RSLogix500 questions

bitmonkey

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Join Date
Oct 2015
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Ohio
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1. What the heck is a "Q"?

2. I stumbled across a program this morning that was obviously written by a very experienced programmer, and it's got me scratching my head.
There are 12 discrete outputs hard wired to a message display panel. They are labeled 1,2,4,8,16,32,64 etc. and they are being used. But yet when I open the output data table and click on usage, it shows them as ...... which is not being used. They don't show up in the ladder and I can't figure out how they are being turned on and off.
 
it's usually just a letter that's assigned to a LBL (Label) or a matching JMP (Jump) instruction ..

you wouldn't be able to "tie" a comment/description to a simple NUMBER - so the software uses the Q so that you can give a description to the JMP or the LBL ...

you'll see the same effect when they use the letter "U" in front of a JSR (Jump to Subroutine) instruction ...
 
Last edited:
Why would you use a mask of 00ff? that converts to 11111111, which is no mask at all, isn't it?
 

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