ABS ALDL comms

[Dale Klein]

	Probably a few people have done this before, but gee it was fun.  Running 
in & out of the house with cables & computers & gizmos.
	I got to borrow a tech 2 today.
	On my '95 Z28 LT1, I plugged that into the aldl port, and had the laptop 
also listening on the aldl.  I had the tech2 ask the abs module for data, 
and recorded the comms on hyperterminal on the laptop.

Does anyone out there have any info about abs module aldl 
communication?  Even if it's just small caliber stuff like below that'd be 
neat.  I'm thinking, (and have heard others thinking) that it would be neat 
to have your logging program (pcmcomm, freescan, carbytes, tts, palm 
pilot!!!, or whatever) ask the abs module for front wheel speed.
Any comments or advice? (see the myriad questions below)

Thanks, Dale

	So a description of what seems to happen:  (let me know if you'd like a 
copy of the 9k file, I'll put it on incoming if anybody wants).

	first there's chatter, I think just from the pcm, but maybe not, before 
the tech2 was set to display abs data

then tech2 - abs module messages, vehicle not moving.  request alternates 
with reply

request, looks like it at least, always the same
F1 AF 01 5F
	header is F1 AF, 01 is mode or message
	5F is normal checksum. add first bytes, not, add 1 = checksum

while not moving, brake off I think
F1 E6 01 20 E9 9E 91 F0
	shorter messages, E6 can't be message length.
	note 20 E9 9E 91 are similar to last 4 data bytes of other messages...
	F0	correct checksum

in between not moving & moving, message gets longer..?
F1 E6 01 01 01 20 E9 9E 91 EE

while driving, low speed, very light accell, reply looks like, message even 
longer...?
F1 E6 01 0F 0F 0F 0F 0F 20 E9 9E 90 AE
	header is F1 E6, 01 is mode or message,
    but second byte E6 is not the same kind of message length (AF + 9 != E6)
    maybe there is a different definition of message length?
	0F 0F 0F 0F 0F looks like 4 wheel speeds & vehicle speed, could be bit=mph?
	20 E9 9E 8F	unknown. one bit somewhere in there may be brake light on/off
	AE 	checksum, but 2's complement checksum would be A6..., right? are there 
other sorts of checksums?  double checked a couple messages from this spot 
in the stream, and they both come out wrong?  doa!
	
while slowing back down, low speed, light decel, brake on
F1 E6 01 07 07 07 07 07 21 E9 99 91 D1
	D1   checksum, but 2's complement checksum would be D1... how bout that.

and coming back to a stop
F1 E6 01 01 01 21 E9 9E 91 E9
F1 E6 01 21 E9 9E 91 EF

later the tech 2 asks for the module calibration number data, etcetera,

and then chatter again, when tech2 not asking for abs data.


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