sequence of program execution

chriswarrn · Jan 13, 2004

I'm confused about when the plc actually changes the state of the output of a rung. I understand that when the rung with a real-world output goes true the output table is updated immediately but the actual output doesn't change until after the scan is finished. What I don't know is if I use an XIC or XIO for that output, does the contact change state immediately when the associated output rung goes true or when the plc actually changes the real output.

Also, what about internal relays and latches. Do they operate/execute differently then real-world I/O?

Ron Beaufort · Jan 13, 2004

this should answer all of the questions you posted ...

this one comes up quite often ... see if this helps ...

link to a previous post

here's a specific quote:

Basic idea: the INSTRUCTIONS in the ladder program are called “instructions” for a reason. They literally are instructions FROM (you) the programmer TO the processor. These instructions tell the processor to “examine a bit to see if it contains a 1” or to “examine a bit to see if it contains a 0” or to “go write a 1 or a 0” into a particular bit. The processor obeys these instructions instantly as soon as the rungs are executed. Specifically, the processor does exactly what the ladder commands it to do and “examines a bit” or “writes to a bit” instantly - whether the bit in question is an output bit (example: O:2/0) or an internal bit (example: B3/0) or even an input bit (example: I:1/0) makes no difference.

post again if you still need help ...

chriswarrn · Jan 13, 2004

Thank you, Ron. That was a very informative post. I tried to search for the answer here, but I was apparently using the wrong keywords for my search. This site is making me look like I know what I'm talking about here at work. It's a great asset. Thanks again.

sequence of program execution

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