from the Automation List department...
Local and wide area networking in factory automation. topic
Posted by Bruno on 1 May, 2005 - 11:45 am
I would like known if modbus have support to multimater message. I need link two master controller, and I don`t known if is possible use the modbus TCP.


Posted by Pat T on 2 May, 2005 - 4:52 pm
I have used ModbusTCP in applications where the PLCs are getting polled from 2 different masters which are not synchronized, and it works well. In fact, I can connect to the PLC with the ladder logic interface program with my laptop while the PLCs are still being polled by the masters with no ill effects. The ethernet speed of the connections on the PLC is what keeps things running, and I would say there are very few instances where 2 different messages are arriving at the PLC at the exact same time. You should have no problems, and if you do notice excessive data collisions, you could reduce polling speed slightly. Once you use this, you won't want to use serial communications any more than you absolutely have to. Good luck!


Posted by Bugi on 2 May, 2005 - 5:42 pm
Base on my experience, you can used HMS - Anybus... please check

Posted by Steve Young on 2 May, 2005 - 6:20 pm
Modbus serial = 1 master per network, modbus ethernet Tcp/IP = peer to peer (ie multi-master). If you must use serial and need multimaster you will need a device in the middle such as a prolinx gateway that can talk to the device, store the data into dual port ram and then allow the data to be accessed on 2 additional ports.

Posted by Anonymous on 13 May, 2005 - 1:50 am
You could also check out Multenet Modbus Bridges.

Posted by Mark on 3 May, 2005 - 11:47 pm
Modbus/TCP is a client/server protocol that supports multiple masters. One thing to watch out for is low-powered embedded devices that only support one connection at a time, in which case only one master would be able to communicate with the slave at a time.

To familiarize yourself with Modbus/TCP, you can download a fully functioning Modbus/TCP master and slave ActiveX Control from Automated Solutions' website

You can run the pre-built example applications on one or more PCs to demonstrate Modbus/TCP capability.

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