Owing to the peculiarities of the PLC Ladder Logic graphic, there may well be the possibilities of the invalidation in the electric chain lockon the circuit.
由于该系列PL C解算梯形图流程的特殊性，容易引起控制电路中电气联锁环节的失效。
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In this paper we discuss how it works about touch switch, to analyse and compare some typical touch switch circuits, to give a detector circuiton two-point 'touch-electronic lock.
Based on discussion on several common elevator door lock circuits with software, this paper points out some hidden peril of the accident in such circuits, and designs a circuit using PLC.