communicating with a modicon 584 using rs422


Thread Starter

Denise Brown

i need to get data from a modicon 584 modbus that is on a process gas chromatograph (gc). it isn't a standard modbus. it has an abbreviated set of function codes available and the only communication protocol it does is half duplex rs422. i've gathered that rs232 is the standard, but my gc modbus is rs422. there's a catch: the communication protocol is half duplex rs422, but somehow the people that made my gc have it set up so the half-duplex signal is carried on full duplex wiring. i think that is one source of my problems because i don't understand how that was done or what it means as far as my computer interpreting signals.

the gc is part of a more complex system, and the control program for the whole thing is in labview. i've been trying to use labview to talk to the modbus on the gc. i started off trying to write my own code, but i ran out of time, so i downloaded a demo of busview and i've been trying to use that within labview to establish communication with the modbus. so far i get an error that tells me that either the controller is not operating or the cables aren't connected properly. either one of those things could be the problem, and i can't tell.

what else would you need to help....ah, i'm reading the signal using a national instruments 485 card, which i know is working properly because i tested it.

can anyone shed any light on this?