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redundant Modbus TCP/IP communication using S7 1200

7/4/2013 2:37 PM	Rate (0)
BungFer Member Joined: 6/22/2013 Last visit: 2/25/2025 Posts: 7 Rating: (1)	Hi everyone! I'm currently working in a project in which I need to establish redundant Modbus TCP/IP communication with a master, in the beginning it was going to be RTU redundant communication, so I was going to use an S7 1200 PLC, but now I think I need to use a S7 300 PLC. Did anybody find the same situation as me? In this case, which components I need to use? Thanks in advance!

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7/5/2013 12:34 PM	Rate (0)
Denilson Pegaia Platinum Member Joined: 9/23/2005 Last visit: 3/14/2025 Posts: 4406 Rating: (1475)	Hi, as product, the MODBUS TCP library was not concepted to manage redundances... However you could try somethings as programmer: If your CPU is server, you can try otbuild two (parallel) connections (two server blocks, each for a different connection, i.e. different ports/ID), and let the client to manage this.
	Denilson Pegaia Siemens Industry Sector Technical Support
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7/8/2013 10:14 AM	Rate (0)
BungFer Member Joined: 6/22/2013 Last visit: 2/25/2025 Posts: 7 Rating: (1)	Good morning Mr. Pegaia, First of all, thanks for your answer. In my application the PLC is server, communicationg with the DCS, and as far as I know the S7-1200 has only one TCP port in the CPU without TCP communication card posibility. I was thinking to use two RTU RS485 cards and two different RS485 to TCP/IP converters. But I'm not sure what kind of problems I can find using this solution, because I never did it and, moreover, I'm so fresh in PLC programming. I think both communications will have the same IP address and the server will need to treat this two communications by separated, using two different switches. Am I right? I'm a bit lost in this topic.

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7/8/2013 11:17 AM	Rate (0)
umencho Diamond Expert Joined: 3/21/2006 Last visit: 3/14/2025 Posts: 10239 Rating: (1183)	Pegaia Hi, as product, the MODBUS TCP library was not concepted to manage redundances... However you could try somethings as programmer: If your CPU is server, you can try otbuild two (parallel) connections (two server blocks, each for a different connection, i.e. different ports/ID), and let the client to manage this. Hello, So, S7-1200 will be Modbus Server. The other side will be Client (two clients). All Modbus clients access the Modbus server through Port 502. In this case, it is necessary to have constant job-controlled establishment and clear-down of connections. The Modbus server can communicate with only 1 Modbus client through Port 502 at any one time. The connection to the first Modbus client must be cleared down and Port 502 released before another Modbus client can access the Modbus server. As soon as Port 502 is released, another Modbus client can access the Modbus server via that port. All these things (managing the connections) to be done in Clients. What is necessary to be done in S7-1200http://support.automation.siemens.com/WW/view/en/59381441(As Pegaia wrote there must call two server blocks). Best regards, Hristo Mihalev

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