I'm new here and my knowledge around electronics is quite limited so bear with me please !

I also don't know if i should be asking this question on these forums, but i thought a lot of knowledgeable people are here so i took the shot.
What i want to do is interface a raspberry pi to a heat pump. ( the brand i do not want to detail at the moment )
I have an interface cable that works fine. I can use it with the manufacturer software in windows.
BUT. I actually want to know what it is. And if possible construct/supply them my own, because i think they are not that special. (probably not worth the price they are asking! )
I have uploaded 2 photos for you to see:
Http://users.telenet.be/bart.e/cable1.jpg
Http://users.telenet.be/bart.e/cable2.jpg
As you can see it uses a max232 chip. So i gues it's TTL to rs232?
It also uses a usb cable solely to supply it with power. Communication is done with the d9 connector.
Can anyone help me analyze this piece of equipment and tell me if i can build my own interface to the raspberry?
Thx!
Bart