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Timers
In a PLC, timers are programming functions that keep track
of time and allow PLC programs to provide varied responses
depending on the elapsed time.
Hard-wired Timing Circuit Timers in a PLC program can be compared to hard-wired timing
circuits, such as the one represented in the accompanying
control line diagram. In this example, normally open (NO) switch
(S1) is used with timer (TR1). When S1 closes, TR1 begins
timing. When the timer’s preset time elapses, TR1 closes its
associated normally open TR1 contact and pilot light PL1 turns
on. When S1 opens, TR1 de-energizes immediately, the TR1
contact opens, and PL1 turns off.
TR1
PL1
TR1
S1
TR1
PL1
TR1
S1
Closes a preset time
after S1 closes
This type of timer is referred to as an on-delay timer. The term
“on-delay” indicates that the timing begins when the timer
receives a signal to turn on. In this example, that happens when
S1 closes.
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