Changing a message instruction on the fly

[Firejo]

Hey all.
I’m curious to see if it’s possible to change a message instruction in a MicroLogix 1100 (or any other AB PLC/PAC) from a read to a write and/or vice versa. It would be a message instruction reading from / writing to a SLC type device. I can change the data table address in the target device (using either a move or sequence) but I can’t find any information as to what the file for the “communications command” is.
Anybody ever done this and/or know what file it is (and if you can access it)?

 

[daba]

I wouldn't bother trying.... just set up two message instructions, one read, one write, and enable them as you wish...

I don't know what you think you'll achieve trying to manipulate the message configuration, which sounds difficult to me, although I haven't looked into it.

 

[Firejo]

At this point it’s mostly a curiosity question. I do have some reasons other than that but the difficulty has more than likely removed them from practicality.
Having said that, if every time I didn’t figure out how to do something different because how I knew how to do it is easily I wouldn’t know how to do most of what I know how to do. There are plenty of things I can do now much easier and more efficiently because I wanted to figure out a new way. Figuring out how to do things is half the fun.

 

[OkiePC]

Set up two identical messages and change one of them from Read to Write. Enable them and put the processor in RUN mode to have them fire off true at least once each. Then try looking at the control words (MG file in Micrologix?) in binary radix (if possible) and comparing the control words bit by bit.

In a SLC, you can use regular N files for the control block so that will definitely allow you to compare the message control data at the bit level. I have not tried to do that with a Micrologix, and there is no guarantee that the different families structure things the same way, but while you are experimenting it is something to try.

It's been a few years ago, but it seems I read in one of the manuals for a SLC the definitions of at least some of those bits, and I do seem to recall descriptions including the type of message. Be aware that you may end up with a fat red fault LED, but if it is standalone test equipment, that may not be of any concern.

If you succeed, how is RSLogix going to react? What will happen if you have the MSG block open on screen and the message type is switched? Will it change the accompanying text description from "Read" to "Write"? These are just a couple of the questions that your experiment raises in my mind at first look.

 

[Firejo]

I’ve tried comparing the “MG” and routing files in my 1100 but I hadn’t thought to try that in the SLC’s (I’ll try that tomorrow). I couldn’t find any difference. I can change the “Target Device, Data Table Address” words and when you watch the message instruction run and they reflect the changes as they happen. Great questions with regards to the other things you bring up. I guess that’s part of the challenge and why I like trying to figure it out.

 

[daba]

I’ve tried comparing the “MG” and routing files in my 1100 but I hadn’t thought to try that in the SLC’s (I’ll try that tomorrow). I couldn’t find any difference. I can change the “Target Device, Data Table Address” words and when you watch the message instruction run and they reflect the changes as they happen. Great questions with regards to the other things you bring up. I guess that’s part of the challenge and why I like trying to figure it out.

Challenging yourself is a great pastime, I do it myself constantly.. but don't over-complicate things for others to unravel.

Store the knowledge for when it is absolutely necessary to use it, but keep things simple for general usage....