Optoisolated ALDL Interface

[Ludis Langens]

In light of the recent ALDL interface threads, I finally drew up a
proper schematic of my optoisolated interface.  It connects the 160/8192
baud ALDL to a Macintosh RS422 serial port.  No battery or external
power source is required, power is drawn from the serial port lines. 
The URL is:
  http://www.cruzers.com/~ludis/interface.gif

I've bench tested this interface with a C3 generation ECM transmitting
at 160 baud.  It hasn't been tested at 8192 baud or with a P4 or newer
ECM.  This interface won't work in cars that use the Service Engine Soon
circuit for ALDL communication.

The circuit can be adapted to a PC style RS232 interface.  Instead of
sucking power from the computer's RS422 transmit data lines, it'll have
to come from two RS232 handshake lines held at opposite levels.  The U2
and U3 circuits also need to be changed.  With Mac RS422 levels, the
LEDs are never reverse biased beyond their limits.  With PC RS232
levels, this wouldn't be true.

The device type of U1 is critical.  It must not be substituted with a
device with different current characteristics.  The type of U2 is also
somewhat critical.  A Darlington output optoisolator must not be
substituted because it isn't fast enough.

-- 
Ludis Langens                               ludis (at) cruzers (dot) com
Mac, Fiero, & engine controller goodies:  http://www.cruzers.com/~ludis/

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