GM ALDL interface

Jennifer Rose javer96 at snowcrest.net
Thu Dec 11 03:47:20 GMT 1997


At 09:49 AM 12/10/97 -0800, you wrote:
>Frederic Breitwieser <frederic.breitwieser at mcione.com> wrote:
>>I created a single-transister circuit to make the proper voltage
>>adjustments (12v to 5v), however over the next day or to I'll have an
>>opto-isolator circuit that's even better.
>
>There are some ECM's where the ALDL 160 data swings between 0 and 5 volts.
>Both of the ones I've looked at (2.5 L4 TBI and 2.8 V6 MPFI) do that.  If
>your's goes to 12 volts, could you trace the ECM schematic for that output
>back to a driver chip?  What series and pullup resistors does it use and
>so on.  I'd like to add support for it in my ALDL interface.
>
>Both of the previously mentioned ECMs have an open collector driver with a
>1500 ohm pullup resistor to five volts.  They also have a series resistor
>between the driver and the ECM output - 300 ohms in one ECM, 120 ohms in the
>other.  This means that the output switches between 300 (or 120) ohms to
>ground and 1800 (or 1620) ohms to five volts.
>
>If you are building an opto-isolated interface without a power source on
>the ECM side, you have to deal with the very limited current available
>during the 'high' state.  When driving the LED portion of an opto-isolator,
>there is a maximum of 1.6 mA available.  I found that most opto-isolators
>won't work or are too slow at such low current levels:  low gain photo-
>transistors essentially don't work at all; high gain phototransistors are
>too slow (for the 160 Hz signal!); even the photodarlingtons are too slow.
>
>Just about the only opto-isolator that worked is an H11L1 "Schmitt trigger".
>(They are $0.99 each from Jameco.)  It is meant to work on exactly 1.6 mA!
>It is also good for up to 1 megahertz, so it even works for the 8192 baud
>signal.  Hook the LED side directly across the ALDL pins A and E with no
>extra series resistor.  To handle the 8192 ECMs, I've placed an 820 ohm
>resistor between the M and E pins, although this hasn't been tested yet.
>
>The H11L1 output side has a unique pinout - it needs power and ground, in
>addition to an OC output.  It can run with a supply voltage of 3 to 16 volts.
>I'm running mine on power stolen from a Mac's serial TxD+ and TxD- signals.
>This results in a 6 volt supply (actually +3 and -3.)  You can do something
>similar on a PC.
>
>>While there is nothing wrong with a
>>complex circuit that converts to a serial port, this solution is just more
>>software based.  There's merit for both actually.
>>
>>I prefer the software method only because I can shove the teeny-tiny
>>interface into the cable end rather than having a box hanging off the
>>notebook.
>
>You can pipe the opto-isolator output to a parallel port and use a
>software polling approach.  It is also just as easy to connect to an
>RS232C serial port and use the UART hack I posted earlier.  An opto-isolator
>works great as an RS232 voltage level shifter.  It also provides the correct
>number of signal inversions for the 8192 mode (I think) or the UART hack.
>
>PS: If you can't find an H11L1 locally, the NTE3090 is a direct replacement.
>
>PPS: Has anyone found a source for the female side of the ALDL connector?
>The pin spacing is non-standard.  I doubt that even a silicon valley junk
>dealer will have anything that can be adapted.
>
>                   unsigned long BinToBCD(unsigned long i) {unsigned long t;
>Ludis Langens         return i ? (t = BinToBCD(i >> 1), (t << 1) + (i & 1) + 
>ludis at netcom.com                 (t + 858993459 >> 2 & 572662306) * 3) : 0;}
>
>Hi

	The 15 pin IP connector from under the dash can be hacked down to fit. Noy
very pretty.

Vance



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