New ALDL 2 transistor circuit (critique)

[Malcolm Robb, LC 0112G]

I'm not sure this is within the scope of the lists intended use, but if you 
want to flame me, please do it off list !

RS232 is almost bound to work at 0V/12V. The nominal threshold level is 1V 
(+/- 0.25V of hysterysis) in my copy of the spec. Take a look at the spec 
for the SN75189 line receiver - the original and IMHO still the best RS232 
line receiver. (I'm biassed coz I've got hundreds of the things in my 
drawer !) The only way you will get a '189 to not work is if you muck about 
with the threshold voltage pin. All the other devices I've ever looked at 
would also work - It isn't good practice to have the threshold of a 
receiver at 0V, because a line short to ground plus a small amount of noise 
will cause false receptions.

I agree that the circuit could be more susceptible to noise than a pukka 
receiver, but I have had no problems with it. I can show using a storage 
scope that the problems I had with corrupted data on my PC were due to the 
ECU transmitting truncated data during cranking & high engine loads. And in 
reality, if you are getting 5 to 10 frames a second, it isn't too important 
if you loose the odd one or two due to corrupt data (IMHO).

The 10 pin ALDL connector is used on all GM/Vauxhall/Opel cars produced 
here in Europe since about 1986. It is also used on a number of Lotus's, 
including the Turbo Esprit, which is what Andy Whittaker has. The full 
pinout of the Vauxhall/Opel ALDL connector is :

A : Ground,
B : Diagnostic Excitation Lead - Engine Electronics
C : Diagnostic Excitation Lead - Gearbox Electronics
D : Diagnostic Excitation Lead - Digital Dash & Trip Computer Electronics
E : Diagnostic Excitation Lead - Engine Telltale ?
F : Battery Voltage (+12V)
G : Bi Directional Data Lead
H : Diagnostic Excitation Lead - Electronic Ride Control & Alarm 
Electronics
J : Diagnostic Excitation Lead - 4 Wheel Drive Electronics
K : Diagnostic Excitation Lead - ABS Electronics

As I said in an earlier EMAIL, most european cars use Bosch electronic 
units throughout. However, a few cars are fitted with the GMP4 ECU, and 
they only have A, B, F and G in the wiring loom, because the Bosch and GMP4 
ALDL data signal (on pin G) is incompatible between the two. My car 
actually has 2 ALDL plugs - one for all the Bosch equipment, and one for 
the GMP4.

Hope this helps,
Cheers,
Malcolm Robb, LC 0112G

-----Original Message-----
From:	Peter Gargano [SMTP:peter at ntserver.techedge.com.au]
Sent:	Monday, May 08, 2000 5:50 AM
To:	gmecm at diy-efi.org
Cc:	mail at andywhittaker.com; amattei at mindspring.com
Subject:	New ALDL 2 transistor circuit (critique)

I've had a few questions about the two transistor ALDL interface
circuit appearing on Andy Whittaker's web site. The page is:

 http://www.andywhittaker.com/ecu/aldl_hardware.htm

and the circuit, attributed to Sanjaya Vatuk is:

 http://www.andywhittaker.com/ecu/images/Simple_ALDL_Interface.gif

I'm sure this circuit will work on certain PCs. The problem
is that the voltage levels on pin 2/9 (RxD) vary between 0
and +12 Volts. Unfortunately, RS232 is not guaranteed to
work reliably at a lowest voltage level of 0. OTOH, most
laptops seem to use CMOS transceivers that will record a logic
LO at a voltage level of 0 Volts.

The two transistor circuit may also be more susceptible to noise
pickup.

Another point, the ALDL connector is shown on Sanjaya's
schematic, as well as Andrew Mattei circuit on Andy's site,
as a 10 pin connector. I'm not aware of any 10 pin connectors,
(but I can be convinced otherwise) just the 12 pin ones
shown at Andrew's (official) home page:

   http://www.mindspring.com/~amattei/aldlif.jpg

I'd be interested in alternative views on this. I was unable
to find any kind of home page for Sanjaya Vatuk.

Peter
------------------------------------------------------------------------  
----
To unsubscribe from gmecm, send "unsubscribe gmecm" (without the quotes)
in the body of a message (not the subject) to majordomo at lists.diy-efi.org



----------------------------------------------------------------------------
To unsubscribe from gmecm, send "unsubscribe gmecm" (without the quotes)
in the body of a message (not the subject) to majordomo at lists.diy-efi.org