MODBUS/RTU RS232 question?

Question

Background- I am installing a MODBUS connection from a RTU, aTelvent 2300, to a DCS for set point control of generator output for ESCC AGC (automatic generation control). I am using Dymec multimode fiber transceivers to haul the RS232 from a Substation control house to the equipment room of the unit.

For test purposes, in the initial phases, I programmed an additional RTU and had good communication between the substation RTU and the test RTU that was simulating the DCS.

When the DCS was connected to the substation RTU the serial analyzer function built into the RTU displays the poll from the DCS as "FR FR FR FR FR FR FR". I know MODBUS protocol, so I don't really need an explanation of addressing, function codes, exceptions, ECT. I am wondering if RS 232 line analyzers normally display framing problems this way? What are some of the problems that would cause framing problems? Connection is 9600 8-1-none. Ideas welcome (DCS factory Rep who programmed DCS is on vacation in Hawaii).

Answer 1

No. I have never seen a serial analyzer that showed framing that way. You might want to rent a standard serial analyzer.

Actually framing errors usually show up as a failure to receive a message and never get through the system to the step at which any kind of exception code is declared. They just look like noise on the line and never pass the firmware tests needed to declare a message received.

Typically they are caused by an RS232 link that has low signal level, has interference from another master, has interference from another RTU replying with the same address, has incorrect baud rate settings, or has an actual hardware failure. Any of those things would show up with a standard data line monitor that would indicate garbled characters on the line and would show whether the problem was with the amster or other units on the network.

Answer 2

It sounds like the RTU might just be hearing crosstalk rather than data. Did you take into account that the RS232 data connections from RTU to DCS involve a crossover Rx > Tx etc which will probably be wired in the DCS? It's easy to inadvertently wire an extra one in when testing using two RTUs!