MSG instruction help

A_G

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Join Date
Jul 2014
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MA
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I'm trying to use a MSG instruction to get data from a SLC into a ControlLogix. I want to read some Boolean tags from the SLC. Is it possible to do this? When I set up the MSG instruction in the ControlLogix, I get this error: "Message references a tag of BOOL data type". Is it possible to read Boolean tags using MSG? Thanks in advance for any help!
 
Try reading INT and later you can use the bit you need. First you need to include your bool into a INT.

Thanks, this is what I ended up doing. So I included the whole B3:0 in the MSG.
 
Thanks for posting the follow-up.

16-bit words in a "B" file in a classic A-B controller (PLC-5,SLC-500 or MicroLogix) are treated just like words in an "N" file and are readable in 16-bit elements.

The reason that BOOL elements and individual bits of any B or N file cannot be addressed by a MSG instruction is a practical one based on technical support experience.

The theory is that if you allow programmers to read individual bits with a MSG instruction, they will be tempted to actually do so, and will use up all the memory and buffers in the controller moving around a handful of bits one at a time.

So Rockwell simply doesn't allow it, forcing programmers to at least move sixteen bits at a time.
 

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