RSLogix500 questions

[bitmonkey]

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.

 

[scott.lawrence]

they're using the bits to cast a binary output into a register

 

[Maxkling]

Could be done from an HMI or MSG from another PLC or device.

 

[Ron Beaufort]

on the Usage Table - do you see an "F" or a "W" at the left end of the word? ...

 

[Maxkling]

they're using the bits to cast a binary output into a register

If you cross reference one of the bits shouldn’t it come up in one of those instructions?

 

[bitmonkey]

Yes there is a W on the left end, meaning "Word" I take it?

 

[bitmonkey]

I found a MOV
SOURCE O:6.0
10310
DEST N7:26
10310

 

[bitmonkey]

And a MVM, looks like I've got some studying to do!

 

[bitmonkey]

I still don't know what a "Q" is in the ladder?

 

[Ron Beaufort]

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 ...

 

[bitmonkey]

Why would you use a mask of 00ff? that converts to 11111111, which is no mask at all, isn't it?

 

[Ron Beaufort]

a recent thread about MVM ...

http://www.plctalk.net/qanda/showthread.php?p=798794&postcount=1

cut to the chase ...

http://www.plctalk.net/qanda/showthread.php?p=52885&postcount=10

looks like I've got some studying to do!

TIP: that NEVER goes away ...

 

[bitmonkey]

Thanks Ron and everyone, it makes a little more sense now.

 

[Maxkling]

If you cross reference one of the bits shouldn’t it come up in one of those instructions?

Edit::: just noticed this was 500 not 5000. Ha.

 

[BruceJW]

Edit::: just noticed this was 500 not 5000. Ha.

yes, me too. I have no idea about the Q.

MVM I assume is the masked move, only move certain bits in word, dword etc.