RsLogix 500 DCD Instuction

Hi Everyone,
I am working on a project with a SLC504. I am using the DCD instruction to use 4 inputs to select 1 of 16 outputs. I have tested my logic and it works.

My question is: Since you have to put in the Source address at the word level i.e. B3:0 in the DCD instuction and I am only using the first 4 B3:0/0 to B3:0/3, is the rest of the word B3:0/4 to B3:0/15 available for other things? Or does the DCD take up the entire word?

Thanks for the help.
Chris

Bullzi said:

Hi Everyone,
I am working on a project with a SLC504. I am using the DCD instruction to use 4 inputs to select 1 of 16 outputs. I have tested my logic and it works.

My question is: Since you have to put in the Source address at the word level i.e. B3:0 in the DCD instuction and I am only using the first 4 B3:0/0 to B3:0/3, is the rest of the word B3:0/4 to B3:0/15 available for other things?

Click to expand...

Yes.

Bullzi said:

Or does the DCD take up the entire word?

Click to expand...

It is non-destructive to the Source so those bits are unchanged, and don't affect the DCD instruction. It only "cares about" or "masks in" the lowest four bits before writing to the destination word in which all bits are used.

Thanks for the help. My testing seemed to indicate what you said but I just wanted to make sure.