Right, Thumper I understand that you want to get the data.. but the reason I am is, its not like you can just put a pi on the network and suck in traffic. I dont know if a PI has the drivers to exchange on Ethernet IP
Rockwell is a CLOSED architecture, with really proprietary data structures called tags. as far as I know there are a couple of driver sets that you could use to read the tag data, but most are for C# or Visual C#. I dont know if there is a mono implementation for ARM that you can leverage to run those.
There was the LibPi PLC project but I dont know if its been kept up….
You COULD.. read modbus values or get OPC data or a bunch of other things to interface the controller and get values and data… but i really think it will be tough to get native AB tags.