PLC5 MSG question

Season greetings friends. I'm at work studying a program and have just found something that is kind of confusing me. We have 8 nodes, all enhanced PLC5s, running on DH+ with RSLogix5 installed on the PC that I am viewing the network with. My question has to with the MSG instruction. I have always thought that the MSG was a one way instruction....that the data was sent from the node that the MSG was in to the target node. Is it possible to use a MSG from say node 1 to prompt node 2 to send the specified data from node 2 to node 1? I hope that makes sense.

MSG instructions can be Writes or Reads.

A Write instruction is simplest; the data payload is sent along with the write command. The reply is a confirmation that the operation was successful.

A Read instruction requires more; A request goes from Controller 1 to Controller 2 for data. Controller 2 replies with the data. Controller 1 confirms the data was received.

There's a little bit of philosophy that goes with this kind of controller-to-controller access. Some folks have been burned by accidental writes to incorrect data tables or incorrect controller addresses, and only use Read instructions. Others are trying to squeeze all the performance they can from their networks and use lower-overhead Write instructions only. Your mileage may vary.

Thanks Ken.

Greetings ptine ...



you said:

I have always thought that the MSG was a one way instruction....that the data was sent from the node that the MSG was in to the target node.

Click to expand...

there are at least two things that might be confusing you here ... I doubt that this first one is of any interest - but I’ll cover it anyway just for kicks ...



it is possible to change an MSG (Message) instruction “on-the-fly” and make it (for example) change the address of the other (source/target) node ... you could even make a single MSG function as a “read” message at some times - and then flip it to function as a “write” message at other times - if you really wanted to do so ... the trick would be to have your ladder logic program alter the data stored in the MSG’s “Control Block” ... some programmers do things like this to minimize the amount of code in their programs ... other programmers - and technicians - HATE this approach because it is extremely difficult to troubleshoot the system whenever any particular message doesn’t “go through” as planned ...



I doubt that this approach is what is confusing you in the program that you’re examining ... just be aware that sometimes you might run into something like this ...



now here’s the thing that really confuses MOST people - until they get used to it ...








in the picture above, the words “This PLC-5” make perfect sense ... now notice that the MSG is being configured to perform a “READ” operation - and not a “WRITE” operation ... the confusing thing is that the words “Target Device” do NOT make sense ... they should instead say “Source Device” ... specifically, this MSG is going to READ/GET data from another SOURCE ... it is not going to WRITE/DATA to another TARGET ...



secret handshake: the words “Target Device” will NOT change to reflect the direction of the data transfer ... specifically, this legend will ALWAYS say “Target Device” regardless of which way the data is flowing ...



tip: always look at the “Communication Command” setting when you need to determine which way the data is supposed to flow ...



is that the part that you found confusing? ... if so, you would certainly not be the first ...



now if I had been on the team who originally created the MSG setup screen, I’d have insisted that we call this particular legend “Other Device” ... that way it would have made sense regardless of which way the MSG was transferring the data ... but obviously the folks at Rockwell didn’t invite me to participate ...



hope this helps ...

Thanks Ron & Ken. You guys are awesome. I really appreciate the effort you both put in on this forum. Thank you very much and I hope you both have a great holiday season. My wife just came into my computer room and told me to hurry up and get ready. Having just read this I want to go and try it on the 5 but I think it's in my best interest to finish this post and jump in the shower. Thanks again gentlemen. Merry christmas.