What You'll Need

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Two-Transistor Interface 

As if things couldn't get more simple! According to its creator, Sanj, this works fine and fits easily within the RS232 D-type shell.

Sanjaya Vatuk can be found at this address: http://turboesprit.tripod.com/

I'm hearing that you may have to add two diodes in series with the 10K resistor in the base of upper transistor in certain applications for reliable operation.

Advanced Interface with Esprit Modifications

This interface was drawn by Andrew Mattei, his website can be found at www.akmcables.com

Andrew also sells an interface through his electronics company:

www.akmcables.com 

Expect to pay in the region of $70 - $100 for a fully assembled unit. 

I've modified the drawing so that it will work with a Lotus Esprit with GM ECU.

The transistors quoted, 2N2222A, are not critical. Any general-purpose transistors will do. I have drawers full of various types including BC107, BC108 and BC109. I built the above circuit using BC109 types and it works great.

What MAX232?

The four capacitors (called pump capacitors) connected to the MAX232-CPE are dependant on the model of MAX232 you buy. The MAX232-ACPE is a "higher speed" device which can use 0.1uF capacitors, this should reduce the size of your interface but the chip is about $5.00 per unit as apposed to $2.00 for the MAX232-CPE.

The MAX-232-ECPE is the Electrostatic Discharge Resistant model, therefore it's more robust and it's about $3.00 per chip.

The MAX232-EPE is the high temperature version of the -CPE chip, and it's about $3.00 per chip as well. 

The MAX233-CPP is the "standard" no external capacitor model, and it's about $7.50 a chip. Although, when capacitors cost $0.16 each, it's easy to buy four of them for $0.64, assemble them with a $3 chip, and end up with the same thing as a $7.50 chip. It's a little more soldering, but costs half as much...

Similar Powered Interface

I found this interesting interface while browsing. It's off Carsten Meyer's ALDL page see it at http://pweb.de.uu.net/pr-meyer.h/aldl.htm 

His software will not work on an Esprit but there's some good background information about the GM ECU. He also has some DOS source code written in Pascal.

Also note here that the capacitors are as I've commented previously. This circuit operates just as well as the more complicated circuit above but remember that you are directly connecting the MAX232 to your ECU ALDL connector.

Professional Interface

Garry Harris, who writes software for GM ALDL systems, kindly suggested this interface. Garry is also working with Ian Levy to decode the Lotus Elan M100 data-stream.

View Professional Interface

Garry sells his software through TTS Power Systems, and makes the software available for free download to encourage its adoption.  It will freely playback any recordings, but has the "free" recording capability limited to 5 runs for evaluation purposes. You may wish to check it out!

A Universal, Ready-Made Interface

Thanks to Bryan Boyle who suggests this:

The MAKInterface is ready-made and you can order one with your VISA card. By all accounts, you could even interface it with your lawnmower! Visit:

http://www.makinterface.de/index_e.htm 

for details.

Maplin Electronics Interface

NOTE: I have been having a few problems with this interface. It appears that the 74HC266 becomes unreliable after transmitting for a while. It's probably due to the maximum current delivery that the device can handle. I suggest trying to source a 74F266 device. I will try this and post my findings here.

UPDATE - 20th November 1999: Remove the 74HC266 and insert a 1K resistor and Diode into the socket. Look at Similar Powered Interface below.

If you live in the UK, I've found that Maplin Electronics sell a RS232 to TTL converter unit. You'll need to order a LM75, RS232/TTL converter kit. This is a nice kit because for £14.95 you get a kit that has the open-collector output drivers (courteous of a 74HC266) that are needed for bi-directional communication. Click here for the circuit diagram. You'll notice that I've scribbled on it. This is because of the following:

	It needs to produce bi-directional TTL data.
	It only needs to be two channel (the unit is four channel).
	It requires 12V.

You'll will also have to order an additional 7805, 5V voltage regulator and a couple of 100nF (0.1uF) ceramic capacitors which need to connected across the input and output of the device.

Testing The Interface

You'll only need 3 wires on your RS232 link:- Gnd, TX and RX. 

We require no hardware flow-control. Therefore, link together DCD, DSR and DTR. Also link together RTS and CTS.

Testing you've succeeded is easy. Power your unit up and connect your PC to it. Run Hyper-terminal and set the baud rate to around 300-14400bps. What you type in at the keyboard should be echoed-back. 

As an extra check, it would be wise to connect an oscilloscope to the data in/out lead and check that TTL (5V) data "blips" can be seen, corresponding to your key-presses. When you do this test, set the baud rate to as low as possible. This gives you a better opportunity to view the data on the oscilloscope.

Links

http://www.maxim-ic.com/ - Maxim Semiconductors.
Rinda Technologies - All sorts of stuff for Marine GM ECUs. Check-out this link and you'll see that it appears that Lotus have used the Marine GM ALDL interface connector.

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