After more than five days of intense search on the internet, whether in Japanese, English or even French, and using Google, Yahoo, Baidu and Bing (shame on me), I didn't find any more information regarding that RAC adapter or the CN7 connector. It's almost as if I'm the first person ever to investigate a Hitachi air conditioner! Giving it a second thought, that might actually be the case...

Let's start again with a different approach. I realized my air conditioner is a Japanese-market-only model because no matter what, I couldn't find even a single document in English. Since I also couldn't find a service manual in Japanese, I thought, "Maybe I should look for a similar model intended for international market, or at least for English-speaking-countries".

So here I am, back to my old friend Google, and I searched for "Hitachi air conditioner service manual". First result seemed to promising: HITACHI RAS-60YH5 SERVICE MANUAL Pdf Download.
I opened it and started reading. December 2005, so it's older than mine but still... and them, by page 35, I'm like, "Are you kidding me?! That's a f***ing circuit diagram of the remote control!". I quickly flipped through the following pages and here it was, the circuit diagram of the air conditioner's mainboard. Unfortunately, no CN7 for this model, just a CN9 labeled HA. Okay, let's go crazy and download as many service manuals for Hitachi air conditioners as I can find.

Well, there are nearly 2000 entries for Hitachi air conditioners on that site, so I just downloaded 30 service manuals. After going through all of them, I found that 20 of them included a circuit diagram with CN7. Among those 20, 13 are for models from 2008 to 2010, and 2 are for models almost identical to mine (same specifications and the same remote control).

Exactly as I confirmed earlier: pin 1 is 12V, pin 3 is unused and pin 5 is GND. There's no label for pin 6, but CN7connector is a 6-pin PA connector from JST (datasheet).
All that's left is to figure out pin number 6...

Well, it's an input with a 5V pull-up resistor directly connected to the microcontroller. I'm willing to bet that the state of this input enables or disables the air conditioner's H-LINK connectivity function. And then, the final clue appearead.

That's not Japanese but Chinese. The character "有" is the same in both languages and means "active/present/valid". However, I didn't recognize the other character so I opened www.qhanzi.com and input the unknown character. It turned out to be "无" which means ”no/not/nothing". So pin 6, the H-LINK pin, is active LOW (inactive HIGH)!

What a great detective job that was! I found the exact pinout of the CN7 connector, and now know it must use serial communication. The last pin is an input that is active LOW to enable H-LINK connectivity function.