Need help - trying to communicate with 4 Allen-Bradley VFDs and 4 york chillers via Modbus RTU to thru a Moxa RTU to TCP bridge to a Kepware OPC server.

I keep getting errors and I get data some of the time. Even when I connect just one device with 10 second poll rate I get message timeout error. ?? I am a HVAC guy, Bacnet and Lon, so Modbus is new to me. Where can I go for help?

 

I'm sure if you call Kepware, the technical support will help you out.

 

I think that's the problem of response timeout in Modbus. When you use the Modbus RTU-to-TCP gateway to convert the data, the gateway plays the RTU-master rule on the serial side. So you have 2 response timeouts to find out before the control network can work well. Normally the resonse timeout on the TCP master (which should be setted on you OPC server) is not big problem. Get spec of the serial slave (VFDs?) and find the recommended response timeout, then configure the response timeout setting in the gateway.

Here is an article about the Modbus response timeout:
http://www.moxa.com/news_events/connection/conissue_2007_03.htm

 

Thank you you Enzo you actually hit it on the head. The Moxa has a time out setting on the serial side and KEPServerEX has a timeout. If the MOXA is in the process of timing out on the Serial side and the request timeout occurs in KEPServerEx then we retry the request. You can get into a loop state where the repsonse from the device do not sync with the requests from the server. They way to resolve this is to set the request timeout for the device in the server so that it is longer then the entire timeout and retry cycle set in the MOXA.

You can can call Kepware Technical support directly or e-mail us at [email protected] if you have any additional questions. You can also use the support form on our web site which is located at http://www.kepware.com/Support_Center/support_technical_form.asp

Fred Loveless
Support Manager
Kepware Technologies
HTTP://www.kepware.com