US2902601A - Latch circuit - Google Patents

Description

SePtl, 1959 F. v. ADAMS LATCH CIRCUIT Filed oet. 18, 1956 INVENTOR.

FRANCIS v. ADAMS ATTORNEY SQmd United Stats LATCH CERCUT Application October *13, '1956, Serial No. 616,765

2 Claims. (Cl. Z50- 27) The present invention relates to an improved bistable device capable of `being set to a first or second stable state.

More particularly, the present invention relates to that class of bistable devices in which a number of tubes are cascaded and a feedback circuit provided so as to form a closed circuit. Each tube is designed to assume a stable condition of maximum or minimum conductivity which may be referred to as an On and Off condition, respectively. The condition of each tube is determined by the condition of the preceding tube so that the condition of one tube will determine the conditions of all tubes. Once set in a given condition, each tube will remain in that condition until some external stimulus is applied. An example of this type circuit is shown in the patent to E. S, Hughes, Jr., Patent No. 2,628,309.

In prior devices, such as shown in Hughes, the circuit comprises three tubes, two inverters and a cathode follower. The plate of the first inverter is connected to the grid of the second inverter, the plate of the second inverter is connected to the grid of the cathode follower and the cathode of the cathode follower is connected to an output circuit in addition to providing a connection back tothe grid of the first inverter. In circuits of this type, where the condition of one tube will determine the conditions of all tubes because of the closed circuit arrangement, it is yapparent that the speed of operation will depend upon the time it takes for all tubes to reach a stable condition. Where the output of such a device is loadedA with a large amount of capacitance, the time it takes for the cathode follower to discharge this capacitance will limit the speed of response of the device since the first inverter will vdepend for operation upon the cathode voltage of the cathode follower.

If the device is one of a plurality in a ring circuit driven by a source of ring drive pulses, failure to assume an On position before the next drive pulse will cause failure of the ring. This is caused by the fact that succeeding devices are operated by a pulse from a preceding device when the preceding device goes from an On to an `Oii position. If the preceding device does not assume an On position, here will be no pulse to operate the succeeding device. An On position is defined `as a situation in which the cathode follower conducts heavily and 4an Off position is defined as a situation in which the cathode follower is only slightly conducting.

Since ring circuits in computer applications characteristically work over long lines, there is a large amount of capacitance associated with the output of each device in the circuit.

It is therefore an object `of this invention to provide an improved bistable device.

It is a further object of this invention to provide a bistable device relatively insensitive to load variations.

It is lanother object of this invention to provide a bistable device which operates at a higher frequency than those previously known.

Other objects of the invention will be pointed out in arent O "ice the following description and claims and illustrated in the accompanying drawing, which discloses, by way of an example, the principle of the invention and the best mode, which has been contemplated, of applying that principle.

In the drawing:

The circuit shown in the figure consists of thee tubes V1, V2 and V3. V1 `and V2 function as inverters and V3 functions as a cathode follower. The normal condition of this circuit, which shall be referred to as an Off condition, is where tube V1 is nonconducting, tube V2 is conducting and tube V3 is only slightly conducting. The output taken from the cathode of V3 at this time will be at some negative potential level. The On condition of this circuit is where V1 is conducting, V2 is nonconducting and V3 is conducting heavily. The output at this time will be 'at some positive potential level.

The circuit is shown as it would be connected into a ring circuit with an output to the next stage taken from tubes V2 and V3. This output would be connected to a terminal in a subsequent stage labeled Set as shown in the instant stage.

A Reset for manually switching from an On to an Off condition is shown connected to the input of tube V1. An advance pulse input is shown which would be connected to a ring drive for switching the stage from On to Off where the circuit is used as a stage of a ring.

With tube V1 nonconducting, the plate of said tube assumes the potential at point i8. The plate of tube V1 is connected to the grid of tube V2. The positive potential at the plate of V1 is suiicient to overcome the negative bias applied to V2 and V2 conducts.

The positive voltage at 9 is connected through va large resistor itl, through diode 13 to the cathode of V3, through resistor 4l to the plate of V2. With V2 con ducting, current will ilow from 9 through V2. With current flow through resistor 4l, the grid of tube V3 will be at a more negative potential than the cathode of V3 and V3 will therefore be biased near cutoff. Resistor 10 is very large compared to the resistor 4l and the resistor 35 in the cathode circuit of V2 so that substantially the entire voltage drop between voltage source 9 and the cathode voltage V2 appears across resistor 10. This voltage drop brings the point 8 below ground potential and keeps V1 cut oif so that the circuit will be stable in the OI position. With V2 conducting and V3 only slightly conducting, the output will be at a negative potential. The diode d4 prevents the line from going too far negative.

To operate the circuit to an On condition, a negative pulse is applied at terminal 2o labeled Setf This pulse is connected to the grid of tube V2 through a voltage dividing network composed of resistors 2l, 23 and capacitors 2li, 22. This negative pulse drives the grid of tube V2 below cutoff and the tube V2 is operated to its nonconducting condition. With tube V2 nonconducting, the potential at point 3S will be the same as that for the cathode of tube V3 (no current iiow through resistor 41) and tube V3 will conduct heavily. With tube V3 conducting, the cathode of V3 will rise to a potential high enough to prevent too large a voltage drop across resistor 10 and the tube V1 will be biased to conduction. With tube V1 conducting, the plate of tube V1 is approximately at ground potential. V2 was cut olf by the pulse on terminal 2o and, since V2 is normally biased to cutoff by source 32, the presence of ground potential on the plate of tube V1 is not sufcient to make tube V2 conduct. The circuit is now in a stable On condition and will remain in this condition until it is reset to an Off condition by a negative pulse on the reset or advance pulse line.

v resistance.

AThe output of this circuit is taken from a parallel combination of the outputs of tubes V2 and V3. From the figure it can be seen that the plate of tube V2 is connected through a -resistor 4l to the output terminal and the cathode of tube V3 is connected directly to the output terminal. Any output pulse therefore will be a direct function of the outputs of tubes V2 and V3.

At this point it may be well to digress for a moment and consider the behavior of a cathode follower when driving a load. When a cathode follower conducts, current will iiow from the plate to the cathode and then to the load. From this it can be seen that a cathode follower will deliver a large amount of power (current) at the positive excursion of an output pulse. When the cathode follower is biased near cutoff, current ow is from the line into the cathode circuit of the cathode follower. When the output contains considerable capacitance, as it does where these devices are used, the capacitance must discharge through the cathode resistor and the load resistance in parallel (shown in broken lines in the figure). in an R-C circuit, lthe voltage decays exponentially in accordance with the circuit constants. One wayto help this condition is to greatly lower the value of the cathode resistor but this necessitates either a larger tube with a higher plate dissipation or tubes in parallel.

Since the cathode volta-ge will decay exponentially in accordance with the capacity of the line and the cathode resistor, it will take a given amount of time before the voltage reaches'some predetermined level. If the instant device is used in a ring circuit driven at some predetermined frequency, it can be seen that a failure of one stage to change condition before the neXt advance pulse arrives will cause the ring to fail.

A plate driver, when connected to an output, functions when conducting to accept current from the output line. When nonconducting, the plate driver is eifectively an open circuit and current to the line would be from the plate supply. Therefore, for a negative excursion of an output pulse, i.e., plate driver conducting, any capacitance of theline would be discharged through the plate driver, while for a positive excursion, i.e., plate driver nonconducting, current would be from the plate supply and the plate driver would have the same disadvantages as the cathode follower' would for a negative excursion.

Y To make the device of the figure more independent of load variations with regard to changes in condition of the device, a plate driver and a cathode follower have been combined to furnish an output pulse which will tum the device On or Gif in a faster time than previously known.

As previously stated, a plate driver such as V2 will have optimum operation when generating the negative excursion of an output pulse taken across the plate since the circuit is through the tube V2 to the cathode circuit where the resistor 35 has a small. resistance. A cathode follower such as V3 will have optimum operation when gen# erating the positive excursion of an output pulse taken across the cathode since the circuit is through the tube V2 from the plate supply where the resistor 40 has a small Equally important is the fact that a cathode follower will present a low irnpedence to the line between pulses and minimize spurious Crosstalk Therefore, in using a plate drive such as V2 and a cathode follower such as V3 in combination, an optimum output waveform is achieved.

Y`From the foregoing analysis it can be seen that the tubes of the output circuit determine the sharpness of the output pulse. Since the time which it takes for the entire circuit to assume a stable state is dependent upon the time it takes for the output pulse to assume a particular voltage level, it can be seen that the frequency at which the circuit will respond is directly dependent upon the output waveform. With the circuit as shown,

the improvement in output waveform has materially increased the response time.

To further improve the response time of the present circuit, a capacitor 14 has been inserted in the feedback circuit. The capacitor allows the grid of tube V1 to follow the feedback voltage from the output more quickly than by merely using the diode.

To prevent the negativeegoing pulse at terminal Z6 from driving the grid of V2 below cutoff and consequently increasing the time that it takes for the grid to approach the normal voltage for cutoi, a connection was made kto the cathode of V2 to the point Z5 of the voltage dividing network. A diode 29 isolates the network from the cathode potential at times when there is no'input pulse at 26. For a large negative pulse, the diode conducts and keeps the point 25 at a predetermined level below the cathode voltage dependent upon the resistor 35.

The diodes 33 and 44 are used to prevent the output waveform from going too high or too low, vi.e., for clipping. 2

The diodes 11 and 12 form an Or circuit wherein terminals 4 and 5 are maintained at a potential high enough to prevent current flow from source 9 through diodes 11 and 12. When it is desired to reset or advance the circuit from an On to an Off condition, a negative pulse is supplied. The resistor 10 will then permit the voltage at the grid of tube V1 to drop below cutoiitl and the circuit will be turned Cif.

While there have been shown and described and pointed out the fundamental novel features of the invention as applied to a preferred embodiment, it will be understood that various omissions and substitutions and changes in l the form and details of the device illustrated and in its operation may be made by those skilled in the art, withf out departing from the spirit of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the following claims.

What is claimed is:

1. A bistable device comprising a first, second and third electron tube, each tube containing -at least a plate, grid and cathode, a iirst and second series connected parallel resistance capacitance section connecting the plateof said tirst tube to the grid of said second tube,

' means connecting the cathode of said second tube to a junction between said first and second series connected sections whereby the voltage at said junction will be a function of the voltage of said cathode, means connecting the plate of the second tube to the grid of said third tube, means connecting the cathode of said third tube to the ,grid of said first tube whereby the conduction of any onetube will determine the conduction of all tubes to set -saiddevice in either a rst or second stable state,

an input connected to the junction between said Erst References Cited in the le of this patent UNITED STATES PATENTS 2,307,308 Sorensen Jan. 5, 1943 2,434,916 Everett Ian. 27, 1948 2,547,213 Johnson et al Apr. 3, 1951 2,586,409 White Feb. 19, 1952 2,628,309 Hughes Feb. 10, 1953 y2,719,227 Gordon Sept. 27, 1955 2,790,076 Mason Apr. 23, 1957

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