Hi. I wish to connect my car's ALDL; (Assembly Line Diagnostic Link, sort of OBD) to my pc via usb 5V TTL board(may add arduino later) I looked in the website it said connect USB TTL's Rx/Tx together and then connect to the ALDL data line.
Then someone raised a warning...
USB TTL drives the output high which could potentially cause the ECU output to blow.
ALDL is half duplex: The ECU output is a 10 kohm resistor to +5V with a switched transistor to 0V. The interface must also have a switched transistor to 0V only. The maximum current that can flow in the ECU output transistor is 5V/10k ~ ½mA.
If the interface is switched via a transistor to 5V in the off state, then when the ECU starts to respond, the maximum current in the ECU transistor is determined by the output capability of the interface and could be 100s of milliamps. You would be advised to put a diode in series (Anode to the interface output and cathode to the ECU). The interface low output will increase by ~0.7V but should still be recognised as a low by the ECU.
So far, I've come up with the circuit shown here. Then someone else said the diode might be the wrong way round.
Can anyone confirm if the protection circuit is correct? Or advise otherwise.