VAG scan tool & codes for V8

[dave <dsaad@micron.net>]

Does anybody out there in Audi land know how the VAG 1551 scan tool
works?
I mean specifically what is the communication protocall between the VAG
and the ECU?
If I can find this info, I am considering building my own tool. I
suspect that it is just a faster version of the blink output but have
not yet put a scope on it to find out.
As far as scan tools go, are there any good ones available for
occasional users like me? (meaning cheap)
And finally, does anybody have a list of fault codes for a 1991 V8Q? I
am getting codes 3433, 3434, 3443, 3444 and can not find them listed
anywhere.

Thanks
    Dave