Abstract: In some circumstances it is important to maintain a d.c. potential within a specific range of a critical value. The circuit shown will provide a fail-safe signal if the potential deviates from the allowed range. Two zener diodes and two optically coupled isolators are used. If the d.c. potential remains within the critical range, an a.c. test signal back biases one optically coupled isolator and forward biases the other on the positive half cycle; and on the negative half cycle back biases the other and forward biases the one. Thus the two optically coupled isolators conduct on alternate half cycles of the a.c. test signal. If the d.c. potential strays from the required range, the optically coupled isolators stop alternate conduction and a fail signal is given. If either zener diode should fail in a mode which alters its characteristics so that the circuit would fail to detect a voltage excursion outside the allowed range, a fail signal is given.

Abstract: An all-digital communication system is arranged to exhibit fail-safe qualities. Each message includes a pair of words, each word separated from every other word by framing information in the form of two bits, either 1/0 or 0/1. Each word in the message is arranged to exhibit a constant ratio of 1's to 0's, so that more than a single change in any bit location is needed to change from one valid message word to another. The second word in each message is the complement of the first. Two decoders are disclosed, a hard-wired embodiment and an embodiment employing a microprocessor. In the hard-wired embodiment, straightforward decoding is employed to determine the apparent message, and the apparent message is encoded to generate a locally generated message which is then compared, bit by bit, with the received message employing vital logic techniques. Assuming each of the received and locally generated bits compare, the message is validated.

Abstract: In a train control signalling system, a wayside signal of an unknown code rate is detected and decoded on board the train to determine the code rate and derive the coded train control intelligence. The code rate is determined by counting the number of cycles of a known higher frequency which occur during one period of the unknown code rate. Before a train command signal is issued, a number of tests are performed to verify that the determination is accurate with no margin for error. The known higher frequency signal is passed through a tuned circuit to verify that it is within predetermined limits of its rated frequency. The counter that counts the cycles of the known higher frequency is tested to verify that it is reset to zero at required intervals. Verification is obtained that one and only one train control signal is obtained. The counting circuits and memory devices are cycled prior to each decoding operation to verify proper functioning.

Abstract: A solid state fail-safe logic system is disclosed including AND and OR gates which are designed as an evolutionary replacement for signal control functions previously performed by vital front and back contacts of vital relays and power check logic. The AND gate is basic and accepts an a.c. and a d.c. input. The a.c. input circuit includes a light emitting diode optically coupled to a light receiving active circuit means. Leakage currents cannot falsely activate the gate since the light emitting diode is poled to be reverse biased by the supply voltage. The d.c. input is protected from leakage currents by proper connections so that any leakage current is of the wrong polarity to produce an output. The d.c. input provides forward bias for light responsive active circuit means. The AND gate is divided into an input module including the light emitting diode and an output module inciuding the light responsive active circuit means.

Abstract: The control system operates vehicles over a right-of-way divided into a plurality of zones and selects the direction of traffic along the right-of-way. The presence of a vehicle is sensed when it enters the zone from either direction and the occupancy of the zone is registered. The occupancy registration is indicated by a magnetically actuated switch to operatively control restrictive aspects in accordance with selected number of zones behind the leading vehicle relative to the selected direction of traffic.

Abstract: A four terminal varistor is shown as a device for providing fail-safe circuit protection under certain conditions. The failure of any lead connected to the four terminal varistor will shut down the protected load so that the load cannot operate under unsafe conditions. By its nature, a varistor can absorb voltage peaks and protect a load from high voltage and/or a current resulting from high voltage. One pair of leads from the varistor is connected to the power supply and the other pair of leads to the load. Thus, if any lead is broken, there will be no power to the load. A variation provides additional protection in the event the metal coating on the semiconductor, to which two of the leads are attached, becomes detached from the semiconductor. In case of such detachement, the electrical connection between the two leads is broken. For this purpose, the metalized coating on the semiconductor is divided into two separate areas, one for each lead.

Abstract: A signaling circuit adapted to be used with railroad tracks or the like for ensuring fail-safe performance in respect to the operation of a control device, such as a track relay, which must function in a fail-safe manner, typically to become deenergized after a set period and to release or drop away when any emergency or failure occurs; thereby to produce a safe action, such as providing a warning signal to the engineer or operator of a train. The signaling circuit includes a quick-acting shunt detector which operates to detect short-duration shunting effects and functions to so extend or enhance them that they produce the desired result of releasing the track relay.

Abstract: There is described a track circuit for use with a track section which may or may not include a switch. The system detects the presence of a railroad vehicle within the track section as it constitutes a shunt across the track. The track section is divided into a plurality of segments with each segment having a common boundary with at least one other segment. Alternating current signals are induced into each track segment and a sensor is provided for sensing and responding to the induced a.c. signals. A modulator is provided for intermittently affecting the sensed signal and a detector is provided for detecting and responding to the changing signal.The detected signals are gated through a logic circuit which provides first and second outputs indicating: the changes are as might be anticipated as a result of the modulation introduced; or, the changes are other than might be anticipated as a result of the modulation, respectively.