ALDL

[Dale Ulan]

> sdbartho at cca.rockwell.com Wrote:
> | 
> I don't think there is a way to send commands to the ECM on the ALDL system, 
> since there is only one data line.  The ECM transmits it's info over the line, 
> the dash listens in for some info it needs.  As far as I know the different 
> resistances on the A-B line set the different modes.  This applies to the 
> 8192 baud stream in late model corvettes, others I don't know.

That sets the ECM diags mode, the serial data mode can be set independantly
of the ECM diags mode. Normally they are linked. The 8192 ALDL
setup indeed uses bidirectional data on the one line.... it's a master-
slave protocol, with the ECM being the slave, and the diag tool the
master. They do this by Hi-Z buffers on the line (or by using an open-
collector output stage, depending on who you talk to).
I believe some test modes change the data stream with the ALDL resistance,
but far more than that is accessable.
Via this bidirectional data line, you can also enable and disable various
test/diag modes... far more than the resistance of the A-B line. Via the
ALDL connector, development versions of the ECM can re-program the EEPROM
while driving, as well as putting in a fast watch on one or two variables
instead of the whole list. This allows you to look at things like the O2
PID controller at the speed it actually runs at (many Hz). I've seen
enough of the code for a late-model vette to see what you can do, but not
how to do it. It would take me longer to figure it out than I feel like
putting into that project.

> As far as the 8192 stream goes, that is a topic that comes up here, on the 
> hotrod list, and on the corvette list regularly, as far as I know no-one has 
> decoded it themselves or pried the info out of GM.

It's a lot of work, and for the same amount of work that goes into
reverse-engineering an ECM to read out the data in a meaningful
manner, you can do your own entire ECM project. It took me two years
to get the C3 ECM (old data stream) figured out enough to be able
to read the data and change all of the tables and constants.

-Dale