AC800m PM856 and Freelance 800f comunication
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in case you have Freelance Version 9.2 or higher as Operator Station you may use the OPC Client functionality, to connect on the AC 800M using the respective OPC Server.
in case you need serial communication the AC 800F Controller must be equipped with FI820F Serial Module, you can use it as MODBUS Slave and have the built in COM port of PM856 as MODBUS master (default)
One good advice is to use the new Panel 800 from ABB as a gateway as it can read directly from AC 800M (ABB Control Network) and Serial MODBUS with the AC 800F (again you need the Serial interface) as it can run two protocols in parallel.
There is no reason why your Modbus link should not work. And there is no simple answer to your question.
Why do you want to link the controllers together ? How much data to you want to move between the controllers ? Are there any interlocks between the 2 systems ?
"Ethernet " is a hardware and basic messaging protcol. There is NO standard format for what the ethernet messages actually contain, and hence saying you want to use ethernet is like saying you want to talk to me by making noises, but not agreeing on which language we will use.
OPC is designed for moving lots of data between the controller and the operator display. however, it requires a PC, which then becomes a critical part of your DCS. You should not use OPC for passing critical signals like interlocks and shutdown commands from another controller.
If you have the hardware already, the Modbus serial link is quite simple to configure and test. Note that the AC800M must be a master and cannot function as a slave.
Profibus is fast, reliable and a good way to pass a lot of information between the controllers. However, both the AC800F and M must be a master, so you will require a master/ master interface. Anybus make a range of very good protocol converters that can do this for you. The anybus hateway will come with GSD files that you can configure and import to both controllers
In this kind of project, there is no substitute for reading the manuals, making a decision about what you need to do and then working through the steps rquited to complete the task.