What protocol should i use to connect PP877K to AC902F?
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I would suggest using Modbus TCP. You can set it up Modbus TCP Server easily with Freelance Engineering (it requires configuring an interface and communication FBs) and then import that configuration into Panel Builder (Freelance can export Modbus configuration in Panel Builder format). I also think there are some apps (solution examples) on AKS for this kind of communication (I found at least one, see below).
If You need more info on how to configure Modbus TCP, check the Freelance 2016 Engineering Manual - Communication and Fieldbusses or just ask a follow up questions.
Hope this helps,