Modbus Subroutine

Here at our station, they recently had to replace a bad S1 controller with a new S1. Now, one single Bristol RTU (serial communication) won’t talk to it. There are 3 RTU’s and the other 2 still work. As far as I know, no changes were made to the programming. It accesses a subroutine called “MBMaster Read Holding RegistersTW”.

The subroutine is a Modbus serial V1.1 program from 2007. It successfully opens a session with the RTU, but when it gets to the transmit and receive portion it delays out and never receives data. Any idea where I can look or what I can check? We verified the RTU is working for other communication to our SCADA network, but it won’t talk to Opto!!

chart snapshot|690x430

Quick first off-the-top-of-my-head thought is if the termination is set the same.
If you still have the orignal PAC S controller, check the dip switch settings and make sure the new controller is set up the same.
(If the Bristol is at the end of the line, or even in the middle, if the termination of the line is not correct it could be seeing all sorts of weird reflections and thus timing out).


Yes, sir,

It is the RS232 connection and all the physical connections were verified and wiring as well. It tries to open a session with the RTU and it looks like it never gets there. I’m thinking with the new firmwares installed maybe I just need an updated subroutine chart. Oh, and the one return message I get is Timeout.