Hello jstolaruk, yes almost all the physical media has been swapped out save for a few things. It's not a big network.
Hello willxfmr, yes I've been using that Turck Tshoot guide, it is rather good but hasn't given me any clues. All device transceivers seem to check out, voltage levels, resistances, etc all check out. I am at the bottom of the rabbit hole now that I'm looking at individual data frames.
Hello Ken, yes about wireshark however that's not what I'm using. I have a 'CANalyst-II' device, that has a dnet script available in its' software but is actually just a generic CAN bus analyzer. It's chinese built and its cheap but it gives me wireshark-like data.
Anyway, looking at the timestamps I have:
-millisecond 407, node 7 response
-ms 412, node 7 response
-ms 434, node 7 response
-ms 467, node 7 response
-ms 473, node 7 response
-ms 497, node 7 response
-ms 507, close connection service request
-ms 507, close connection service response
-ms 525, node 7 response
-ms 531, node 7 response
My EPR is the standard 75 ms, however this particular node is a Rockwell safety block (1732 type) and so has a corresponding RPI setting in the I/O tree of the PLC, which is 30 ms for input, 60 for output.