By the way, I sent Lynn some email on this but I tested a PLC-5 over the weekend with the version F firmware on an IAP to do Modbus/TCP <–> AB/Ethernet bridging.
The PLC-5/40E is an older vintage (rev. A) so it doesn’t support SLC messages or multihop, which limits usefulness. I’ll do a more recent vintage very soon.
Anyways, PLC-5 formatted MSG blocks work just fine communicating to a Modbus/TCP I/O card on type 4 Modbus addresses only! It does not appear to work with type 4 addresses (N250:XX AB PLC addresses). With this particular card, that’s not a real limitation.
I also tested the IAP vs. the actual I/O card using Modbus/TCP <–> Modbus/TCP with no issues (mostly as a troubleshooting aid).
My next two sets of experiments will be with a newer vintage PLC-5 (which supports SLC messages and Multihop), and also with a card that has type 0 and type Modbus registers.
I didn’t test PLC-2 (“raw”) messages either because I didn’t see much point in doing so.
One warning about PLC-5’s: RS-Logix is kind of “stupid”. It will not error check against revisions so you can easily enter a Multihop or SLC-formatted MSG statement. Depending on what you do, it will give you an error in the MSG block, fault the processor, or outright crash and blank the memory! I managed to get it to do all 3 while testing.