To begin with, what am I dealing with?
It's an air conditioner made by Hitachi, model RAS-LJ22Y, manufactured in 2009.
First, let's have a look at the manual and see if:

• there is any mention of connectivity options
• there are accessories that can connect to it (like a wall remote control, for example)
• there is any details regarding its electronics (I highly doubt that but who knows!)

With the help of my friend Google and the magic keywords "取扱説明書" and "RAS-LJ22Y", here is the manual.
→ https://kadenfan.hitachi.co.jp/support/raj/item/docs/ras-lj22y_b.pdf

After thorough reading, I found something quite interesting on page 26.

The text in bold enclosed in the small black rectangle says:
●To connect to HA system, the HA connection cord sold separately is required (service part number RAS-2810RX100).
●To connect to H-LINK system the RAC adapter sold separately is required.
It also mentions that HA system connects to CN6 connector and the RAC adapter to CN7 on the mainboard.

That's a promising result! 
Despite being quite old, this model supports two types of wired connectivity.

Next step is to look for what the HA and H-LINK system are and what kind of functionality they offer. Again, with the help of Google, I found this PDF from Hitachi explaining what HA system is and how it works. HA stands for "Home Automation" and it is a system for controlling the power operation and monitoring the power status of home appliances equipped with an HA terminal. In other words, it allows you to remotely turn a device on or off, and know at any time whether it's on or off.

That's not bad but very limited, so let's have a look into H-LINK system. 

After a brief search, I found another PDF from Hitachi specifically about H-LINK and the RAC adapter. Google search engine power? Just some luck? After a closer look, I realized that PDF is hosted on the same site where I found the previous one about HA system... I'll need to look deeper into it, as I might find more useful information. Anyway, H-LINK is a Hitachi proprietary two-wires communication system for connecting air conditioners to Hitachi's building air conditioning management system called CS-NET. Here's the best part so far:
●From CS-NET, you can control and monitor not only the power mode (ON/OFF) but also the set temperature, operation mode and air flow mode.

This small explanatory diagram shows how a remote control is connected through H-LINK to the RAC adapter, which then connects to the air conditioner.

As a conclusion, this model has two types of wired connectivity, one of which is for connecting to a commercial management system that allows to remote control of its various settings. The next step will be to gather more information about the RAC adapter in order to figure out what kind of communication protocol is implemented between it and the air conditioner.