How to identify the Modbus master & slave?

Hi all, I’m using in my current project an RTU composed of a pair of redundant CPUs (a master and a standby) that I need to connect to a panel view via a MODBUS RTU protocol (using RS232). My question is: who would be the modbus master and who would be the modbus slave knowing that data is shared in both directions in this case (read data from the rtu to display it on the panel view HMI and command field instruments connected to the rtu from the same HMI)? and how to identify the master and the slave in case i decided to connect my system to another third party ? Regards.

Use a heartbeat bit. Heartbeat bit will help you to identify whether the PLC is running or not. If your redundancy is passive,only one PLC will be running at a time.

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In Modbus, a slave is silent unless spoken to by the master. A slave can not start/initiate a Modbus query/message/poll.

If the HMI is used for issuing commands, like start or stop, then the HMI must be the master and the PLC must be the slave.

Field instruments or analyzers are almost always Modbus slaves. To accomplish Modbus RTU communications between a PLC and Modbus field instruments, then you need either

  • a PLC that can operate as a master on one comm port to communicate with the field instruments, and as a Modbus slave on a 2nd comm port to communicate to the HMI, or

  • a transfer data transfer scheme where the master HMI gets data from the slave field instruments and then writes that data to the slave PLC using separate transactions.

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