for each of the units, are used for coupling the receivers to the pairs of conductors within the ribbon cable. Each of the connector means includes at least a first and a second electrical connec-tor for engaging the cable. Each of the connectors has a plurality of spaced-apart pins which receive the signals from the cable. The pins in the first connector are arranged to receive signals from pairs of the conductors which are separated by other pairs of conductors. The pins in the second connector are arranged to receive signals from the other pairs of conductors. In this manner, the capacitance coupling associated wlth coupling to the cable is substantially reduced, providing an increased band~idth.
~. :
In the presently preferred embodiment, the bus drives include a pre-emphasis network for emphasizing predetermined frequencies from the transmitters. The transmitters operate in a burst mode, thus permitting transmission without the assumption that all the signals are "in phase".
The receivers employ emitter-coupled logic (ECL) circuits which are connected to the cable through series resistors.
These receivers are mounted in close proximity (on a printed circuit board) to the connectors engaging the cable.
A bandwidth of 100 MHz has been realized with this arrangement with ribbon cable lengths up to 80 feet and with 16 units coupled to the cable.
sRIEF DE'SCRIPTION OF T~IE DRAWINGS:
Figure 1 is a block diagram illustra-ting -the manner in which the electrical units (such as PBXs) are coupled to the ribbon cable in a broadcast mode.
Figure 2 is a circuit diagram which illustra-tes the interconnection between a receiver and conductors of the ribbon cable.
Figure 3 is a plan view of a connector which illustrates the coupling between a connector engaging the ribbon cable and receivers mounted on a printed circuit board.
Figure 4 is a perspective view illustrating the layout for connectors on the cable, corresponding mating connectors on a printed circuit board, receivers and series resistors.
Figure 5 is a circuit diagram of one embodiment of the driver circuit used to drive the conductors in the cable.
Figure 6 is an alternate embodiment of the driver circuit o~ Figure 5.
DETAILED DESCRIPI'ION OF TIIE INV~NTION~
A bus structure is described which employs an ordinary ribbon cable. In the following descrip-tion numerous specific details, particularly those relating to the use of the bus structure in a computerized ~BX, are set Eorth. ~lowever, i-t will be apparent to one skilled in the art that -the invention may be practiced without -these spec:ific details and in applications other than for PBXs. In other instances, well-known circuits have been shown in block diagram form in order not to obscure the present invention in unnecessary detail.
10Referring first to Figure 1, the manner in which a plurality of electrical units, such as computerized PBXs (or other electrical units, such as central processing units,memories, etc.,) are coupled to the fla-t ribbon cable 10 in a broadcast mode is illustrated. Three units, units A, B, and C are connected to the various conductors in the ribbon cable.
Each unit includes a driver, such as driver 20, which drives a single pair of conductors in the cable. For example, driver A drives leads lla and llb, driver s leads 12a and 12b and driver C leads 13a and 13b. Each of the units includes a plurality of receivers for sensing the signals impressed on the cable by the drivers of the various units. (As presently implemented, all the receivers are identical.) For example, referring to uni-t B, receiver lB is connected to the leads lla and llb, receiver 2B to the leads 12a and 12b and receiver nB to the leads 13a and 13b. Each pair of leads is separated by a lead which is coupled to a constant potential or to ground. These are shown as leads 15, 16, 17 and 18 in Figure 1. The isolation provided by these leads can also be obtained by provicling a space between each pair of active leads, however, -this is not so easily realized where a standard cable is used.
In the presently preferred embodiment, in order to provide symmetry of operation, each unit includes a receiver which "listens" to the driver of that unit. By way of example, the receiver nC is coupled to leads 13a and 13b, and thus receives signals from the driver C. In some applications, it may be feasible to eliminate receiver nC,and like receivers, with an internal connection within the unit.
With the arrangement of Figure 1, each of the units has its own "private" transmission line and each unit has a receiver coupled to each of the active lines in the cable.
This eliminates all bus arbitra-tion problems, since all the units can asychronously transmit, thereby fully utilizing the capacity of the cable.
~.
In the presently preferred embodiment, the rlbbon cable is a flat (co-planar) cable wi-th 50 conductors.
This permits 16 units to be interconnected by active conductor pairs with the described isolation between each palr.
Referring to Figure 2, in the presently preferred embodiment, each of the receivers 21 is a commercially available emitter-coupled logic (ECL) circui-t. The input to these circuits are coupled to the conductor pairs of the cable (such as conductors 12a and 12b) through series resistors 32. The pin assemblies 23 are shown press ~it to the conduc-tors and engaging circuit board -traces 29.
One end of these traces engage the female pin members 26, while the other end of these traces are connected to one end of the resistors 32. The traces 30 interconnect the other ends of the resistors to the input terminals of the circuit 21.
The advantages gained from use of the series resistors 32 and the spacing technique used for the pin assemblies of the connectors (described below) can best be illustrated by way of example. Typically, the input resistance RI for an ECL circuit is approximately 6k ohms. The input capacitance CI is approximately 5 pF. At 100 MHz, CI
obviously dominates. In the presently preferred embodiment, ir~t~
wide pin spacing (as will be descrihed below) is provided in the connectors by removing pin assemblies from a standard flat cable connector, such as the 3M Part No. 3307-000.
With this removal, Cc can be reduced to 0.2pF'. The capacitance associated with the traces 29 and 30 (CB) is approximately 0.5pF. CR, the capacitance associated with the resistors 32 is approxima-tely 0.4pF.
Assume firs-t that the series resistors 32 are not employed. (This is the typical prior art structure.) In this case, CR is eliminated, however, there is an increased trace capacitance, making CB equal to approximately 0.6pF.
Without the resistors, the total parasitic capacitance loading on the conductors is approximately 5.8pF or a reactive component of approximately 300 ohms at 100 MHz. This produces intolerable reflections into the 160 ohm cable at these frequencies.
In the presently preferred embodiment, the resistors 32 are each 330 ohms. The use of these resistors does increase the effective input impedance of the receiver and to some extent, it slows the response time of the receivers. However, this resistance has the advantage of changing the impedance of the receiver, as seen from the cable, from highly capacitive to mostly resistive. This greatly decreases the distortion of pulses (for digital transmission) received by receiver 21 and improves system rise time. The increased cable propagation obtained through use of these resistors greatly outweighs the slight degradation of receiver performance. The value of -the resistors 32 is not particularly critical;
the selected values of 330 ohms for the present embodiment was obtained by simply weighing the improved cable performance against the loss of receiver (input) performance. An optimized value may be readily selected by examining the various capacitances and resis-tances as described in conjunction with Figure 2.
The capacitances Cc and CB for the prior art bus structure are not of great significance (when compared to CI); however, these capacitances assume a major role because of the series resistors 32. For this reason, much greater care must be taken to reduce these capacitances as shall be described.
In a typical ribbon cable connector, the female part of the connector includes a plurality of pressure contacts 24 as shown in Figure 2. Typically, these contacts are pressure fit onto the cable and include cylindrical openings 25 which receive pins 26 from the male part of -the connector.
To reduce the capacitance coupling (Cc) some of the contact assemblies 23 are removed from the male and Eemale parts of the connectors.
Referring to the connector 35 shown in plan view in Figure 3 only every third contact assembly remains within the connector 35. The larger circles in Figure 3 denote the remaining pins 23, which are separated from one another by two apertures 37. In practice, the contact assemblies are simply removed Erom both the male and female parts o~
the connectors, leaving the apertures 37. Thus, to make contact to each of the conductors in a cable (for the described embodiment) three connectors are required instead of the usual single connector.
For the above described 3M connector, leaving only every third pin assembly increases the spacing between the contact members in the connectors from approximately 0.1 inches to approximately 0~3 inches. The same result, of course, can be achieved by fabricating connectors having a wider pin spacing, however, it has been found much more economical to use currently available commercial connectors and to remove the desired pins.
Referring to Figure 4, the male part 35a of connectors is shown coupled to the cable while -the female part 35b is moun-ted on a printed circuit board 42. For the described embodiment, the two outer connectors are used for coupling -the conductor pairs -to the receivers while the center connector is used for grounding -the conductors which separa-te each of the conductor pairs.
As shown best in E'igure 3, one contact from one side of each outer connec-tor of Figure 4 and another contact from the other side of the connector (such as contacts 45 and 46) are coupled to a receiver mounted along one side of the connector. The next pair of con-tacts provide coupling to a receiver mounted on the other side of the connector. (In Figure 3, only those receivers mounted on one side of the connector are shown, however, the leads 39 provide the coupling to the receivers mounted on the other side of the connector).
The layout of the receivers and connectors are best illustrated in Figure 4. Two ECL circuits are included 2~ on each package 41. Two such packages are mounted on each side of the outer connectors. The resistors 32 are mounted between the male part 35b of the connectors and the packages 41. As illustrated, the ECL circuits and resistors are mounted in close proximity on the printed circuit board 42 to the connector part 35b and are interconnected through the traces 29 and 30 shown in Figure 2.
With this connector arrangement, each connector engages conductors, which are separated by other conductors to which no con-tact is made. For the illustrated embodiment, for the two outer connectors, a conductor pair and grounded conductor lies between every contac-ted conductor pair.
In the case of the center connec-tor, which provides the ground connections, two pairs of conductors separate each of the contacted conductors.
The substantial reduction in the parasitic capacitance Cc obtained with this connector arrangement and the reduction of CB obtained by mounting the ECL circuits and resistors in close proximity to the connectors, makes possible the advantageous use of the resistors 32 of Figure 2.
In Figure 5, an embodiment for a driver circuit, such as the drivers 20 of Figure 1/ is illustrated. Two receivers 21 are shown coupled to a conductor pair of the ribbon cable 10.
-These conductors (at the end of the cable) are coupled to a positive potential through the resistors 48, 49 and 50 which act as loads. The conductor pair is driven in a push-pull mode through the npn driver transistors 54 and 55.
The bases of these transis-tors are coupled to an amplifier 53 which provides the differential outputs needed to drive the transistors 54 and 55. The amplifier 53 is driven by a pulse-generator 57 which of course receives the di~ital data or other data to be transmitted from an 2;~
electrical uni-t such as a PB~. The transistor 60 provides a cons-tant current source since its base i5 biased at a constant potential through the resistors 51 and 52.
Importantly, this driver circuit provides high frequency pxe-emphasis (at approximately 100 M~lz for the described embodiment). The resistor 58 and inductor 59 which are coupled across the conductor pair (along with the distributed.parasitic capacitance and distributed resistance associated with the cable) provides this pre-emphasis. This pre-emphasis increases the high frequency performance of the cable since it compensates for the high frequency "roll-off" of the cable and receiver circuits.
The circuit of Figure 5 operates best when only a single transmitter is used on each of the conductor pairs. Thus, this circuit may be used for the configuration of Figure 1. In some applications, it is desirable to have more than a single transmitter or driver on each of the conductor pairs. This requires those drivers not in use to be turned-off. At first it may appear that by simply disconnectinq the constant current source associated with transistor 60, the driver circuit will be effectively disconnected from the conductor pair. However, the reverse bias, collector-to-base capacitance of transistors 54 and 55 and the parasitic capacitance associated with the driver circuit layout will continue to load the line.
In Figure 6, a driver circuit is shown which substantially decouples itself from the line when not transmitting. This permits a plurality of drivers to be coupled to a single conductor pair. Aqain, in Figure 6, a pair of npn driver transistors 61 and 62 are driven through their base terminals via lines 72 with a differential signal. The collectors of these transistors are coupled to the conductor pair through diodes 63 and 64. These conductors (at the end of the cable) are connected to a positive potential V2 through resistors 75, 76, and 77 which act as loads for the transistors. The emitters of the transistors 61 and 62 are again coupled to a constant current source. This constant current source is provided through transistor 69 which has its base biased at a constant potential through resis-tors 66 and 67. The circuit also provides pre-emphasis through the resistor 73 and inductors 74 as described in conjunction with Figure 5.
When the driver circuit of Figure 6 is not in operation, the transistor 70 conducts since a transmit signal is applied to its base. When this occurs, the constant current to the driver transistors 61 and 62 is removed. This draws the cathodes of diodes 63 and 64 to a positive potential Vl through resistor 68 (and resistor 73 in the case of diode 63) reverse biasing the diodes. Note Vl must be larger than V2 to assure reverse biasing of the diodes. Then the conductor pair is isolated from transistors 61 and 62 by the capacitance associated with these reverse biased diodes. Thus, the driver circuit is substantially decoupled from -the cable when not trans-mitting. Note that during transmission, neither diode 63 nor 64 are reverse biased so that the switching -time of these diodes are not critical.
Thus, a bus structure has been disclosed which permits the use of an ordinary, inexpensive, ribbon cable in a high speed application. A bandwidth of 100 MHz has been obtained wi-th approximately ~0 feet of cable and up to 16 receivers coupled to each active conductor pair in the cable.
There is an ever increasing need in the electronics industry for long, high speed buses, particularly for communicating digital information. Current technology provides numerous such buses~ However~in many instances, these buses employ relatively expensive cables, such as co-axial cables and expensive bus driving circuitry and receiving circuitry.
Flat ribbon cables are often used for buses. These cables are relatively inexpensive, moreover, there are numerous cable accessories, such as connectors which are readily available.
However, the bandwidth of these cables, particularly as they become longer, is limited.
As will be described, the present invention employs ribbon cables in a high speed bus. Through use of a combination of unique geometries, isolation and circuitry, a greatly increased bandwidth is obtained. By way of example, a bandwidth of lOOMHz is obtainable with cable lengths up to 80 feet and with 16 receiving stations on the bus.
7~
The high speed bus of the pxesent invention may be used in coun-tless applications. For example, the bus may interconnect computers housed in separa-te cabinets, inter-cabinet connections or intracablnet connec-tions may be made between a memory and a central processing unit, as well as to a host of other equipment.
The present invention resulted from a development program for improving a bus structure in a computerized private branch exchange (PBX). Specifically, the bus was designed for both intershelf connections and intercabinet connections.
For this reason, and also to provide more insight into the invented bus, a specific prior art bus structure shall be discussed, along with the improved bus structure in a PBX
environment. It will be apparent to one skilled in the art that the invented bus may nonetheless be used in a plurality of other applications.
In one prior art PBX, time division multiplexed signals are transferred along a multi-conductor bus in parallel during each time slot. For this structure, it is assumed that the data is "in phase" along -the entire bus. That is, a data bit originating at any station is expected to be propagated to any receiving station within each of the time slots. This restricts the maximum bit rate of the system to a function of the total cable length. As will be seen in the present invention, to eliminate this particular problem, the data is -transmit-ted serially (burst mode transmission). Each - time slot then may be shor-ter -than the propagation time along the cable. To prsvent bus arbitra-tion problems, each station or unit is allocated its own "private"
transmitting line.
In the prior art structure, receiving circuits were separated from the bus (ribbon cable) by relatively long traces on circuit boards. Also, the connector arrangement added to the total effective parallel admittance as seen from the bus, further degrading performance. The present invention provides unique configurations and circuits which substantially improve this aspect of overall bus performance.
SUMMARY OF Tl-IE INVENTION:
.. . . _ A bus structure which employs a ribbon cable for interconnecting a plurality (n) of elec-trical units or sta-tions is disclosed. A plurali-ty of driver circuits, one for each of the units, is used for transmit-ting signals from its respective unit to a pair of conductors wi-thin the cable. At leas-t n-l receivers are associated with each of the units in order that each unit is able to sense the signals transmitted by a:Ll of the other uni-ts (broadcast transmission mode). Connector means, one for each of the units, are used for coupling the receivers to the pairs of conductors within the ribbon cable. Each of the connector means includes at least a first and a second electrical connec-tor for engaging the cable. Each of the connectors has a plurality of spaced-apart pins which receive the signals from the cable. The pins in the first connector are arranged to receive signals from pairs of the conductors which are separated by other pairs of conductors. The pins in the second connector are arranged to receive signals from the other pairs of conductors. In this manner, the capacitance coupling associated wlth coupling to the cable is substantially reduced, providing an increased band~idth.
~. :
In the presently preferred embodiment, the bus drives include a pre-emphasis network for emphasizing predetermined frequencies from the transmitters. The transmitters operate in a burst mode, thus permitting transmission without the assumption that all the signals are "in phase".
The receivers employ emitter-coupled logic (ECL) circuits which are connected to the cable through series resistors.
These receivers are mounted in close proximity (on a printed circuit board) to the connectors engaging the cable.
A bandwidth of 100 MHz has been realized with this arrangement with ribbon cable lengths up to 80 feet and with 16 units coupled to the cable.
sRIEF DE'SCRIPTION OF T~IE DRAWINGS:
Figure 1 is a block diagram illustra-ting -the manner in which the electrical units (such as PBXs) are coupled to the ribbon cable in a broadcast mode.
Figure 2 is a circuit diagram which illustra-tes the interconnection between a receiver and conductors of the ribbon cable.
Figure 3 is a plan view of a connector which illustrates the coupling between a connector engaging the ribbon cable and receivers mounted on a printed circuit board.
Figure 4 is a perspective view illustrating the layout for connectors on the cable, corresponding mating connectors on a printed circuit board, receivers and series resistors.
Figure 5 is a circuit diagram of one embodiment of the driver circuit used to drive the conductors in the cable.
Figure 6 is an alternate embodiment of the driver circuit o~ Figure 5.
DETAILED DESCRIPI'ION OF TIIE INV~NTION~
A bus structure is described which employs an ordinary ribbon cable. In the following descrip-tion numerous specific details, particularly those relating to the use of the bus structure in a computerized ~BX, are set Eorth. ~lowever, i-t will be apparent to one skilled in the art that -the invention may be practiced without -these spec:ific details and in applications other than for PBXs. In other instances, well-known circuits have been shown in block diagram form in order not to obscure the present invention in unnecessary detail.
10Referring first to Figure 1, the manner in which a plurality of electrical units, such as computerized PBXs (or other electrical units, such as central processing units,memories, etc.,) are coupled to the fla-t ribbon cable 10 in a broadcast mode is illustrated. Three units, units A, B, and C are connected to the various conductors in the ribbon cable.
Each unit includes a driver, such as driver 20, which drives a single pair of conductors in the cable. For example, driver A drives leads lla and llb, driver s leads 12a and 12b and driver C leads 13a and 13b. Each of the units includes a plurality of receivers for sensing the signals impressed on the cable by the drivers of the various units. (As presently implemented, all the receivers are identical.) For example, referring to uni-t B, receiver lB is connected to the leads lla and llb, receiver 2B to the leads 12a and 12b and receiver nB to the leads 13a and 13b. Each pair of leads is separated by a lead which is coupled to a constant potential or to ground. These are shown as leads 15, 16, 17 and 18 in Figure 1. The isolation provided by these leads can also be obtained by provicling a space between each pair of active leads, however, -this is not so easily realized where a standard cable is used.
In the presently preferred embodiment, in order to provide symmetry of operation, each unit includes a receiver which "listens" to the driver of that unit. By way of example, the receiver nC is coupled to leads 13a and 13b, and thus receives signals from the driver C. In some applications, it may be feasible to eliminate receiver nC,and like receivers, with an internal connection within the unit.
With the arrangement of Figure 1, each of the units has its own "private" transmission line and each unit has a receiver coupled to each of the active lines in the cable.
This eliminates all bus arbitra-tion problems, since all the units can asychronously transmit, thereby fully utilizing the capacity of the cable.
~.
In the presently preferred embodiment, the rlbbon cable is a flat (co-planar) cable wi-th 50 conductors.
This permits 16 units to be interconnected by active conductor pairs with the described isolation between each palr.
Referring to Figure 2, in the presently preferred embodiment, each of the receivers 21 is a commercially available emitter-coupled logic (ECL) circui-t. The input to these circuits are coupled to the conductor pairs of the cable (such as conductors 12a and 12b) through series resistors 32. The pin assemblies 23 are shown press ~it to the conduc-tors and engaging circuit board -traces 29.
One end of these traces engage the female pin members 26, while the other end of these traces are connected to one end of the resistors 32. The traces 30 interconnect the other ends of the resistors to the input terminals of the circuit 21.
The advantages gained from use of the series resistors 32 and the spacing technique used for the pin assemblies of the connectors (described below) can best be illustrated by way of example. Typically, the input resistance RI for an ECL circuit is approximately 6k ohms. The input capacitance CI is approximately 5 pF. At 100 MHz, CI
obviously dominates. In the presently preferred embodiment, ir~t~
wide pin spacing (as will be descrihed below) is provided in the connectors by removing pin assemblies from a standard flat cable connector, such as the 3M Part No. 3307-000.
With this removal, Cc can be reduced to 0.2pF'. The capacitance associated with the traces 29 and 30 (CB) is approximately 0.5pF. CR, the capacitance associated with the resistors 32 is approxima-tely 0.4pF.
Assume firs-t that the series resistors 32 are not employed. (This is the typical prior art structure.) In this case, CR is eliminated, however, there is an increased trace capacitance, making CB equal to approximately 0.6pF.
Without the resistors, the total parasitic capacitance loading on the conductors is approximately 5.8pF or a reactive component of approximately 300 ohms at 100 MHz. This produces intolerable reflections into the 160 ohm cable at these frequencies.
In the presently preferred embodiment, the resistors 32 are each 330 ohms. The use of these resistors does increase the effective input impedance of the receiver and to some extent, it slows the response time of the receivers. However, this resistance has the advantage of changing the impedance of the receiver, as seen from the cable, from highly capacitive to mostly resistive. This greatly decreases the distortion of pulses (for digital transmission) received by receiver 21 and improves system rise time. The increased cable propagation obtained through use of these resistors greatly outweighs the slight degradation of receiver performance. The value of -the resistors 32 is not particularly critical;
the selected values of 330 ohms for the present embodiment was obtained by simply weighing the improved cable performance against the loss of receiver (input) performance. An optimized value may be readily selected by examining the various capacitances and resis-tances as described in conjunction with Figure 2.
The capacitances Cc and CB for the prior art bus structure are not of great significance (when compared to CI); however, these capacitances assume a major role because of the series resistors 32. For this reason, much greater care must be taken to reduce these capacitances as shall be described.
In a typical ribbon cable connector, the female part of the connector includes a plurality of pressure contacts 24 as shown in Figure 2. Typically, these contacts are pressure fit onto the cable and include cylindrical openings 25 which receive pins 26 from the male part of -the connector.
To reduce the capacitance coupling (Cc) some of the contact assemblies 23 are removed from the male and Eemale parts of the connectors.
Referring to the connector 35 shown in plan view in Figure 3 only every third contact assembly remains within the connector 35. The larger circles in Figure 3 denote the remaining pins 23, which are separated from one another by two apertures 37. In practice, the contact assemblies are simply removed Erom both the male and female parts o~
the connectors, leaving the apertures 37. Thus, to make contact to each of the conductors in a cable (for the described embodiment) three connectors are required instead of the usual single connector.
For the above described 3M connector, leaving only every third pin assembly increases the spacing between the contact members in the connectors from approximately 0.1 inches to approximately 0~3 inches. The same result, of course, can be achieved by fabricating connectors having a wider pin spacing, however, it has been found much more economical to use currently available commercial connectors and to remove the desired pins.
Referring to Figure 4, the male part 35a of connectors is shown coupled to the cable while -the female part 35b is moun-ted on a printed circuit board 42. For the described embodiment, the two outer connectors are used for coupling -the conductor pairs -to the receivers while the center connector is used for grounding -the conductors which separa-te each of the conductor pairs.
As shown best in E'igure 3, one contact from one side of each outer connec-tor of Figure 4 and another contact from the other side of the connector (such as contacts 45 and 46) are coupled to a receiver mounted along one side of the connector. The next pair of con-tacts provide coupling to a receiver mounted on the other side of the connector. (In Figure 3, only those receivers mounted on one side of the connector are shown, however, the leads 39 provide the coupling to the receivers mounted on the other side of the connector).
The layout of the receivers and connectors are best illustrated in Figure 4. Two ECL circuits are included 2~ on each package 41. Two such packages are mounted on each side of the outer connectors. The resistors 32 are mounted between the male part 35b of the connectors and the packages 41. As illustrated, the ECL circuits and resistors are mounted in close proximity on the printed circuit board 42 to the connector part 35b and are interconnected through the traces 29 and 30 shown in Figure 2.
With this connector arrangement, each connector engages conductors, which are separated by other conductors to which no con-tact is made. For the illustrated embodiment, for the two outer connectors, a conductor pair and grounded conductor lies between every contac-ted conductor pair.
In the case of the center connec-tor, which provides the ground connections, two pairs of conductors separate each of the contacted conductors.
The substantial reduction in the parasitic capacitance Cc obtained with this connector arrangement and the reduction of CB obtained by mounting the ECL circuits and resistors in close proximity to the connectors, makes possible the advantageous use of the resistors 32 of Figure 2.
In Figure 5, an embodiment for a driver circuit, such as the drivers 20 of Figure 1/ is illustrated. Two receivers 21 are shown coupled to a conductor pair of the ribbon cable 10.
-These conductors (at the end of the cable) are coupled to a positive potential through the resistors 48, 49 and 50 which act as loads. The conductor pair is driven in a push-pull mode through the npn driver transistors 54 and 55.
The bases of these transis-tors are coupled to an amplifier 53 which provides the differential outputs needed to drive the transistors 54 and 55. The amplifier 53 is driven by a pulse-generator 57 which of course receives the di~ital data or other data to be transmitted from an 2;~
electrical uni-t such as a PB~. The transistor 60 provides a cons-tant current source since its base i5 biased at a constant potential through the resistors 51 and 52.
Importantly, this driver circuit provides high frequency pxe-emphasis (at approximately 100 M~lz for the described embodiment). The resistor 58 and inductor 59 which are coupled across the conductor pair (along with the distributed.parasitic capacitance and distributed resistance associated with the cable) provides this pre-emphasis. This pre-emphasis increases the high frequency performance of the cable since it compensates for the high frequency "roll-off" of the cable and receiver circuits.
The circuit of Figure 5 operates best when only a single transmitter is used on each of the conductor pairs. Thus, this circuit may be used for the configuration of Figure 1. In some applications, it is desirable to have more than a single transmitter or driver on each of the conductor pairs. This requires those drivers not in use to be turned-off. At first it may appear that by simply disconnectinq the constant current source associated with transistor 60, the driver circuit will be effectively disconnected from the conductor pair. However, the reverse bias, collector-to-base capacitance of transistors 54 and 55 and the parasitic capacitance associated with the driver circuit layout will continue to load the line.
In Figure 6, a driver circuit is shown which substantially decouples itself from the line when not transmitting. This permits a plurality of drivers to be coupled to a single conductor pair. Aqain, in Figure 6, a pair of npn driver transistors 61 and 62 are driven through their base terminals via lines 72 with a differential signal. The collectors of these transistors are coupled to the conductor pair through diodes 63 and 64. These conductors (at the end of the cable) are connected to a positive potential V2 through resistors 75, 76, and 77 which act as loads for the transistors. The emitters of the transistors 61 and 62 are again coupled to a constant current source. This constant current source is provided through transistor 69 which has its base biased at a constant potential through resis-tors 66 and 67. The circuit also provides pre-emphasis through the resistor 73 and inductors 74 as described in conjunction with Figure 5.
When the driver circuit of Figure 6 is not in operation, the transistor 70 conducts since a transmit signal is applied to its base. When this occurs, the constant current to the driver transistors 61 and 62 is removed. This draws the cathodes of diodes 63 and 64 to a positive potential Vl through resistor 68 (and resistor 73 in the case of diode 63) reverse biasing the diodes. Note Vl must be larger than V2 to assure reverse biasing of the diodes. Then the conductor pair is isolated from transistors 61 and 62 by the capacitance associated with these reverse biased diodes. Thus, the driver circuit is substantially decoupled from -the cable when not trans-mitting. Note that during transmission, neither diode 63 nor 64 are reverse biased so that the switching -time of these diodes are not critical.
Thus, a bus structure has been disclosed which permits the use of an ordinary, inexpensive, ribbon cable in a high speed application. A bandwidth of 100 MHz has been obtained wi-th approximately ~0 feet of cable and up to 16 receivers coupled to each active conductor pair in the cable.
Claims (11)
1. A high speed bus for interconnecting and asynchronously transmitting data between a plurality (n) of electrical units, comprising:
a ribbon cable having a plurality of conductor pairs;
a plurality of driver circuits for transmitting signals serially, each of said driver circuits being coupled to one of said electrical units and coupled to a different conductor pair of said cable;
a plurality of receivers coupled to each of said electrical units for receiving said signals, each of said electrical units having at least n-1 receivers, each receiver being coupled to a different conductor pair of said cable;
at least one resistor coupled in series between each of said receivers and said conductor pairs for reducing the capacitive coupling between said cable and said receivers;
a plurality of low capacitance connector means, one for each of said units for providing said coupling to said conductor pairs for said receivers, each of said connector means comprising:
at least a first and a second electrical connector for engaging said cable, each electrical connector having a plurality of spaced apart pins for receiving signals from said cable, said pins of said first connector arranged to receive signals from a first conductor pair, said pins of said second connector arranged to receive signals from a second conductor pair separated from said first conductor pair;
whereby the capacitive coupling of said receivers to said cable is reduced, and said electrical units may simulta-neously and asynchronously transmit and receive data-on said cable at high speed.
a ribbon cable having a plurality of conductor pairs;
a plurality of driver circuits for transmitting signals serially, each of said driver circuits being coupled to one of said electrical units and coupled to a different conductor pair of said cable;
a plurality of receivers coupled to each of said electrical units for receiving said signals, each of said electrical units having at least n-1 receivers, each receiver being coupled to a different conductor pair of said cable;
at least one resistor coupled in series between each of said receivers and said conductor pairs for reducing the capacitive coupling between said cable and said receivers;
a plurality of low capacitance connector means, one for each of said units for providing said coupling to said conductor pairs for said receivers, each of said connector means comprising:
at least a first and a second electrical connector for engaging said cable, each electrical connector having a plurality of spaced apart pins for receiving signals from said cable, said pins of said first connector arranged to receive signals from a first conductor pair, said pins of said second connector arranged to receive signals from a second conductor pair separated from said first conductor pair;
whereby the capacitive coupling of said receivers to said cable is reduced, and said electrical units may simulta-neously and asynchronously transmit and receive data-on said cable at high speed.
2. The bus as defined by claim 1 wherein data is transmitted between 30 and 100 Mhz.
3. The bus as defined by claim 1 wherein each of said driver circuits comprises:
a transistor;
a diode coupled between said transistor and said conductor;
a load for said transistor coupled to said conductor;
current means coupled to said transistor for selectively providing current for said transistor;
biasing means coupled to said diode, for reverse biasing said diode when said current means is not providing said current for said transistor;
whereby said transistor is decoupled from said conductor by said diode when said current means is not enabled.
a transistor;
a diode coupled between said transistor and said conductor;
a load for said transistor coupled to said conductor;
current means coupled to said transistor for selectively providing current for said transistor;
biasing means coupled to said diode, for reverse biasing said diode when said current means is not providing said current for said transistor;
whereby said transistor is decoupled from said conductor by said diode when said current means is not enabled.
4. The circuit defined by claim 1 wherein said diode is coupled between the collector of said transistor and said conductor.
5. The bus structure defined by claim 3 wherein each of said connectors includes a male part and a female part, one of said parts being coupled to said cable, the other of said parts being coupled to a board.
6. The bus structure defined by claim 5 wherein each of said pairs of conductors in said cables is separated by another of said conductors which is coupled to a constant potential.
7. The bus structure de-fined by claim 6 wherein said constant potential is ground potential.
8. The bus structure defined by claim 7 wherein each of said drivers includes a pre-emphasis means for pre-emphasizing predetermined frequencies.
9. The bus structure defined by claim 8 wherein each of said drivers, drives said pairs of conductors in a push-pull mode.
10. The circuit defined by claim 3 wherein said current means is a constant current source coupled to the emitter of said transistor.
11. The bus structure defined by claim 10 wherein a pair of said driver circuits are coupled to a pair of said conductors and wherein said transistors are driven with a differential signal.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US13730980A | 1980-04-04 | 1980-04-04 | |
| US137,309 | 1980-04-04 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1167121A true CA1167121A (en) | 1984-05-08 |
Family
ID=22476787
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000374567A Expired CA1167121A (en) | 1980-04-04 | 1981-04-03 | High speed ribbon cable bus |
Country Status (3)
| Country | Link |
|---|---|
| CA (1) | CA1167121A (en) |
| DE (1) | DE3112863A1 (en) |
| GB (1) | GB2074824B (en) |
Families Citing this family (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5716220A (en) * | 1994-12-22 | 1998-02-10 | Lucent Technologies Inc. | Backplane arrangement for electrical equipment |
| DE19736704C1 (en) * | 1997-08-20 | 1999-02-18 | Krone Ag | Method of designing a telecommunications and data network |
| DE19833928B4 (en) * | 1998-07-28 | 2005-05-25 | Infineon Technologies Ag | Biometric sensor device with electronic components integrated in a flex conductor strip |
| DE19913932C1 (en) * | 1999-03-26 | 2000-07-13 | Stn Atlas Elektronik Gmbh | Signal processing unit has additional board with integral special wiring, plug strips for insertion into bus board plug positions, flexible sections at least between plug strips |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE1215776B (en) * | 1963-12-05 | 1966-05-05 | Telefunken Patent | Ribbon cable with shielding wires for multiple wiring of components of a communication system that are arranged one above the other and carry voice wires, in particular for use in switching networks of telephone exchanges |
| FR1589721A (en) * | 1968-09-05 | 1970-04-06 | ||
| GB1262691A (en) * | 1968-11-12 | 1972-02-02 | Burroughs Corp | Data processing system having current drive for transmission line |
| US3858010A (en) * | 1973-04-06 | 1974-12-31 | Gte Automatic Electric Lab Inc | Intermediate communication link for use in electronic systems |
| JPS52137222A (en) * | 1976-05-12 | 1977-11-16 | Hitachi Ltd | Interface system |
-
1981
- 1981-03-17 GB GB8108232A patent/GB2074824B/en not_active Expired
- 1981-03-31 DE DE19813112863 patent/DE3112863A1/en not_active Withdrawn
- 1981-04-03 CA CA000374567A patent/CA1167121A/en not_active Expired
Also Published As
| Publication number | Publication date |
|---|---|
| GB2074824A (en) | 1981-11-04 |
| DE3112863A1 (en) | 1982-01-07 |
| GB2074824B (en) | 1984-02-08 |
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| MKEX | Expiry |