Being dependent upon RS485, actually using the USB-port of the RUT955 with an externally powered RS485 converted attached, which works very well.
However, coming back to the original problem, to directly use the RS485 port, first of all I have to note, that I did quite a lot of custom programming on the RUT955, running special LUA-code to do the ModBus/RTU comms, i.e. to do fast polling, pre-processing of process data etc. Systematically checking the non-working setup, using the built-in RS485 port, I have the following technical question: Because the problematic setup is running in the USA, any possible issue because of 110V/60Hz power supply ?
BTW: I found another post here on the forum, stating possible engineering issue on the RUT955 regarding RS485 port. Search for topic 'RUT955 RS485 tty port issue'.