For a full schematic of the circuit, see  Schematic for RS232 to ALDL Converter

Complete parts list included in kit

1        10 Foot DB9F to DB25M external modem cable
1        Cigarette lighter adapter with cord and LED indicator for power
1        Null modem shell box as a case
1        DB25 shell for ALDL side connector
2        DB25F solder cup connectors (one for the RS232 side of the converter, one for ALDL side shell)
1        DB25M solder cup connector (for the ALDL side of the converter)
1        Printed circuit board
1        7805 Voltage Regulator
4        2N3904 Transistors
6        10 uF 16V Capacitors
9        10K Resistors
1        MAX232 IC
1        22GA set of wires (not pre-cut to length, just some wire included)
4        Plastic tie wraps (if ya need 'em, they might be handy)

Assembly notes:

PCB Tips:

1)  Note the orientation of capacitors in the main picture. It is important to get the polarity right.

2)  I have reconsidered the "port powered" interface, and have decided to make it an option (either cig lighter or port powered - no cost difference). I measured the working current of the circuit, and it was just under 3 mA, which is acceptable for a port powered interface on *MOST* computers. If this port powered interface does not work on your computer, you may always fall back on the cigarette lighter option.

The Carputing software automatically sets the DB25 pins 4 and 20 to +V (usually around 12 Volts, but can be as low as 6V). So, by placing a diode on pin 4, and another diode on pin 20, we can have our port powered interface. The stripe of the diode (cathode) needs to be on the printed circuit board side. I am using DigiKey P/N 1N5256BDICT-ND, a Zener 30V DO-35 package unit. No ALDL software I've ever seen needs these control lines, so your software should work properly with these lines set to a logical 0 (the +V). If the software you're using doesn't do this, well, it NEEDS to. ;) A simple software correction and recompile should work fine. As long as one of these lines is set to +V, it should work, but you're safer if both of them are set to this +V.

3) Here is a picture of a unit where I ran power through the pins 12 and 13 of the ALDL side of the converter, and it's also shown inside its shell... Left side is RS232, Right side is ALDL. Notice that the board actually lays below both rows of pins on the DB25 connectors... It does not sandwich between them.

4)  For the ALDL side wires, I cut them about 1.5" long. For the RS232 wires (that run over the top of the chip), I cut them about 2" long. I used small enough wire (22 gauge) that it would be easy to push them back so that they tucked and fit in the case.

5)  I used Pin 1 on a DB25M for ALDL GND and Pin 5 for ALDL Data when building my cables. Just FYI. You're free to use any wire you wish. :)

6) GND for RS232 is pin 7 on the DB25 connector (and FYI, pin 5 on a DB9). Please note that in the primary picture above, I've listed the pins for both the DB25 and DB9 connectors. The kit will include only DB25 connectors - but there may be people building this from just the bare printed circuit board, and they may want to use a DB9 instead - so I leave this option open. Going by the name "TXD" and "RXD" may be confusing (the ones I listed are actually for the RS232 port, not the MAX IC) - so simply wire these solder pads to the DB25  (or DB9) pins indicated.

7) TESTING TIP: The circuit, when properly assembled, will echo back characters when you're using a terminal program (like Hyperterminal or Procomm). Simply plug the assembled unit in to the serial port, apply the 12 volts to the unit, start Hyperterminal, set the baud rate to something reasonable (9600) and the port to the one you're using, and type on the keyboard. If you see what you're typing, then the interface is working properly. If you don't see what you're typing, then there may be a problem with the board (excess solder shorting together traces is pretty easy to do!).
 

Note that there is no OBD connector included in the kit. I am working on putting together an OBDII style kit, though, and will be introducing it within a couple of weeks. Pricing is expected to be in the $50 range (it will include an OBDII connector and pins). Still no cheap source for OBDI connectors... :^(

Any questions at all, please email me!  Email Andrew