Thanks in advance,
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A couple of hints, not knowing what youre intentions are:
1) Use a 3-party OPC Server and PLC Connect System Extension, go over Ethernet to Siemens PLC for most applications. Gives you 800xA look-and-feel DCS abilities.
You have to design your own object-oriented library to make it fit to the 800xA object model. This alternative is very scalable and integrated, but PLC Connect comes with a price (license).
2) There are also built-in serial protocol to AC800M controllers or possibilities to use Profibus DP-DP. This is if you want the communication to be done on the PLC-level.
A bit more cumbersome but doable (if there are timing issues/demands from process control). Profibus could require extra hardware, depending on your setup.
3) If you only are going to deploy a few signals you could also go for generic-OPC, you still need a (3-party) OPC. It´s a cheaper solution with limitations to scalability.
Answer depends on, what kind of communication cards AC800M and Siemens PLC have?
If both of them have Profibus DP card, then simplest way is Profibus DP, if both of them have ModbusTCP then simplest way is Modbus TCP.
If you can give more information about your architecture, we can give you deep advise.
Hope it helps you.