PLC Program

During the scan of a program, the inputs are scanned if they are already on when the scan takes place, but are the inputs seen if they turn on while the scan of inputs is taking place?

Prayder said:

During the scan of a program, the inputs are scanned if they are already on when the scan takes place, but are the inputs seen if they turn on while the scan of inputs is taking place?

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If the scan has passed that point then no, not until the next scan cycle, should be pretty fast though, evaluating all the inputs is very quick.

Thanks for the quick response....

Prayder said:

During the scan of a program, the inputs are scanned if they are already on when the scan takes place, but are the inputs seen if they turn on while the scan of inputs is taking place?

Click to expand...

Depends on the PLC.

Consider Allen-Bradley for example:

With SLC500/MicroLogix series (RSLogix 500), the input image table is updated at the beginning of each program scan. If the physical inputs change state during the scan, it won't be reflected until the next scan.

With Compact/ControlLogix (RSLogix 5000), when you address an input point in an instruction it will return the state of that input at that instant in time. If this behavior is not desired, you can copy the input words to memory words at the beginning of the scan and reference those in your instructions.