Design of fail safe computer systems for railway control

In the last ten years, control computers have been introduced for railway control in Poland. The previous, relay versions of railway control systems have good safety measures derived directly from reliability theory because the base element of such systems-an electromagnetic relay-has nonsymmetrical faults and appropriate fail-safe rules of design may be applied. Typical digital circuits in electronic devices have symmetrical faults (1→0 or 0→1), so corresponding safety criteria may be satisfied using fault tolerant methods: modular redundancy and self-diagnosis. Introduction of low-cost high integrated circuits (microprocessors, memories, input-output adapters, etc.) allowed one to design highly reliable microcomputers, realizing important railway control tasks like interlocking or dispatcher supervision. Computer communication standards give the possibility of implementation of some additional railway control functions related to data and command transmission between different points of the system. The new tasks correspond to optimization of traffic, visualization and remote control