800xA, PLC Connect (redundant servers) with Modbus TCP/IP (redundant PLCs)
I am looking for solution on how to connect the 800xA to third party redundant PLC controllers (two different IP addresses).
The 800xA PLC Connect service is configured to run in redundant configuration on two servers (CS1 and CS2) with redundant ethernet connections. One PLC Connect service has to be able to communicate with two PLC controllers on both networks.
Additionally - to make it more complex - the Primary PLC controller is identified by a bit value (coil register) received via Modbus TCP. Moreover the commands / setpoint etc have to be sent down to both PLC controllers so they are both updated with latest data in case switchover occurs.
Is it possible to make this kind of configuration using only 800xA or it has to be done via third party OPC-Modbus server?
Thank you in advance
Hi. I guess the first thing that should be clarified before such a connection is made is that Modbus does not inherently support redundancy. Therefore all such issues are then "engineered" to suit and thus are not 100% the way the end user may like it. I think you need to consider this following scenarios: Failure of PLC1 or PLC2 and then letting the CS's know where to get their data. Same for failure of either ethernet connections and how does this effect your data integrity.
From your diagram I would suggest a single dedicated network for the Modbus and a separate network for 800xA. This is a single network (as the PLC's do not have redundant ethernet anyway, there is no sense in having dual networks). That is have a single connection to each CS to each PLC, with no cross over connections to each PLC nor CS (think of a single wire). This way any "connection" issues are covered and the 800xA PLC connect will have the redundancy covered within the 800xA software.
PLC connect is fully redundant and therefore loss of Communication is covered (Ethernet loss), however there is no easy way to force the CS's over when a PLC fails, as the "connection" may still be intact at that time (ie ethernet has not failed), unless there is something in your setup that can give you this "failed data state".
As much as this will add to your cost, from my experience I would use OPC and get a Modbus OPC server, then have 800xA PLC connect connect to this. You have better control and better redundancy concepts. The Modbus OPC server would run on your CS's directly, so you will need to worry a little about about compatibility but should not be a big issue.
Anyway good luck.