Owing to the peculiarities of the PLC Ladder Logic graphic, there may well be the possibilities of the invalidation in the electric chain lock on the circuit.

In this paper we discuss how it works about touch switch, to analyse and compare some typical touch switch circuits, to give a detector circuit on 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.