maybe you should have a look here
It sure is! The diode anode would need to go to the connection between RXD and the ALDL data while the TXD line connects to the cathode of the diode.A 4k7 resistor between the ALDL data and the RXD line might be extra protection.
Would it be silly to ask why Rx and Tx are shorted together on FTDI side ?Is that supposed mean ALDL Tx => FTDI Rx ? If so why is drawn that way ?
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.
look, here is how they should be wired, they aren't shorted, butALDL has only one pin for both Tx and Rx , the other side has to deal with it and I think they should be wired the same way if FTDI is used
Why have you posted an RS-232 interface circuit when the OP referred to "communicating with ALDL via USB"?
Thanks Paul__B. You mean like this:-
So the ECU output is also well protected as per this quote right?
..... The interface low output will increase by ~0.7V but should still be recognised as a low by the ECU.
The ECU output is a 10 kohm resistor to +5V with a switched transistor to 0V.
I think it is as well protected as it is going to be.Pity the last paragraph in that quote is garbled with the description of the diode the wrong way round!
because of the second sentence :either the first or the 2nd sentence is wrong
The guy he might mean the worst case when FTDI send 'high' to the ECU without protection circuit.