laptop scantool.

["Gregg Gruen" <gruegre@hotmail.com>]

I've just started really digging into the details of the ECM on my truck.  I
would really like to start taking a look at my stock calibration and so
forth, but first I wanted to put a scantool together so I can get some idea
of what my truck is doing before I go and think of making any changes.  I
plan to buy a laptop soon so I want to use a software based scantool.
Here's my trouble.  I've been reading everything I can find on the web about
how to interface and I have a few questions that I really hope someone
wouldn't mind answering for a beginner.

First off, I have a '93 S10 2.8 5 speed ECM 1228062.  I've looked at the
schematics for the serial interface and I started to put it together a bit,
but then I found another website that showed a parallel interface that can
be constructed with merely two wires.  Has anyone heard of this?  If so, why
does the serial interface seem so popular?

Secondly, assuming I have to construct the serial interface, do I have to
use a MAX232 chip for the interface, or can I used the less complex version.
I apologize that I don't have the URLs to put here where I found the
schematics, but I'm writing this offline and I don't have them handy.  I've
seen info on the 5v system which is what I believe mine to be.  The site I
read wasn't real clear on whether the less complex circuit design (I think
it just used a couple transitors) would work with the 5v system.

Also, I've read that some ECMs are capable of the 160 baud and the 8132 (??)
baud.  I took the liberty of running my multimeter on the E pin and I saw
some output just as I thought I would.  I ran it again on pin M not knowing
if there would be higher baud rate output.  I didn't see anything, but my
DVM is very old and has terrible refresh rate.  I'm assuming I only have 160
baud output.  If it turns out that I do have both, can I use either?

Finally, is there one final authority on what the output signal looks like
from the ECM?  I've seen a couple different diagrams of the output signal.
I understand that it's 6.25ms (at least for the 160 baud) but exactly when
should I read the bit?  I've seen a couple different ideas on this.

Thanks so much for the help.
___________________________________________
Gregg Gruen
Mechanical Engineering major, Illinois Tech
ICQ 15085467


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