MCR instruction.

[mellis]

Just ran into a SLC500 program last week that had multiple MCR zones. Had to look it up to be sure how it worked. In this case, conditional subroutines would have done just as well. In some cases it looks like the original programmer was just trying to make the program execute faster by skipping sections that didn't need to execute all the time. A JMP would have been better if this is what he wanted.

I understand that a conditional subroutine is not a "drop in" replacement for a MCR. But I can get the same function even if I have to code it a bit differently. The MCR zones I encountered recently had a lot of RTOs and UTL/UTO pairs in the logic. It would have probably been simpler in a subroutine with TONs and OTEs.

Bottom line, It's a programming technique that is not used much and there are alternatives that are more common and readily understood. I think it is a hold over from PLC2 days where there were no subroutines.

 

[JeffKiper]

I know a bunch of material handling companies that use MCR for every subroutine.

They usually use them with a System started or system running bit. I am not a fan of them. But when the man who approves my T&M sheets said do it like this you follow suit. They aren't that bad once you use them and start to understand them.

 

[daba]

...... They aren't that bad once you use them and start to understand them.

critical words highlighted ^^ a lack of understanding is what causes confusion and mistakes.

Which is why respectable training is needed for maintenance personnel who may find themselves digging deep holes into which everything falls.