The only answer I can give you is that it could be absolutely anything.
What you need to do is open the machine up and find out where the data wire goes/what it connects to and based on that you should be able to drawn out a bit of the circuit at one or other of the ends. Then you need to know what voltage it's all working at; normally it will eventually reach TTL levels (0V low to +5V high). Then connect an oscilloscope or logic analyzer, capture the data trace and from there you can deduce the operating frequency and from there the baud rate of the data.
If you're lucky it will (as in my case) turn out to be a standard RS232 speed, you may need to analyse the data frame to determine what the data structure is, stop bits, parity bits etc. as there are many possible combinations. In my case the Daikin was 1 start bit, 2 stop bits and odd parity and running at 1200 baud.
A cheap logic analyzer is probably the easiest way to capture the data frames for analysis - you can find them on ebay.
It's a challenge but can be done. If you have no electronics/software (you will need this next) knowledge then you'll probably need to enlist the help of someone who does.
Nigel
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