ALDL timing (was GM 2.8L V6 ALDL)

["Paul Blackmore" <paul_blackmore@xtra.co.nz>]

I have learnt much about ALDL timing over the last few months, mostly
due to the hard work of Barry Tisdale, thanks Barry :)

I cannot gaurantee that what I say here is 100% correct
it's just what I and others have observed.

There are two types of 8192 baud ALDL communication schemes:

1.  Passive, where the ECM/PCM waits silently for a command. This
is by far the easiest scheme to deal with.

When a scan tool is connected it merely send commands when it
wants to and waits for the reply - pretty simple.

2.  Chatter, where the ECM/PCM and various other modules are
polled at regular intevals and are expected to reply to the
poller. The poller is sometimes called the bus master.

When a scan tool is connected and wishes to send a command it
must wait until there is silence on the bus. I have found that
most (if not all) chattering buses will send (at regular intervals)
a heartbeat. This heartbeat is usually a three byte frame consisting
of an ID byte, the hex constant $55 and a chacksum byte. Following
the transmission of a heartbeat frame the bus master will wait for
a period of time for a scan tool to request silence on the bus.
The window in which the request may be sent can be as small as 2ms and
as large as 50ms. (The exact timings appear to be ECM/Vehicle specific).

After the "silence" request ALL chatter will cease for about 5 seconds.
Chatter will stay suspended as long as successive, valid commands
(including mode $01) are received.

This raises problems with other modules that expect to see the chatter.
They will invariably log diagnostic trouble codes indicating that they
cannot communicate with the bus master. The scan tool should take over
the roll of polling these other modules.

On some vehicles the request for silence is the Mode $01 command itself.
On most other vehicle I have dealt with it is a Mode $08 command with
a Mode $09 command re-enabling chatter.

You can test out your ALDL interface using:
ftp://www.diy-efi.org/incoming/Scantool.zip


Not wanting to abuse the list with "advertising" here's some info on
my products and what they can/can't will/wont' do with chatter:

CarBytes does not support ALDL interfaces that chatter.

EFIlive v3 does. (with one exception: any chatter windows
less than 10ms is NOT supported - that means the 749) This is a Microsoft
Windows
limitiation.

EFIlive v3 is not free. (sorry I'd love to make it free
but I need to pay the bills)

EFIlive V4 should support any chatter window down to 1ms (using hardware
high resolution timers). It will be available later this year.

Cheers
Paul Blackmore


Date: Wed, 13 Sep 2000 17:45:49 -0500
From: "Shawn R. Lin" <slin01@mail.orion.org>
Subject: GM 2.8L V6 ALDL

I can get chatter from my '89 Olds Cutlass Supreme (2.8L V6 VIN W).
However, I can't get it to spit out Mode 1 data (or any other mode for
that matter).  I've tried sending it the Mode 1 string for the Lotus
4-cyl, '93 LT1 Camaro, and even tried sending it the string that's
preprogrammed into ScanTool.exe.  No luck.  Carbytes 2.0 shows that it
just echoes back.  For some reason, repeatedly transmitting the Mode 1
string for LT1 causes Carbytes 2.0 to lock up.  CTRL-ALT-DEL shows
"CarBytes [Not Responding]" and "MsgSrv32 [Not Responding]".

Anyone have specs for this motor?  The bin is ATFB3833 running in a
1227727 ECM.  The car has the U51 (Driver Info Computer) option.

I tried searching the archives but didn't come up with anything.  The
only archives I found was GMECM99.TXT, are there any older/newer
archives yet?

Shawn


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