US3569730A

US3569730A - Logic circuitry for railroad crossing systems

Info

Publication number: US3569730A
Application number: US844229*A
Authority: US
Inventors: Authur W Wetmore
Original assignee: General Signal Corp
Current assignee: SPX Technologies Inc
Priority date: 1967-10-23
Filing date: 1969-05-21
Publication date: 1971-03-09
Anticipated expiration: 1988-03-09

Abstract

Logic circuitry utilizing transistorized switching circuits arranged to provide a check on their own proper operation. Each switching circuit monitors a single track section and provides three electrical outputs indicating respectively, train absence, train presence, and the proper functioning of the device itself. These outputs are fed into other transistorized circuit elements consisting of stick circuits, AND circuits and OR circuits logically arranged so as to activate a motorist warning system when a train is approaching a crossing from either direction, when a train is traversing the crossing, or when a circuit failure occurs and to deactivate the warning system as soon as a train has cleared the crossing.

Description

United States Patent [72] Inventor Arthur W. Wetmore [56] References Cited Ormond Beach, Fla. UNITED STATES PATENTS I211 PP 844,229 3,229,117 1 /1966 Chilton 307/216 [22] F1Ied May 21,1969

Division Of Ser. N6. 677,139, Oct. 23, 1967, Primary ExammerD nald Form Patent No. 3,471,687. Assistant ExaminerB.P. Davis [45] Patented Mar. 9, 1971 Attorney-Harold S. Wynn [73] Assignee General Signal Corporation Rochester N ABSTRACT: Logic circuitry utilizing transistorized switching circuits arranged to provide a check on their own proper operation. Each switching circuit monitors at single track section and provides three electrical outputs indicating respec- [54] LOGIC CIRCUITRY FOR RAILROAD CROSSING tively, train absence, train presence, and the proper function- SYSVQEMS ing of the device itself. These outputs are fed into other 8 Clams 5 Drawmg transistorized circuit elements consisting of stick circuits, 52 0.5. Ci 307/216, N ci and O circuits logically arranged so as to 328/ 159, 307/218 activate a motorist warning system when a train is approach- [51] Int. Cl H03k 19/20 ing a crossing from either direction, when a train is traversing [50] Field of Search 307/216, the crossing, or when a circuit failure occurs and to deactivate 2 I 8; 328/93, 159; 340/47 the warning system as soon as a train has cleared the crossing.

W7 H i 1 TC e CTR J BTR ATR gig (o 1+) +lo-q-irl CHECK OUTPUT CIRCUIT 0R L SWITCHING I CIRCUIT I CIRCUIT 2 D 1- STICK STICK 20-) CIRCUIT CIRCUIT 2 AB 2 ABC X R SWITCHING 1 CIRCUIT 2 AND t T STICK I I STICK CIRCUIT CIRCUIT 2 CB 2 CBA SWITCHING 1 OR T mmun CIRECUIT 3 PATENTEU MA 9 SHEET 2 BF 5 NOE INVENTOR ARTHUR W. WETMORE HIS ATTORNEY PATENIED MAR 9 1911 SHEET 3 [IF 5 II;IVENTOR ARTHUR w. WETMORE HIS ATTORNEY LOGIC CIRCUITRY FOR RAILROAD CROSSING SYSTEMS This is a division of application Ser. No. 677,139, filed Oct. 23,1967 now Pat. No. 3,471,687.

BACKGROUND OF THE INVENTION I. Field of Use This invention relates to highway crossing systems for railroads, and more particularly relates to solid state logic circuitry for use in such systems with the organization arranged to check the operability of the system.

2. Description of Prior Art The purpose of the logic circuitry found in a typical railroad crossing protection system is to automatically cause the warning system for motorists to be activated during the time when a train is approaching a crossing from either direction, to remain activated while the train is traversing the crossing and to be deactivated as soon as possible thereafter, even though the train may still be influencing the circuitry as it is leaving the crossing area. A further requirement imposed upon railroad crossing protection system circuitry is that it be fail-safe. In other words, if a malfunction occurs, it should result in the activation of the motorist warning system. At present these logic functions are accomplished largely through the use of relays. Although the relays employed for this purpose are highly reliable, it is nonetheless desirable to replace them, if possible, with solid state devices. Since solid state devices do not have moving parts, increased reliability is attainable through their use. Furthermore, by the use of solid state devices the cost o'f installing, maintaining and operating the logic circuitry for crossing protection systems may be lowered.

One reason why transistors have not been used heretofore in the logic circuitry of crossing protection systems is the difficulty of achieving fail-safe operation with their use. In present day crossing protection systems train presence in a track section in approach of a crossing is detected and used to initiate an electrical signal which, in turn, energizes a directional stick relay. This electrical signal is easily obtained by passing current through the back contact of a track relay. This is considered to be safe because the materials and design used in such track relays make it extremely improbable that the back contact will fuse and provide a false electrical signal. Those versed in the art will realize that if such a malfunction does take place when a train passes in a given direction, a second train subsequently approaching the crossing in the opposite direction will not be protected. Thus, great care must be taken to insure that a stick circuit, once established, will be deenergized after a train has passed. A transistor can be used to supply an electrical signal analogous to that obtained from the back contact of the track relay. But the degree of certainty that a transistor will cease to provide such a signal on command is not sufficient for use in railroad work. As is well known, a sufiiciently high spurious voltage can cause a transistor to fuse, thereby rendering it incapable of being turned off. This invention overcomes this-problem through the use of novel circuit arrangements which cause the warning system to be activated in the event of a circuit failure, including the type mentioned.

It is accordingly an object of this invention to provide logic circuitry for railroad crossing protection systems which takes advantage of the characteristics of transistors.

It is a further object of this invention to provide transistorized logic circuitry for railroad crossing protection systems which will be fail-safe in operation.

SUMMARY OF INVENTION The invention contemplates the use of switching means to monitor the occupancy conditions of track sections on and about a crossing. Each switching means provides an indication of train absence, of train presence and of its own proper functioning. Check output means, responsive to the switching means are provided to activate the motorist warning system when a train is present in a track section in approach of or on the crossing or when any switching means is not operating properly. Stick circuit means, also responsive to the switching means, are provided todeactivate the motorist warning system while a train which has traversed the crossing continues to travel in a monitored track section.

BRIEF DESCRIPTION OF THE DRAWINGS A fuller understanding of the present invention will be had by reference to the following description taken in connection with the accompanying drawings briefly described as follows:

FIG. I is a schematic drawing of a portion of the invention showing the logic functions performed by the invention.

FIG. 2 is a general wiring diagram of the invention.

FIG. 3 is a schematic drawing typical of the switching circuits shown in FIGS. 1 and 2.

FIG. 4 is a schematic drawing typical of the stick circuits shown in FIGS. 1 and 2. 7

FIG. 5 is a schematic diagram of the check output circuits shown in FIGS. 1 and 2.

Throughout the drawings conventional symbols have been used for brevity. For example, a center tapped DC supply is employed in the invention and the symbol is used to indicate its positive terminal; the symbol is used to indicate its negative terminal; and the symbol normally employed to designate ground is used to indicate the center or common terminal of the supply.

A general understanding of the overall operation of the invention and of the logic functions it performs will be obtained from a studyof FIG. 1. A detailed explanation of the invention will be given in connection with the other FIGS. At the top of FIG. 1 there is shown a stretch of railway track intersected by a highway II. This stretch of track is divided into three sections, two approach sections TA and TC separated by an intermediate section TB. Each section comprises a track circuit and the associated relays, normally energized, are designated ATR, BTR and CTR. Connected to the front contact of each track relay there is shown a separate switching circuit means designated A, B or C, each having three outputs. Output 1 of each switching circuit is connected to the check output circuits shown in the FIG. For example, output 1 of circuit A and output 1 of circuit C are connected to the OR circuits D and E respectively. Output 1 of switching circuit B is connected directly as an input to the AND circuit. The output of the AND circuit is connected to the winding of relay XR, shown as being normally energized. The back contact of relay XR is connected to a motorist warning system. The stick circuits shown have inputs designated 1 and 2. Stick circuits AB and CB receive their inputs, as shown, from certain ones of the outputs marked 2 of the various switching circuits. Stick circuits ABC and CBA receive their inputs from outputs 2 of switching circuits C and A respectively, and from the stick circuits AB and CB. The output of stick circuits ABC and CBA are connected to the check output circuit at OR circuits E and D respectively. Output 3 of each switching circuit forms a direct input to the AND circuit.

Before discussing the overall operation of the crossing system, the mode of operation of the individual circuit components shown in FIG. 1 will be explained. The switching circuits A, B and C are identical in structure and operation. When an input is supplied from its associated relay, each switching circuit will provide its output 1 while not providing its output 2. When the input is removed, output 1 will cease and output 2 will appear. Output 3 is always given unless a malfunction develops in the switching circuit. Thus, output 1 may be thought of as a train absence indication; output 2 may be thought of as a train presence indication, and output 3 may be thought of as a check upon the switching circuit.

The stick circuits AB, CB, ABC and CBA are likewise identical with one another in structure and operation. If inputs I and 2 are simultaneously applied to a given stick circuit, it will be energized and will provide an output. Once energized, the output of a stick circuit will continue as long as input 2 is maintained. Any time input 2 is removed, the stick circuit will be deenergized.

The circuit components collectively designated Check Output Circuits have been illustrated with conventional logic symbols. When one or both inputs are supplied to an OR circuit, it will provide an output; when and only when all inputs are simultaneously supplied to the AND circuit, it will provide an output.

When no train is present, all track relays are energized and all switching circuits are supplied with an input. Switching circuits A AND C each provide output 1 to their respective OR circuit which, in turn, supplies the AND circuit with inputs 1 and 3. Switching circuit B provides AND circuit input 2. Output 3 of the respective switching circuits form AND circuit inputs 4, and 6. All inputs being present, the AND circuit provides an output which maintains relay XR energized and the crossing signal is not activated. No stick circuit is energized at this time.

When a train, travelling from east to west enters track section TA, relay ATR will release. With its input thus removed, switching circuit A will provide output 2 and output 1 will cease. Input 1 is supplied to stick circuitAB and input 2 is supplied to stick circuit CBA. Input 1 of the AND circuit is removed. This causes its output to cease, dropping relay XR and activating the warning system.

When the train enters track section TB, relay BTR will release, removing the input to switching circuit B. Output 1 of switching circuit B will cease and output 2 will commence. The condition of switching circuit A will remain unchanged, at least momentarily, because the train, even if short, will straddle track sections TA and TB. Therefore, stick circuit AB is supplied with inputs 1 and 2 simultaneously and it provides input 1 to stick circuit ABC. Input 2 is supplied to stick circuit CB. The motorist warning system remains activated because AND circuit input 2 is not available as long as'the train occupies track section TB. 1

When the train enters track section TC, output 2 of switching circuit C will begin and output 1 will cease. During the time when the train occupies track sections TB and TC simultaneously, stick circuit AB remains energized since its input 2 is maintained. Stick circuit ABC is therefore energized because inputs 1 and 2 are supplied to it simultaneously. Stick circuit CB likewise is energized at this time. Stick circuit CBA may be energized if all three track sections are being occupied, but it will be deenergized as soon as the rear of the train clears track section TA, removing its input 2.

As mentioned above, while the train occupies track section TB, the warning system will remain activated because input 2 will not be supplied to the AND circuit. When the rear of the train clears track section TB, switching circuit B provides output 1 and output 2 ceases. Stick circuits AB and CB are deenergized since no input 2 is supplied to either of those circuits. Stick circuit ABC is maintained energized as long as the train occupies track section TC. At this point therefore, the situation is as follows: switching circuit A supplies output 1 which passes through OR circuit D to input 1 of the AND circuit; switching circuit B supplies its output 1 to input 2 of the AND circuit; and stick circuit ABC supplies an input through OR circuit E to input 3 of the AND circuit. Assuming that the switching circuits have continued to operate properly, AND circuit inputs 4, 5 and 6 are maintained. Consequently, the warning system is deactivated. It can be noted in passing that if a following train were to enter track section TA at this time,

the warning system would be activated. Switching circuit A would not, in that case, supply its output 1 to the AND circuit.

After the rear of the train clears track section TC, switching circuit C will nolonger provide an output 2 and output 1 will be restored. Stick circuit ABC is therefore deenergized. The various circuits are thus returned to their original condition of holding the warning system deactivated. For trains travelling from west to east, a cycle of operation similar to that described above takes place.

Referring now to,FIG. 2, there is shown a general organization and wiring diagram of the logic circuitry of the crossing system. It is generally similar to FIG. 1. In addition to the circuit components shown in FIG. 1, however, there is shown a clock, an amplifier and a rectifier. Also, the switching circuits and stick circuits are shown as providing for various outputs on conductor pairs. As mentioned earlier, the switching circuits are identical with one another in structure and operation, as are the stick circuits. For brevity therefore, only switching circuit A and stick circuit CBA are shown in detail in FIGS. 3 and 4 respectively. The check output circuits are shown in detail in FIG. 5, together with the amplifier and rectifier. The clock is shown in detail in FIG. 3. The same designations used for the conductors shown in FIG. 2 are used in the remaining FIGS. For example, conductors l0 and 11 in FIG. 2 are the conductors bearing the same designations in FIGS. 3 and 5.

Conductors

12 and 13 in FIG. 2 are the conductors bearing the same designations in FIGS. 3 and 4.

Conductors

14 and 15 in FIG. 2 are the conductors bearing the same designations in FIGS. 3 and 5.

Conductors

60 and 61 in FIG. 2 are the conductors bearing the same designations in FIG. 4.

Conductors

50 and 51 in FIG. 2 are the conductors bearing the same designations in FIGS. 4 and 5.

FIG. 3 shows, in detail, switching circuit A and the clock circuit. The clock may be any circuit capable of producing a square wave of suitable frequency and having two separate outputs, one referenced to common, the other to negative. A simple multivibrator is shown by way of example-only. Its mode of operation is familiar to those versed in the art.

The switching circuit is supplied with two inputs, one from the front contact of track relay ATR and one from the clock. There are three output conductor pairs designated 10 and 11, 14 and 15, and 12 and 13. As shown, the base of transistor T1 is connected to common through resistor R15 and to the clock pulse source through resistor R16. Its emitter is connected to common also, while its collector is connected to the base lead of transistor T2. The emitter of transistor T2 is connected to common and its collector is connected through resistor R7 to positive. The collector of transistor T2 is coupled through resistor R1 and capacitor C1 to the base lead of transistor T3. The collector of transistor T3 is connected to a DC restoring circuit comprised of resistor R2, condensers C2 and C3 and diodes D1 and D2. The output of this branch of the circuit appears across conductors 10 and 11. The other branch of the circuit which provides an output across conductors l2 and 13 is comprised of similarly arranged circuit elements. As shown in FIG. 3, the base lead of transistor T7 is connected to the emitters of transistors T3 and T6. Its emitter is connected to negative while its collector is connected to DC restoring circuit comprised of resistor R6, capacitors C7 and C8 and diodes D5 and D6. An output appears across conductors l4 and 15.

The square wave signal impressed upon conductor 70 by the clock is applied to the bases of transistors T1 and T5. This signal varies between common and positive and the bias conditions are such that it will have the effect of turning transistor T1 on and off at the clock frequency. With track relay ATR energized, a positive DC signal is applied through its front contact to the base of transistor T2, biasing that transistor on. However, with transistor T1 switching on and off, this DC signal will be chopped. The result of this chopping action is that transistor T2 will likewise be switching on and off at the clock frequency. When transistor T2 is off, condenser C1 will charge by way of a path from positive through resistors R7 and R1, and through the base emitter junctions of transistors T3 and T7 to negative. The initial effect of this forward biasing current is to turn transistors T3 and T7 on. When transistor T2 subsequently turns on, the voltage at the junction between resistors R7 and R1 will decrease below that of the charged capacitor C1. At this time the capacitor will discharge and current will fiow in the opposite direction from negative through resistor R3, resistor R8, capacitor C1, resistor RI, transistor T2 to common. The effect of this is to bias transistors T3 and T7 off. It can thus be seen that transistors T3 and T7 will be switching on and off at the clock frequency. It should be noted that if transistor T2 should fail by opening from collector to emitter, capacitor C1 would, in effect, be continuously connected across the positive and negative sides of the power supply. A current would then flow through the base emitter junctions of transistors T3 and T7 only long enough to bring capacitor C1 to a full charge. Thus, transistors T3 and T7 could not be switching. Furthermore, if condenser C1 shorted, the bases of transistors T3 and T7 would receive a signal directly from the collector of transistor T2 through resistor R1. This would cause the voltage applied to the bases of transistors T3 and T7 to swing between positive and common. Since the emitters of these transistors are connected to the negative supply they would not be switching, but would remain steadily on. Again the switching action is lost.

When transistor T3 is on, capacitor C2 will charge from commonthrough diode D1, transistor T3, resistor R3 and transistor T7 to negative. When transistor T3 is off, capacitor C2 will discharge through diode D2, capacitor C3 and resistor R2. Capacitor C3 is a four terminal capacitor. The terminals on each side are located at either end of the capacitor plates. Thus, as capacitor C2 discharges, the upper plate and conductor will be driven positive while the lower plate and conductor 11 will be maintained at common. At the same time capacitor C3 will receive a charge. During the times when capacitor C2 is charging, capacitor C3 will discharge across any load placed between conductors 10 and 11. The result is that conductor 10 will be continuously maintained positive while conductor 11 will be maintained at common.

As explained above, with a DC input through the front contact of relay ATR, an output will appear across conductors 10 and 11. At this time transistor T4 will be driven to saturation by the positive DC input, thus biasing transistor T5 off. When relay ATR releases, transistor T4 will be off. The clock pulse on conductor 70 will therefore be applied to the base of transistor T5 causing it to switch on and off. Transistors T6 and T7 will switch on and off also, causing outputs to appear across

conductors

14 and and 12 and 13. At this time however, no output will appear across conductors 10 and 11 because there will be no positive input to the base of transistor T2 to turn it on. Although transistor T1 will be switching, the potential of its collector will never rise above common.

in summary, it can be seen that with a DC input from the front contact of relay ATR, a DC output will appear across conductor pairs 10 and 11 and 14 and 15 while no output will appear across conductor pairs 12 and 13. When the DC input is removed, no output will appear across conductor pair 10 and 11 while a DC output will appear across conductor pairs 14 and 15 and 12 and 13.

The clock signal goes through one more stage in reaching transistor T3 than it does in reaching transistor T6. As can be readily seen, the signal applied to the base of transistor T6 is out of phase with the clock, while the signal applied to the base of transistor T3 is in phase with the clock. If a failure should occur, such that both T3 and T6 should be driven simultaneously, the sum of their respective driving signals would be applied to the base of transistor T7. Under those circumstances T7 could not switch and its output appearing across

conductor pair

14 and 15 would cease. Of course, if neither transistor T3, nor transistor T6, is being driven, T7 could not switch and again, the output across

conductor pair

14 and 15 would cease. The readers attention is directed to the fact that conductors 14 and 1.5 feed directly into the AND portion of the check output circuit, as shown in FIGS. 1 and 5. This serves as a check upon the switching circuit. [f the output across

conductors

14 and 15 should cease at any time, the output from the check output circuits would cease, causing the crossing relay XR to release and activating the motorist warning signal. The switching circuit could also fail in such a manner that an intended output would not appear across conductor pairs 10 and 11 or 12 and 13. But such a failure is on the side of safety as will be more fully appreciated hereinafter.

FIG. 4 shows in detail the circuitry of stick circuit CBA. Conductor 71 is connected to the clock. The square wave impressed upon this conductor by the clock swings between positive and negative. The baseof transistor T8 is connected to conductor 71 and to negative through resistor R9. Its emitter is connected to negative while its collector is connected through resistor R10 to the base of transistor T9. The base of transistor T9 is also connected to conductor 60. The emitter of transistor T9 is connected to conductor 61 while its collector is connected to positive through resistor R12. The base of transistor T10 is coupled through capacitor C9 to the collector of transistor T9. lts emitter is connected to negative while its collector is connected to the base of transistor T11. Also connected to the base of transistor T11 is conductor 12. The emitter of transistor T11 is connected to conductor 13 while its collector is connected to positive through a resistor. The base of transistor T12 is connected to the collector of transistor T11. Its emitter is connected to common while its collector is connected to positive through a resistor. The base of transistor T13 is capacitively coupled to the collector of transistor T12. lts emitter is connected to negative while its collector is connected to a DC restoring circuit comprised of capacitors C11, C12 and C13, resistor R1], and diodes D7, D8 and D9.

The clock signal is continuously applied to conductor 71. The base of transistor T8 is therefore made to swing between positive and negative. Transistor T8 will therefore be switching on and off at the clock frequency. When -.stick circuit CB, shown in FIGS. 1 and 2, is energized, conductor is maintained positive while conductor 61 is maintained at common. This would ordinarily turn transistor T9 on. But the switching action of transistor T8 chops the input to the base of transistor T9, causing it to switch at the clock frequency. When transistor T9 is off, capacitor C9 will charge from positive through resistor R12, resistor R13 and transistor T10 to negative. The result of current flowing in this direction will turn transistor T10 on. When transistor T9 is on, capacitor C9 will discharge in the opposite direction through transistor T9 to common and through resistors R14 and R13. At this time transistor T10 will be biased off. Transistor T10 therefore switches at the clock frequency when transistor T9 is supplied with the input across

conductor pair

60 and 61.

As noted in the above discussion of switching circuit A, when an output appears across

conductor pair

12 and 13, conductor 12 will be maintained positive while conductor 13 is connected to common. With such an input present, transistor T11 will be switching at the clock frequency due to the chopping affect of transistor T10. The base of transistor T12 will be made to swing between positive and common and it will likewise switch at the clock frequency.' Transistor T13 will also be switching and an output will appear across

conductors

50 and 51. Conductor 50 will be maintained positive while conductor 51 is connected to common. Part of the positive output of the DC restoring circuit is fed back to the base of transistor T9 by way of conductor 17. Therefore, once the circuit has been supplied with simultaneous positive inputs on

conductors

12 and 60, the removal of the positive signal from conductor 60 will have no effect of the output of the circuit. The positive feedback signal on conductor 17, in effect, replaces the positive input on conductor 60. if the positive signal on conductor 12 is removed however, transistor T11 will no longer switch. lts emitter is connected to common through conductor 13, as shown in FIG. 3, and transistor T10 is referenced to negative. Hence, in that case the output across

conductor pair

50 and 51 would cease.

In summary, the stick circuit is energized by the simultaneous application of two positive DC input signals. Once energized, the output of the stick circuit will continue even though a certain one of the inputs is removed. Should the other input be removed at any time however, the output from the stick circuit will cease.

P16. 5 shows in detail the arrangement of the check output circuits. Conductor 71 is connected to the clock. The square wave impressed upon this conductor swings between positive and negative. The base of transistor T14 is connected to negative and to conductor 71. Its emitter is connected to negative while its collector is connected to the base of transistor T15. The emitter of transistor T15 is connected to conductors 51 and 11. Its base is connected to conductors and 50. The collector of transistor T is connected to positive through resistor R17 and capacitively coupled to the base of transistor T16. The base of transistor T16 is also connected to negative through a resistor. Its emitter is connected to negative while its collector is connected to the base of transistor T17. It can be seen that the same pattern is repeated throughout the remainder of the check output circuits. One difference to be noted however, is that transistors T19, T21, T23 and T25 are each connected to only one conductor pair. For example, transistor T19 is connected to conductor pair and 21. Transistors T15 and T17 on the other hand are connected to two conductor pairs.

With a continuous input on conductor 71, transistor T14 will be switching at the clock frequency. If either conductor 10 or conductor 50 is maintained positive while conductors 11 and 51 are maintained at commomtransistor T15 will also be switching at the clock frequency. This is so because the switching action of transistor T14 chops the input to the base of transistor T15. When transistor T15 is off, current will flow from positive through resistor R17, capacitor C14, resistor R18 and transistor T16 to negative, thus causing T16 to go on. When T15 is on, capacitor C14 will discharge, reversing the direction of current flow and transistor T16 will go off. Transistor T16 will therefore be switching at the clock frequency. The remainder of the circuit operates in a similar fashion. It can be seen that if all of the odd numbered transistors are supplied with a positive signal on the associated even numbered conductors, each of those transistors will be switching. An output will thus appear on the collector of transistor T in the form of the clock signal. It should be noted at this point, that transistor T15 and its'associated conductor pairs 10 and 11 and 50 and 51 corresponds to the OR circuit D of FIG. 1. Transistor T17 and its associated conductor pairscorresponds to OR circuit E of FIG. 1. The remainder of the check output circuit corresponds generally to the AND circuit of FIG. 1.

The output from the collector of transistor T25 is amplified by transistor T26. It is then transferred to the isolated secondary of transformer 18. After rectification in the usual manner, it is used to hold the crossing relay XR energized.

It should be clear from the above description that this invention provides for the use of transistorized circuitry in a crossing system in a safe manner. The transistors used throughout the circuitry are relied upon only so long as they are switching. Should one fail, an essential input to the check output circuitry will cease and the motorist warning system will be energized. Failure in other circuit components would produce the same result.

While there has been described what is at present considered to be the preferred embodiment of this invention it will be obviousto those skilled in the art that various changes and modifications may be made therein, without departing from the invention, and it is therefore aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

Iclaim:

1. A logic circuit element comprised of:

a switching means having an input terminal and adapted to provide a first output only when an input is supplied to said terminal, a second output only when no input is supplied to said terminal, and a third output when either, but not both, of the first and second outputs are provided; and

check output means responsive to the switching means for providing an output when the switching means provides its first and third outputs or when the switching means plrovides its second and third outputs.

2. he invention according to claim 1 wherein said nput terminal is a signal input terminal and wherein said switching means is further comprised of:

an operating energy input terminal;

a first switching device connected to both terminals and adapted to provide the first output only when the operating energy is applied and there is an input on the signal input terminal;

a second switching device connected to both terminals and adapted to provide the second output only when operating energy is applied and there is no input on the signal input terminal; and

a third switching device connected to the first and second switching devices and adapted to provide the third output when either, but not both, of the first and second switching devices is providing its output.

3. The invention according to claim 2 wherein said switching devices are solid state devices.

4. The invention according to claim 3 wherein said solid state devices are transistor devices.

5. The invention according to claim 1 wherein the switching means comprises:

an AC terminal in addition to the input terminal;

a first plurality of transistors connected to both terminals and adapted to switch on and off in response to the AC energy only when an electrical input is supplied to theinput terminal, whereby to provide the first output;

a second plurality of transistors connected to both terminals and adapted to switch on and off in response to the AC energy only when no electrical input is supplied to the input terminal, whereby to provide the second output; and

a third transistor connected to the first and second pluralities of transistors and adapted to switch on and off in response to the AC energy only when either, but not both, of said pluralities is switching on and off, whereby to provide the third output.

6. The invention according to claim 5 wherein each output is connected to a DC restoring means for providing said outputs as DC outputs.

7. The invention according to claim 5 wherein the input terminal is a DC terminal.

8. The invention according to claim 1 wherein the check output means is comprised of:

an AC input terminal and first, second and third other input terminals;

a first transistor connected to the AC terminal and adapted to switch on and ofi in response to the AC energy;

a second transistor connected to the first transistor and to the first and second other input terminals and adapted to switch on and off when the first transistor is switching on and off while an electrical input is supplied to either of said first and second other input terminals; and

a third transistor connected to the second transistor and to the third other input terminal and adapted to be switching on and off, whereby to provide an output, only when the second transistor is switching on and off while an electrical input is supplied to said third other terminal.

Claims

1. A logic circuit element comprised of: a switching means having an input terminal and adapted to provide a first output only when an input is supplied to said terminal, a second output only when no input is supplied to said terminal, and a third output when either, but not both, of the first and second outputs are provided; and check output means responsive to the switching means for providing an output when the switching means provides its first and third outputs or when the switching means provides its second and third outputs.

2. The invention according to claim 1 wherein said input terminal is a signal input terminal and wherein said switching means is further comprised of: an operating energy input terminal; a first switching device connected to both terminals and adapted to provide the first output only when the operating energy is applied and there is an input on the signal input terminal; a second switching device connected to both terminals and adapted to provide the sEcond output only when operating energy is applied and there is no input on the signal input terminal; and a third switching device connected to the first and second switching devices and adapted to provide the third output when either, but not both, of the first and second switching devices is providing its output.

5. The invention according to claim 1 wherein the switching means comprises: an AC terminal in addition to the input terminal; a first plurality of transistors connected to both terminals and adapted to switch on and off in response to the AC energy only when an electrical input is supplied to the input terminal, whereby to provide the first output; a second plurality of transistors connected to both terminals and adapted to switch on and off in response to the AC energy only when no electrical input is supplied to the input terminal, whereby to provide the second output; and a third transistor connected to the first and second pluralities of transistors and adapted to switch on and off in response to the AC energy only when either, but not both, of said pluralities is switching on and off, whereby to provide the third output.

8. The invention according to claim 1 wherein the check output means is comprised of: an AC input terminal and first, second and third other input terminals; a first transistor connected to the AC terminal and adapted to switch on and off in response to the AC energy; a second transistor connected to the first transistor and to the first and second other input terminals and adapted to switch on and off when the first transistor is switching on and off while an electrical input is supplied to either of said first and second other input terminals; and a third transistor connected to the second transistor and to the third other input terminal and adapted to be switching on and off, whereby to provide an output, only when the second transistor is switching on and off while an electrical input is supplied to said third other terminal.