A safety-related digital input system based on the analysis of the architectural constraints

As a large-capacity public transport tool, the railway system needs high stability and reliability. In order to reduce cost and satisfy the safety requirements of train control system, a novel safety-related digital input system is designed and implemented based on architectural constraints of safety-related systems and the analysis of the working principle of the specific digital input system. This system is designed as 2-out-of-3 architecture on the basis of vital computer. The method of fault diagnosis has been improved after the analysis of the practical situation. That is, the detection signal sent by the external pulse transmitter module contains the circuit fault information after the signal pass through the input circuit, and the real-time self-diagnosis will be done after using the supporting software to deal with timing diagram. The system uses less CPU resources to achieve the high diagnostic efficiency, which improves safety failure rate. And its failure analysis is discussed. The Markov Model is used to verify the Safety Integrity Level (SIL) of the safety-related system. The analysis shows that this design, with easy implementation and low cost, meets the requirements of SIL4 in railway and is more reliable.