sathackr
05-24-2012, 14:46
I'm not an electronics expert but I've tinkered enough that I *think* this will work.
DON'T TRY THIS UNLESS YOU'RE SURE OR LIKE TAKING RISKS, I HAVE NOT TESTED IT AND IT MAY RESULT IN A COOKED ECM!!
Maybe someone with more knowledge of electronics and/or the ALDL interface on the pre-1996 models can help validate it.
My understanding is the data is RS-232 based except with 0v and 5v levels instead of the -/+ 12v in RS-232. In addition, TX and RX are on the same pin.
It's been my experience in the past that PC-based RS-232 interfaces will work when fed TTL(0v/5v) signals as the logic low is around 1.5v and high is around 3.5v(IIRC). That being the case, the signal from the ALDL *should* be able to be fed directly into the RxD pin. Since the output from the PC could be +/- 12v it needs to be clamped to prevent damage to the ECM. Putting a 5.1v zener in reverse, across the TxD pin and Ground *should* accomplish this. The 5.1v zener will clamp the output of the PC to 5.1v as this is the breakdown voltage of the diode. If the voltage swings to negative, it will function as a normal diode and clamp it to 0v/ground.
Can someone with more electronics knowledge than myself confirm this? See attached professionally drawn schematic :)
DON'T TRY THIS UNLESS YOU'RE SURE OR LIKE TAKING RISKS, I HAVE NOT TESTED IT AND IT MAY RESULT IN A COOKED ECM!!
Maybe someone with more knowledge of electronics and/or the ALDL interface on the pre-1996 models can help validate it.
My understanding is the data is RS-232 based except with 0v and 5v levels instead of the -/+ 12v in RS-232. In addition, TX and RX are on the same pin.
It's been my experience in the past that PC-based RS-232 interfaces will work when fed TTL(0v/5v) signals as the logic low is around 1.5v and high is around 3.5v(IIRC). That being the case, the signal from the ALDL *should* be able to be fed directly into the RxD pin. Since the output from the PC could be +/- 12v it needs to be clamped to prevent damage to the ECM. Putting a 5.1v zener in reverse, across the TxD pin and Ground *should* accomplish this. The 5.1v zener will clamp the output of the PC to 5.1v as this is the breakdown voltage of the diode. If the voltage swings to negative, it will function as a normal diode and clamp it to 0v/ground.
Can someone with more electronics knowledge than myself confirm this? See attached professionally drawn schematic :)