Replace the PLC5 with a ethernet model like a PLC5/20E (you can get them used for AUS $5k) probably cheaper in singapore . its a simple slot mount replacement of hardware,the software migrate for the PLC 5 is easy (a 3 minute job with rs logix, open the file offline and change processor type, resize data table when prompted) then use ethernet for comms. i have done this many times in my plant