Dear List Members,
We are a Petroleum Products Pipeline. We are using S-3 SCADA for monitoring and control purposes. The Master Control Centre (MCC) within our system gets data from RTU's at block valves and from PLC's at the booster stations. Earlier the MCC was polling individual PLC's at the booster stations over MODBUS and the system was working satisfactorily. Now we have gone in for local server to MCC server communications via a router. Thus there is a LAN to LAN connectivity. We are using the DDS driver within the SCADA to do it. The problem is that the Local servers tend to lock up rather frequently, twice to thrice a day. We suspect a bug in the driver for the protocol but are unable to nail it. As we are using V.35 connections to the routers, we checked them up and found that there were errors in the communications. The number of errors has been brought down but not eliminated. This also has not solved our problems. Currently we have gone back to direct polling of the PLC's and system is stable but there is a serious loss of functionality.

Stats:
MCC (Master Control Centre) and SCC (station Control centre) servers are Digital Alphas running Windows NT 4 SP5. One MCC is networked with 11 SCC's. The backbone is optic fibre, WAN protocol is V.35. Communication channel errors used to be 200-300 times a day, have been brought down to 10-12 a day.

We are thinking of trying out the MODBUS/TCP driver available in the application. As we are using modbus over RS-232, could somebody point us in the right direction to get started. The second question is has somebody done it differently with this SCADA.

I have kept out a lot of details to keep this short but i think i failed ;-). Thanks for getting this far.

regards,
Tomy Zacharia
Telecom and Instrumentation Engineer

 

V.35 is a synchronous interface, performance of which directly depends on synchronisation of clock signals at both ends. Stability of WAN channels directly depends on the V.35 interface provided.Once the WAN interface gets stabilised, the data transfer should become smooth. I also believe, the same mechanism is under operation in other pipelines with few SCCs. It would be interesting to know the feedback from those sites. Of course, with huge number of WAN channels, performance at MCC directly depends the corresponding driver. It would be better if the views of the manufacturer of S/3 is sought on this scheme. When SCC to MCC communication is through WAN (through DDS driver of S/3), having MODBUS protocol in TCP/IP will not help the cause in any way. As you have pointed out, you have a ready made alternative in going back to first scheme of polling directly from MCC on Asynch. channels!!!. The key to stability is the clock synchronisation between MCC and all SCCs. It would be better to establish stability of V.35 interface before doint anything at SCADA end. Of course, there is no ready made solution to project specific problem.

This answer has become detailed in view of the detailed nature of question!!!!

 

My former company did a lot of communication work between PLC's and PC's. check out "www.comtime.com":http://www.comtime.com and give them a call. Ask for Bill Hayes