How do i interface an ABB 800xA (using an AC450 PLC) with an ABB PM592-ETH PLC and be able to view and control
You add PLC Connect to your 800xA system and talk to the PLC through the OPC server.
If you only have a few signals in the AC500 ( say less than 100 or so) then use Generic OPC Connect - which is included in every 800xA system and needs no extra license.
The AC450 controller is not part of the HMI for the AC500.
If you want to communicate directly between the AC450 and the AC500, then probably a Modbus serial connection is the easiest and most reliable, but not cheap unless you already have a modbus MVI in the AC450.