Modbus - Redundant Master (Redundant PLC)

[wirelessliquid]

Hi guys,

Just wondering if anyone has tackled an issue similar to this before: creating a redundant master on Modbus so I can have a redundant PLC in my unit.

I need to design a system where I have 2 PLCs (a shadow/redundant PLC with the same program on it) so if the primary PLC is to fail then the secondary PLC can take over.

The Comms protocol used is Modbus RTU (Carel Modbus RTU) and the controller of choice is Carel cPCO Mini. Each Controller has 2 Modbus ports.

With Modbus being a single master protocol, could I use the first Modbus port to connect the primary and secondary controllers together to create a heartbeat between the two controllers to detect if the primary is still alive and communicating. The Primary Controller will use the second Modbus Port and be set as a master to connect to all sensors and devices.

The Secondary Controller will also be set as a master on its secondary Modbus port but the port will be disabled if the heartbeat between the controllers on Modbus port 1 is healthy. If the heartbeat has an error detected, could I deactivate the Primary PLC Modbus port 2 and enable the Secondary PLC Modbus port 2. This way I should have a redundant master and only 1 master active at a time talking to the sensors and devices.

Will this work? am I barking up the wrong tree? am i missing some key info? I am not so familiar with Modbus as I have been mainly using Ethernet IP for the past 4 years.

Kind Regards,
Wirelessliquid

 

[OkiePC]

I know nothing about that controller, but as long as the backup RTU master remains silent when not in charge of communication and has a unique Station ID number on the RTU network, I think it should work. Those are big IFs. Some PLCs don't let you set up a port as Modbus Master and assign a Station ID. I'd be concerned that if both RTU masters have the same Station ID number, then they will both become active when any slave responds to a poll. If that happens it will interfere with communication.

 

[Steve Bailey]

Is Modbus the only option available to to you? It's not really a great choice for a redundancy setup. I can't see how your transition between primary and secondary in control of the outputs can ever be bumpless. The PLC not in control of the I/O won't know the states of the I/O devices.

I think you would need three ports on each controller to make it work. One port for I/O communications with both PLCs on the network wire with the I/O as masters, but only one at a time actually sending the Modbus commands. For the other two ports, one would have the primary as master and the secondary as slave. The other would have the secondary as master and the primary as slave. On those two ports you would exchange status data and in addition the PLC controlling the I/O would have to transmit the I/O states to the secondary along with any other data required to keep the programs synchronized.