I have a main computer that has a RS-485 board. It communicates with its sub systems via this comminication board.
I have also a test computer thas connected to main board via RS-485 channel. It will be used for test and update the sub systems using this RS-485 channels.

I want the main computer behave transparently to test computer so that the main computer wouldn't know about test algoritm. I don't want to update the main computer when the test algorithms change.
If I'd designed my hardware I would use a FPGA to connect test computer to sub systems physically so that the test computer would connect the sub systems directly and the main computer wouldn't know about tests, but I use standart computer and a communication card.

What would you do to connect the test computer and the sub systems without adding test related algorithms to main computer.