Since I now have an easy access to the CN7 connector, I decided to hook up my oscilloscope to check if there's any signals on pins 2, 4 and 6.

I connected my Rigol DS1054Z and checked for any possible signal or voltage changes when:

• connecting the air conditioner to main AC
• turning the air conditioner on
• turning the air conditioner off
• pressing a button on the IR remote control

Sadly, this test didn't yield me any results. Once the air conditioner is connected to the main AC, these 3 pins stay at a constant 5V don't change. I also probe the 12V and NC pins, but as expected, the 12V pin remains constant once the air conditioner is powered up, and there's absolutely nothing on the NC pin.

I began to doubt whether this old air conditioner could really be connected to CS-NET via H-LINK, but after digging through that site I found earlier, I came across this schematic/wiring diagram.

It's clear that this air conditioner can be connected to CS-NET via H-LINK. I will just have to keep playing detective for a bit longer and continue looking for clues.