indirect address for AB PLC5


Thread Starter

Jay Collosi

Does anybody have experience in using a indirect address for the AB PLC-5 MSG instruction.
I want to be able to dynamically change the Destination address on a read from another PLC-5.

Hello Jay,

There is no way to directly use an indirect address inside the configuration
of a MSG instruction. THANK GOODNESS.

You could use an indirect address in the control block of the message
instruction. NOT RECCOMMENDED.

You could also use the MOV instruction to manipulate the destination value
in the control block of the MSG instruction. NOT RECCOMMENDED EITHER.

I would simply create multiple message instructions to perform the reads
that you want to do. It will be easier in the short run and in the long run
the people that come after you will appreciate the multiple instructions
versus the game playing.

My two cents worth.

Bradley G. Hite
Intertech Inc.
I have a routine that uses one Msg block that does radio communications to
30 sites. Email me if you want more info.
I think the best way to solve this one is to sacrfice one word of the
message you are trying to send/receive. This word becomes the "MessageID",
and you can then direct the message content where you want by the use of EQU
instructions (if MessageID is equal to this - do that, if it's equal to
that - do something else).

Although, it saves PLC memory and reduces the size of the Ladder listing, I do not recommend Indirect addresses unless its really necessary. It is a big pain to trouble shoot later.
Sorry it took me so long to respond. I am very interested in what you are doing. please send any info.


Yosef Feigenbaum

Hello Jay,

You can dynamically change change the destination address if you really want to. It's stored as a string somewhere in the MSG block. I did this a few years ago as a demonstration. I wouldn't recommend doing it though because of supportability. The best thing to do IMHO would be to MUX the data going in/out by adding an ID number at the beginning of each data block.