The Nematron 120's and 200's SUCK VERY LOUDLY AND SUCH! They have very little tolerance for heat - the kind of heat that people can tolerate. And from what I hear from Nematron, those models are no longer supported.
I'm replacing one myself. Fortunately, I have a copy of the "display" pages with the associated Key assignments and the C-Relays associated with I/O. As part of the set-up, a few V-Mem locations in the PLC are dedicated to the communication channel. The V-Mem locations are specified during the initial creation/set-up of the Nematron Program.
If the V-Mem locations are not recorded somewhere, or, at least, identified by means of a V-Mem Name-Tag, then you don't know which V-Mems are being used. You can not retrieve that info from the Nematron - Nematron Tech Support says so.
Somehow, those V-Mem locations control the states of a range of C-Relays. Reading the manual has not shown me how this is being done! I haven't removed mine yet, but I'm concerned that I might have to re-write all of the affected code with new C-Relays.
Please keep us (me) posted with your progress!