EEM vs. MSG Instructions - when to use which one?

[hpsustar]

I have been working on a project recently to control a PowerFlex525 drive using a SLC 505. By control I mean controlling the starting, stopping, frequency output, etc. of the drive using this SLC 505. I have written programs before to control this kind of VFD using ControlLogix and CompactLogix but never a SLC.

Through some research I have read that there are a couple different ways to go about this and I am just trying to figure out the practical and theoretical differences between the two. When is it better to use EEM instructions and when is it better to use the MSG instruction and are there pros and cons to using one over the other? What is the difference in using these two different msg types?

I ended up getting the program to work using MSG instructions and the N41 and N42 data files but still am very interested in the differences and pros and cons in using the EEM instructions and when that should be done instead of MSG instructions.

 

[ASF]

Just a heads up that controlling a Powerflex 525 over ethernet to an SLC is generally a bad idea regardless of which instruction you use. Ken Roach will probably be along shortly to emphasise this much more eloquently, but if you search the forum for "Ken Roach Powerflex 525 SLC" you'll probably get the gist of it.

Beyond that caveat - I don't think it makes a lot of difference which of the two you use. The manual even states "The instruction is similar in operation to a standard MSG instruction."

I would choose the MSG instruction purely because the EEM is largely deprecated as far as I know, whereas the MSG instruction still lives on in modern controllers. Although looking bigger picture - I'd choose not to do it at all. I would wire the start and stop to digital I/O, and just read status information back from the drive over Ethernet. If you need a speed reference, I would *consider* sending this over Ethernet, but if it were a critical, dynamic reference I would probably go looking for analog outputs to use instead.

 

[joseph_e2]

I had a colleague use a MicroLogix 1400 to do something similar with a PF525 over Ethernet. As I recall, he was just sending a speed reference with the start/stop handled via relays. The ML only supports MSG, not EEM. I remember it worked...kinda. It was klunky and he had to send multiple MSG instructions to update the speed command and it didn't recover nicely from a power cycle. Sometimes, it would start with a 0 reference even though the MSG instruction to set the speed would complete correctly. I don't remember his workaround but I think he just made it a habit to send everything multiple times regardless of outcome.


All that to say...I'm with ASF on this one. Use physical outputs from the PLC to control the drive and use the MSG instruction to read back status information.