HMI Modbus Master-Slave


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I have trouble with multi HMI communication using Modbus RS485. I'm trying to communicate with 3 HMIs. As idea is that 1 HMI is Master device, that control 2 others, that Will power on/off lights or something like that. I have manuals how to do that, but with PLC. Is it possible to do it without PLC? I am using Weintek HMIs, one MT8070iE and two MT8050iE. Do anyone can give me some hint to solve this problem?

In order to "power on/off lights" you have to include the PLC on the Modbus network, unless it already communicates to one of the HMIs with another protocol on another comm port.

I can't help on the specifics of the HMI configuration/programming, but the rule is that an RS-485 network can support one, and only one Modbus RTU master.

With 3 HMIs and a PLC needing to communicate with one another, only one can be the Modbus master. Usually an HMI is the master in order to respond to local changes in things like pushbutton states. Otherwise, the master has to constantly poll the HMI(s) and analyze the replies for updated changes. But that constant polling and analyzing has to be done for at least 2 of 3 HMIs, if one of the HMIs is the master.

My SCADA work doesn't involve multiple HMIs so I'm stuck for deeper detail. Anyone else do multiple HMIs who can comment?

Bob Peterson

Most HMI terminals can only be masters (they initiate and control communications with the slave devices such as PLCs).

If you need multimaster capability on the same network, there are adapters that you can buy to do that, but straight modbus only supports a single master unit.