Abstract:
A driver circuit for a digital data transmission system is provided with a transistor current limiting network, the network being bypassed by a capacitor. The network is selectively supplied with a disabling signal via another transistor to disconnect the driver circuit from its power supply in the event of a short circuit condition.

Description:
The invention herein described was made in the course of or under a contract or subcontract thereunder, with the Department of Defense. 
    
    
     CROSS-REFERENCE TO RELATED CASES 
     This is a continuation of application Ser. No. 267,748, filed June 30, 1972, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates generally to digital data transmission systems and more particularly to such systems wherein a transmission line is used to interconnect a plurality of modular subsystems, each one thereof having an associated input/output device arranged in what is commonly called a &#34;party-line&#34; organized system. 
     As is known in the art, digital data transmission systems sometimes include a plurality of modular subsystems, interconnected by a transmission line, which transfer binary data from one such subsystem to a selected number of other ones therof. 
     Each one of such modular subsystems generally includes an input/output device incorporating a transmit section and a receive section. The transmit section is connected to the transmission line through a driver circuit which generally includes an output transistor interposed between such transmission line and a voltage source. Such a transistor is responsive to binary control signals for coupling or decoupling the voltage source and transmission line in accordance with the binary control signals. The driver circuit is connected to the transmission line in what is commonly called a &#34;wired-OR&#34; arrangement, that is, a binary &#34;1&#34; (or &#34;high&#34; voltage) produced at the output of any one of the driver circuits will apply a binary &#34;1&#34; signal on the transmission line. Thus, if a short circuit occurs to the output transistor in any one of the driver circuits an erroneous signal (i.e., a &#34;1&#34;) is applied to the transmission line. For proper operation of the digital data transmission system as a whole, it is necessary to remove any faulty driver circuit from the transmission line. 
     Because of the amount of current flow in each driver circuit conventional logic gates cannot be interposed between the output transistor and the transmission line to disable any faulty driver circuit. It has been necessary, therefore, sequentially to manually disconnect each one of the modular subsystems to identify any faulty driver circuit. Such procedure is obviously costly and time consuming on the system. 
     The driver circuit used in the input/output device of the type described above generally includes protective means operative if a short circuit occurs in the transmission line because such short circuit ordinarily results in excessive, and damaging, current flow through the output transistor of such driver circuit. One known protective means limits the amount of current flow through the output transistor of a driver circuit. Such means include a transistor network disposed between the voltage supply and such output transistor. The use of such a transistor network however, reduces the overall switching response time of the driver circuit, with the concomitant disadvantage of reducing the speed of the digital data transmission system. 
     An additional problem with digital data transmission systems of the type described above is that of &#34;noise immunity.&#34; That is, because the receive sections of the input/output devices are generally connected to the transmission line through relatively low impedance logic gates, such receive sections draw current, even when not selected, thereby causing a relatively large voltage to be developed across the transmission line when a binary &#34;0&#34; (or low voltage) is transmitted thereon. Such relatively large voltage, together with any noise which may be coupled into such transmission line, thereby produces a signal on such transmission line which may appear as a binary &#34;1&#34; instead of a binary &#34;0&#34; to the receive section of a selected input/output device. Therefore, noise immunity protection is generally required to eliminate ambiguity in the binary signal developed across the transmission line. Known &#34;noise immunity&#34; protection techniques generally include the use of a network between the transmission line and each one of the receive sections. Such a network requires a voltage source of opposite polarity from that which is used with the driver circuit. The network and additional voltage source add to the cost of the digital data transmission system. 
     SUMMARY OF THE INVENTION 
     With this background of the invention in mind it is therefore an objective of this invention to provide an improved digital data transmission system wherein a transmission line is used to interconnect a plurality of modular subsystems having associated input/output devices. 
     It is a further object of the invention to provide an improved driver circuit wherein a transistor network is interposed between an output transistor and a voltage source, such driver circuit being faster than any such driver circuit known heretofore. 
     It is a further object of the invention to provide a digital data transmission system adapted to remove a faulty driver circuit from such system in a faster and less costly manner than has been known heretofore. 
     It is a further object of the invention to provide an improved noise immunity mechanism for use with a digital data transmission system. 
     These and other objects of the invention are attained generally by providing in a digital data transmission system wherein a plurality of input/output devices are interconnected through a transmission line, a driver circuit for each one of the input/output devices, such circuit having a first electrical circuit including a voltage source, a drive transistor and a capacitor and a second electrical circuit including the transmission line, an output transistor and the capacitor, such output transistor coupling the voltage source to the transmission line in response to binary signals. The capacitor is common to both the first and the second electrical circuits and arranged such that the capacitor charges when the voltage supply is decoupled from the transmission line and discharges when the voltage supply is coupled to the transmission line. The capacitor stores a voltage for initiating the turning &#34;on&#34; (or &#34;coupling&#34;) response of the output transistor to the binary signal and also acts as a sink for minority carriers in turning &#34;off&#34; (or &#34;decoupling&#34;) such output transistor. 
     Means, including an electronic switch, disposed between the base electrode of the driver transistor and the voltage source, is described for automatically disconnecting each one of the plurality of input/output devices from the transmission line at mutually exclusive times and for indicating which one, or ones, of such devices may have a short circuited output transistor. 
     In one embodiment a noise immunity technique is provided wherein a transistor, biased in its inverted mode of operation, is disposed to shunt a matching impedance connected across the transmission line. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing features of this invention, as well as the invention itself, may be more fully understood from the following detailed description read together with the accompanying drawings, in which: 
     FIG. 1 is a diagram of a digital data transmission system employing the features of the invention; 
     FIG. 2 is a schematic diagram of a driver circuit for use in the digital data transmission system shown in FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, binary data is transferred between a plurality of modular subsystems (not shown but here contained within a processor 10) when a controller 12 applies binary control signals to selected ones of n identical input/output devices 14 1  - 14n. The input/output devices 14 1  - 14n are interconnected through transmission line 16. The operation of each one of the input/output devices 14- 14n is determined by the controller 12 selectively actuating a receiver enable line R 1  - R n , a transmit enable line T 1  - T n , or a failure monitoring line FM 1  - FM n . Each one of the input/output devices 14 1  - 14 n  is connected to each modular subsystem of processor 10 by a different transmit data line TD 1  - TD n , respectively, and a different receive data line RD 1  - RD n , respectively, as shown. Transmission line 16 is terminated at each end by a matched (or characteristic) impedance, here by resistors R 1 , R 2 . A transistor 18 1  has its collector electrode and emitter electrode connected to a different terminal of resistor R 1 . A similar arrangement is shown for resistor R 2  and transistor 18 2 . For reasons to become apparent, each one of the transistors 18 1 , 18 2  is biased in an inverted mode of operation. When a transistor is so biased, current flows into the emitter electrode and out of the collector electrode for an NPN transistor (and vice versa for a PNP transistor). The base electrode of each one of the transistors 18 1 , 18 2  is coupled to a +V voltage source through a different resistor, not numbered. 
     In operation if, say, digital data in a modular subsystem is to be transferred via input/output device 14 1  to another modular subsystem, via input/output device 14n, a transmit enable signal is applied to transmit enable line T 1  and a receive enable signal is applied to receive enable line R n  by controller 12. Therefore, digital data on transmit data line TD 1  passes through AND gate 20 1 , driver circuit 22 1 , transmission line 16, AND gate 24n and receive data line RD n  to the modular subassembly connected to input/output device 14 n . In this normal mode of operation the signal on each one of lines FM 1  - FM n  is a binary &#34;0&#34;. 
     Referring now to FIG. 2 an exemplary one of the driver circuits 22 1  - 22 n , say 22 1 , is shown to include an output transistor 26 1 . The base electrode of such output transistor 26 1  is in circuit with: the AND gate 20 1  (FIG. 1) (by line 27 1  through a conventional R-C coupling network 29 1 , as shown); and a +V voltage source through a resistor 30 1 . The collector electrode of output transistor 26 1  is connected to transmission line 16 and AND gate 24 1  (FIG. 1) via line 32 1 . The emitter electrode of output transistor 26 1  is connected to: the +V voltage source through a drive transistor 34 1  and a biasing network 36 1  ; and a capacitor 38 1 . The driver circuit 22 1  may therefore be viewed as two electrical circuits intercoupled through capacitor 38 1 . One such electrical circuit includes capacitor 38 1 , output transistor 26 1  and a &#34;load,&#34; such &#34;load&#34; including the transmission line 16 (and its matching resistors R 1 , R 2  and transistors 18 1 , 18 2  and the input/output devices 14 1  - 14 n  coupled to such transmission line 16. The other such electrical circuit includes the capacitor 38 1 , drive transistor 34 1 , biasing network 36 1  and +V voltage source. Biasing network 36 1  includes: a resistor 40 1 , between the emitter electrode of drive transistor 34 1  and the +V voltage source; a transistor 42 1 , the base electrode thereof being connected to the emitter electrode of drive transistor 34 1 , the collector electrode thereof being connected to the base electrode of drive transistor 34 1  and the emitter electrode of transistor 42 1  being connected to the +V voltage source; and a transistor 44 1 , the emitter electrode thereof being connected to ground (and therefore to the +V voltage source ground), the collector electrode thereof being coupled to the base electrode of drive transistor 34 1  through resistor 46 1 , and the base electrode thereof being coupled to controller 12 (FIG. 1) via failure monitor line FM.sub. 1 through conventional R-C coupling network 48 1  and NOR gate 49 1 . 
     In operation, when the +V voltage source is activated (output transistor 26 1  being assumed &#34;off&#34; because a binary &#34;1&#34; signal is applied to line 27 1 ), biasing network 36 1 , being arranged to forward bias drive transistor 34 1 , enables current to flow from such source, through resistor 40 1  and the emitter-base electrodes of drive transistor 34 1  to (1) resistor 46 1 , transistor 44 1  (which is assumed to be &#34;on&#34; because the signal on line FM 1  is a binary &#34;0&#34; and the +V voltage source; and, (2) capacitor 38 1 , such capacitor therefore charging towards a voltage, V-V CE .sbsb.s.sbsb.a.sbsb.t ≈ V. When a binary signal &#34;0&#34; is applied to line 27 1 , output transistor 26 1  turns &#34;on&#34;, capacitor 38 1  discharges through such output transistor 26 1  and such capacitor 38 1  thereby provides the initial current to develop a binary &#34;1&#34; signal on the transmission line 16. The current to such transmission line is sustained by current flowing to such line from the +V voltage source through the collector electrode of drive transistor 34 1 . A little thought will make it apparent therefore that capacitor 38 1  acts as an energy source for output transistor 26 1 , and such transistor&#39;s initial &#34;turning on&#34; is not dependent on the inherent delay in having collector current produced in drive transistor 34 1 . That is, no delay is introduced by drive transistor 34 1 . Further, capacitor 38 1  acts to remove base-emitter storage charge when output transistor 26 1  is turned &#34;off&#34; by the binary signal &#34;1&#34; on line 27 1 . That is, capacitor 38 1  charges when output transistor 26 1  is &#34;off&#34; (such charge being from the +V voltage source and carriers in the base - emitter region of output transistor 26 1 ) and discharges when output transistor 26 1  is &#34;on,&#34; such discharge being through such output transistor to the &#34;load.&#34; 
     Let us now consider the condition where output transistor 26 1  is &#34;on&#34; and current flows, as previously described, from +V voltage source through resistor 40 1  to drive transistor 34 1 . This is the normal operating condition when driver circuit 22 1  applies a binary &#34;1&#34; to transmission line 16 (that is, the +V voltage source is coupled to such transmission line). If, for some reason, a short circuit develops across transmission line 16 an excessive amount of current will tend to flow through output transistor 26 1 . The current flow through such output transistor 26 1  is limited because any &#34;excessive&#34; current flow will cause an increase in the normal voltage developed across resistor 40 1 . Such increase in voltage will provide a forward bias across the base - emitter region of transistor 42 1 , turning such transistor on. Therefore, the base electrode of drive transistor 34 1   has applied thereto a higher positive voltage than the voltage applied to the emitter electrode of such drive transistor, thereby reducing the current flow through the output transistor to the load. The maximum current which may be supplied to the load under this condition is ≈ 0.6V/R 3  where R 3  is the resistance of resistor 40 1 . 
     It should here be noted that if a short circuit develops across the collector - emitter region of output transistor 26 1 , a binary &#34;1&#34; will be applied continuously to transmission line 16 (FIG. 1) independent of the signal on line 27 1 . This is because the input/output devices 14 1  - 14 n  are connected to such transmission line in what is commonly called a &#34;wired - OR&#34; arrangement. Such transistor failure therefore results in an erroneous signal being produced on transmission line 16. When such transistor failure is suspected, or at any other convenient time, a test signal is applied to controller 12 in any convenient manner. Controller 12, in response to such test signal, disables lines T 1  - T n  and R 1  - R n  and produces an enabling signal (i.e. a binary &#34;1&#34;) on line FM 1 , FM 2  . . . FM n  consecutively, at mutually exclusive times. Such enabling signals may be produced by conventional logic circuitry arranged with a shift register. The test signal also causes controller 12 to generate a signal on line R to reset flip-flops 50 1  - 50 n , thereby producing a binary &#34;0&#34; on lines D 1  -D n . The enabling signal on an exemplary failure monitoring line FM 1 , (and referring also to FIG. 2), turns &#34;off&#34; transistor 44 1 , thereby decoupling the +V voltage source from line 32 1  in the event that output transistor 26 1  has a short circuit across its emitter - collector region. That is, such enabling signal on exemplary failure monitoring line FM 1  will electrically decouple the driver circuit 22 1  from the transmission line 16. A little thought will make it apparent that each one of the other driver circuits 22 2  - 22 n  will successively be decoupled from such line 16. Indicator 52 (FIG. 1) is connected to transmission line 16 and lines FM 1  - FM n . Such indicator provides an indication of which input/output device 14 1  - 14 n  is short circuited. Thus, indicator 52 here includes a lamp means, 54 1  - 54 n , each one thereof connected to a different AND gate 56 1  - 56 n  respectively. AND gates 56 1  - 56 n  are coupled to transmission line 16 through an inverter, 57 1  - 57 n  and to failure monitoring lines FM 1  - FM n  respectively. Therefore, in this test mode of operation when the enabling signal on an exemplary line, say FM 1 , is applied to the AND gate 56 1  and if such enabling signal removes the faulty binary &#34;1&#34; from transmission line 16, (and therefore a binary &#34; 0&#34; now appears on such line) the lamp means 54 1  connected to such AND gate 56 1  is activated, thereby indicating that input/output device 14 1  is faulty. (If such enabling signal does not remove the faulty binary &#34;1&#34; from the line, the lamp means is not activated.) Further, the flip-flop 50 1  which is connected to such AND gate 56 1  changes state when the lamp means becomes activated, such flip-flop producing a binary &#34;1&#34; signal on the disabling line D 1  thereby disconnecting the faulty input/output device 14 1  from the system. Such disconnection results because a binary &#34;1&#34; signal on, say line D 1 , assuming input/output device 14 1  is faulty, is fed to NOR gate 49 1  (FIG. 2) thereby turning &#34;off&#34; transistor 44 1 . When the test signal is removed from controller 12, the faulty input/output device will remain decoupled from the transmission line because of the binary &#34;1&#34; signal on line D 1 . 
     Let us now consider the terminating arrangement at each end of transmission line 16 as shown in FIG. 1. As previously mentioned, at each end a matching resistor R 1 , R 2  is shunted by a transistor 18 1 , 18 2 , each such transistor being biased in the inverted mode of operation. This terminating arrangement is to provide noise immunity for the system. When a transistor is biased in this mode of operation, a very low (typically 40 mv) emitter to collector saturation voltage is developed. Further, the inverted beta of a transistor so biased (that is, the ratio of emitter current to base current) is generally less than the normal beta (that is the ratio of collector current to base current) by a factor of 10 or more. Let us first consider the case when a binary &#34;0&#34; is applied to transmission line 16 by one of the input/output devices 14 1  - 14 n . Current will flow from the voltage source (not shown) connected to AND gates 24 1  - 24 n  through the matching resistors R 1 , R 2  to ground and also through the transistors 18.sub. 1, 18 2  to ground. Using conventional AND gates 24 1  - 24 n , a conventional voltage source of +5 volts, and a matching resistor of 50 ohms for resistor R 1  and resistor R 2 , the amount of current flow to ground will be about 16 ma. The major portion of such current will flow through the transistors 18 1 , 18 2  and not through the resistors R 1 , R 2  and the voltage appearing on transmission line 16 will be limited to about 40 mv, thereby insuring good noise immunity protection for the system. When a binary &#34;1&#34; is applied to the transmission line 16 by one of the input/output devices 14 1  - 14 n , the activated driver circuit supplies typically 192 ma to such line. Because of the low inverted beta of each transistor 18 1 , 18 2 , however, no more than 16 ma will flow through each one of such transistors. Each 50 ohm matching resistor R 1 , R 2  will pass 80 ma of current, thereby developing a voltage of +4 volts across the transmission line 16. Such voltage is sufficiently large to represent a binary &#34;1&#34; condition. It has been found that a satisfactory ratio of binary &#34;1&#34; current to binary &#34;0&#34; current is one which is greater than 3. 
     While the salient features have been illustrated and described with respect to the embodiment above, it should now be readily apparent to those of ordinary skill in the art that modifications can be made; for example, when a unidirectional line is used only one matching impedance to such line is needed. Further, only one transistor, say transistor 18 1 , is needed in the bidirectional line described in FIG. 1 even though such line requires two different resistors 18 1 , 18 2 . Further, such transistor 18 1  may be included in one of the input/output devices. 
     It is felt, therefore, that this invention should not be restricted to the proposed embodiments, but rather should be limited only by the spirit and scope of the following claims.