modbus responce problem and repeater problem


Thread Starter


We are using modbus protocol in Ge multilin relays for all the drives above 75 Kw and for HT drives 1500 Kw we use alstom relay 241 series. We control, start and stop through modbus communication. Our PLC is AC450 of ABB system. After commissioning the system ABB now says the responce time is for start and stop will be 5 seconds. The five seconds also varies up to 12 sec in alstom relays to stop the coresponding drives. We already taken care by intoduced hardware tripping for the most critical drives. We like to know whether the response time will be 5 to 10 sec as claimed by ABB? We hardly exchage three to four register per slave. And we connected to maxmimum of 12 slaves in critical bus cables. Can any one give us sugest us how to increase the responce time in modbus communication

2nd question is: the communication always trips in the bus which have repeaters. this is hapening ones in two days? we solve by disconnecting the connector in the repeater and give start from PLC and reconnect the connector in the repeater.
I have implemented Modbus Communication Between ABB AC450 and different PLCs for a Thermal Power Plant.
As you have mentioned, if the number of slaves connected through a single port increases, the response time or the update time for each slave will decrease.
In case of AC450, the actual polling must be triggered from PC program. And at a time you can poll only one device.
And above all your network traffic and communication cables also tend to decrease the data flow rate.
Summing up all, I feel this 5 to 10 seconds can not be avoided.

You can get back to me at "[email protected]", mailto:[email protected] or "[email protected]", mailto:[email protected]
for any clarifications in this regards.
This does not seem like a Modbus problem unless you are running Modbus very slow. Sounds more like an interface program problem. Ask ABB for a timing map to show where all the communication timing delays are.

Bill Mostia
William(Bill) L. Mostia, Jr. P.E.
Excellence in Dependable Automation
[email protected](b) [email protected](h)
These opinions are my own and are offered on the basis of Caveat Emptor.
I know it is a while since you posted this question. Have you resolved this problem yet or are you still looking for a soution?

I have worked any dozens of ABB Modbus links and I disagree with the other replies to your queries that say you are going to have to live with a 5 to 10 second delay on executive actions on ANY ABB modbus link.

It will be possible to engineer a solution to your problem.