ABS ALDL comms

[Len Sabatine]

       P - VCM talking to BCM. BCM "sleeps" after 20 minutes of inactivity. 
Intiial comms send command to "wake up ".
       and "Ok I'm Awake " reply, via UART. This area can be defined as 
inter module communications. The comfort and
       entertainment bus is another module, if option are in place , you 
can tune up your radio features, for example.
       Proceed with caution in communicating with BCM ,very undesirable 
side effects can be initiated. Nuff said.
       Len

>         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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