A plc ladder logic program consists of a number of rungs with each rung controlling an input12/1/2023 ![]() A short pulse on the input X000 can be extended into a longer pulse appearing at the contacts of the output Y400. The ladder logic above acts like a pulse extender. ![]() When the countdown period has expired, the normally closed contacts of T300 become open, thus interrupting the logic continuity to the output Y400 and so Y400 is de-energised. During the countdown period, the output device Y400 remains energised by the latched path through the contacts Y400. Now, when X000 goes false, a true signal is sent to the timer to begin the countdown period. ![]() Thus, when the input X000 goes false, the output Y400 remains on, it is latched by its own contacts. This true state is fed back into the input to the normally closed contacts of the timer. In this example, when the input X000 is true, there is logic continuity through the normally closed timer contacts T300 to the output Y400.
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