View Full Version : When exactly does a timer get energized

April 13th, 2008, 07:57 AM
A timer must get energized whenever the set time is reached. This may happen in the middle of the program scan. Then, in the same scan, the same contacts (say, N/O type) may be logic 0 in the beginning of the ladder, and logic 1 towards the end. Or, is the effect of energizing goes to the next scan? Please clarify.

Steve Bailey
April 13th, 2008, 08:29 AM
The evaluation of whether the timer's accumulated value has reached its preset takes place at the point in the ladder where the timer function is located. In the scan in which the timer's accumulated value reaches its preset, a NO contact addressed to the timer's done bit will be false in all rungs before the timer function and true in the rungs following. In all subsequent scans, the same contacts will all be true until the timer is reset. The same analysis applies in the scan in which the timer is reset.

In the example below, in the scan when the timer's accumulated value reaches its preset, Output 1 will be false and Output 2 will be True. In subsequent scans both outputs will be true. In the scan where "True Logic" is no longer true and the timer is reset, Output 1 will be true and Output 2 will be false. After that, both outputs will be false.

Timer Output
Done 1
-] [----------------------( )-

True Timer
Logic Done
-] [------------|Timer|---( )-

Timer Output
Done 2
-] [----------------------( )-

April 13th, 2008, 08:41 AM
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April 13th, 2008, 11:57 AM
The PLC program is a computer program. It is a list of instructions and memory addresses what a processor executes one at a time. Unless the processor acts on the instruction the contents of the memory address do not change. It is not a collection of devices.