GM's ALDL info

[Ven Polychronakos]

In article <1992Jan5.084232.22926 at informatik.uni-erlangen.de>
orscholz at immd4.informatik.uni-erlangen.de (Oliver Scholz) writes:
>Here's what I know:
>
>The ALDL uses a TTL level output (1=H=5V, 0=L=0V) with 8192 Baud.
>A word consists of 10 Bits, the startbit (L), 8 data bits (LSB first) and
>one stop bit (H). The line is idle high.
>The ECM sends packets that look like this:
>
>1 byte device code
>1 byte: 85+N (N being the length of the data part of the packet)
>N data bytes
>1 byte checksum
>
>between two packets the line goes idle (H) for at least 10 bit times.
>
>Information still missing includes: A list of device codes, the interpretation
>of the data bytes and how to compute the checksum (which I probably could
>find out myself... I guess it's just adding up the data part of the package.
>
>So, once again, if anyone has any further information, please let me know
>(or better yet, post to the net. I know a *bunch* of people is interested
>in this!)
>
>-Oliver
>
>
Thanks for posting this information. I had figured out (the hard way)
the basic protocol but had (and still do) trouble interpreting what
I was reading. At least now I can stop trying to decipher the checksums
and device codes.  So here is my minor, I'm afraid, contribution
towards solving the puzzle. Below is a record of what the serial link
on my '88 Celebrity (2.8L, v6,VIN W engine)  tells me.  Car at "PARK",
engine idling. All digits hexadecimal.

0A 58    00 03 53    48
0A 58    00 03 61    3A
0A 58    00 03 6A    31
05 5F    00 00 00 3C 8C 39 62 01 EC 72    DA
0A 58    00 03 69    32
F0 55 BB

The first line, for example, shows device code 0A (10 Decimal), 3bytes
of data (58 hex=88 decimal, therefore 88-85=3), then the three bytes
of data follow and ,finally, the checksum byte 48. By the way these 
three bytes are zero when ignition is ON but engine not running.This 
device code seems to be repeated 3 more times in the same record!?
The device code 05 gives 10 bytes of data from which the only one 
I was able to figure out is the fifth one (8C= 140) which looks like 
the battery voltage, 14.0 V, when interpreted as tens of millivolts. 
When engine is not running it becomes about 12.0 V.Finally the device 
code F0 has zero data(55) and seems to be some sort of end-of-record
marker (BB by the way is the checksum of F0 and 55). 
The checksum byte is the two's complement of the 8-bit sum of all 
bytes in a given packet (except the checksum itself) not only the 
data part.  In plain english if you sum all bytes now including the 
checksum the eight least significant bits of this sum should be all zero.
If anyone has more information on the contents of these packets, 
I would appreciate hearing it.
Thanks in advance,  Ven.