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Old October 9th, 2002, 05:06 AM   #1

s_viswesh is offline
Join Date: Oct 2002
Posts: 13
Internal contacts/bits update...PLEASE HELP...


I understand that all physical inputs are scanned during the "input scan" before the "program scan" phase.
But is this true for internal relay, contacts & registers also?

i.e.(1) Are internal relays energised during "Output scan phase" or do they get energised immediately as the rung logic goes true?

2) The state of contacts corresponding to internal relays: Is it checked only at the beginning of the program or is it checked each time the element is encountered?

The "How a ladder is scanned" topic in your contents seems to suggest that internal contacts are checked each time they are encountered in a rung. Your help will be highly appreciated.

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Old October 9th, 2002, 10:59 AM   #2
Eric Nelson
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I realize this can be tough to understand at first, so instead of thinking about the physical I/O as being "scanned", think of it as being "updated".

Without getting too confusing, the answers to your questions (based on how you seem to see it) would be...

1.)All coils (inputs, outputs, internal coils) are "energized" (or "de-energized") immediately as the rung logic goes true/false.
2.)The state of contacts are checked each time they are encountered.

At the end of the scan, the physical outputs are "updated" with the current state. Therefore, the outputs will reflect the last state they are in at the end of the scan. Then the PLC gets "updated" with the physical state of the inputs, and the state of the inputs will remain constant for the current scan.

Get it?...


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