Importance

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Abstract

A railroad train moving along an approach to a grade crossing (or other location of interest) is detected by apparatus that is responsive to the change in effective track impedance caused by the moving shunt. A constant current AC signal is applied to the track adjacent the crossing, and the resulting voltage changed in amplitude and phase is picked off by a receiver on the opposite side of the crossing. Voltage and current information derived from these signals is inputted to a microcomputer which calculates the track impedance and determines whether or not a warning device or devices at the crossing should be activated. Such determination is made on a repetitive basis in accordance with a repeating program loop and stored in memory. To assure that the system is failsafe, the program loop is interrupted at predetermined intervals and an interrupt service routine checks the memory address at which the determination is stored and causes an independent computer output to change state each time the memory is checked and activation of the warning is not required. Accordingly, the independent output is toggled to produce an AC output signal, the loss of which initiates activation of the warning. The program executes each loop in a fixed time period which is monitored, and if the program is not executed within such period, the system is forced to reset and the warning is activated until the proper time constraints are again met. A self check also assures that the system will respond to the loss of the AC output signal.

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First Claim