Not nessecarily true, if using half duplex current loop is using 2 wires instead of 4, rs422 is based on balanced voltage and uses 4 wires.Originally Posted by pippolippi
But then both current loop and rs422 would need 4 wires, so I was probably wrong
mmmh, I cannot swear it (I don't have my notes here) but the led on my adapter lights up when I connect pin 2 to 14, so either there's a pull-up resistor there [see the edit] or it's the Tx line, not the Rx.
I'll try tonight or tomorrow to connect everything as you say and try (again) to send frames to the HU.
Edit: if is there a pull up resistor it's not going to work with a diode in series on the rs232 tx line, so I'm not going to try.
Vicne, is it possible for you to make a relatively simple test?
Disconnect the spying circuit (or leave it connected, it shouldn't make a difference), leave the connection between the CDC TX and the HU RX but disconnect the opposite one (HU TX->CDC RX).
Leave the scope in place and check, when you connect the remote cable (edit: or if it is permanently connected when you turn on the HU), if you can see the signal from the CDC and the reply from the HU and the levels.
If you can see the reply at the same level we can definitely be sure that it's voltage and not current based. You can also check if the start sequence from the CDC is enough for the HU to recognize the CDC is connected, but that's not the main goal of this test.
My god, I followed the wires twice, taking notes, and am 99% sure that's correct, but as it's in contradiction with what you say and also CristianC's original post - and I'm missing a bit of sleep - , I'll check a third time, disconnecting one of the wires to be sure.
I should add a disclaimer here (and maybe on the first post of the thread) : Please note that I don't take any responsibility bla bla...
Don't worry, if I fry my HU rx it'll be no worse than it is now (since I'm not going to install a changer). If it works, good, if it doesn't, bad luck but not a big deal.
Besides, it's not my car
OK, I wanted to be sure : I just disconnected pin 14 of the blue connector and started a capture session, and only saw acknowledges from the head unit.
So these acknowledges from the head unit come out of pin13 of the connector, that is the one farthest from the front of the head unit.
Maybe my schema is ambiguous regarding pin numbering as it depends whether you look "at" or "through" the connector. Numbering refers to the one of pinout below.
So I confirm that if you want to emulate a CD changer :
You have to link pin 3 (Transmit Data) of the serial port to pin 14 (RxD) of the blue connector to send PLAYING frames.
You have to link pin 2 (Receive Data) of the serial port to pin 13 (TxD) of the blue connector to read HU commands.
So what is the HU acknowledging?, since, according to what you just said, it shouldn't see anything from the CDC.
No, no, that's not what I meant. Everything was still connected between HU and CDC. I only disconnected the derivation from pin 14 to the MAX. I opened the circuit before the 10K resistor if you like.
Here's version 3, with full PLAYING frame decoded.
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