GM E&C Protocol

Mask of Destiny · 04-12-2005, 10:00 PM

I have a 97 Saturn which has the option of having a trunk mounted CD changer. I'd like to attach a computer or PDA of some sort to this interface in such a way that not only is the audio passed through to the head unit, but so that the computer will respond to the standard CD controls. I've seen adapters which will handle the audio, but none of them seem to have any provision for passing the data along.

Doing a bit of searching around I figured out that the head unit talks to the CD changer using GM's E&C serial protocol, but I haven't found a lot of details about it. This is about all I know:

Single Wire Serial Protocol
PWM Encoding
Parity Error Correction
11-12 bit header
1-8 bits of data
1000 b/s

The pinout seems to vary from one line of GM vehicles to the next, but the color coding is relatively consistant so with the help of my multimeter I've more or less figured it out for my Saturn, but that's currently where I'm stuck.

Does anyone know anything more about this protocol or know where I could possibly find more info?

wizardPC · 04-12-2005, 10:41 PM

<threadjack>
Hey, you got a screen for your saturn yet?
</threadjack>

lgbr · 04-12-2005, 11:05 PM

Getting the audio to come through shouldn't be a problem, though it is expensive. Check a local sound shop. The adapter is pricey. You're going to have a hard time with the controls, since it is an analog signal coming into the computer, it may require custom software.

Mask of Destiny · 04-12-2005, 11:08 PM

Not really sure if I'll add a screen. I don't usually have a lot of passengers in my car so adding DVD playback isn't really a concern. I might consider adding GPS navigation at some time in the future, but at the moment I can't really justify the expense given how little driving I do in unfamiliar territory (and I'm pretty handy at old fashioned navigation anyway).

All I really want to do is find a better way to play my Ogg Vorbis CD rips than what I'm doing currently (PDA connected to a tape adapter). I'd like to avoid unneccessary clutter which is why I want to fully utilize the CD changer interface. If possible I might even rig it up so that the name of the track scrolls across the tiny little LCD screen on the head unit (this is possible in newer GM vehicles, not sure about mine).

If I really feel the need to have a separate status display screen of some sort, I could just make a mount for my PDA (a Zodiac Tapwave) and have it talk to whatever is in the trunk via Bluetooth (assuming I don't use it to play the music directly).

Mask of Destiny · 04-12-2005, 11:21 PM

Quote: Originally Posted by lgbr

Getting the audio to come through shouldn't be a problem, though it is expensive. Check a local sound shop. The adapter is pricey. You're going to have a hard time with the controls,

The audio inputs are actually quite simple. The adapter just fakes a CD changer so that the head unit will actually switch to the appropriate audio inputs. If I can figure out how the serial protocol works I can avoid using the adapter and get the controls working in one blow.

Quote: Originally Posted by lgbr

since it is an analog signal coming into the computer, it may require custom software.

It's a digital signal, just not one easily fed into a standard serial port. Because of this it will probably require custom hardware as well as software, but that's not the big issue (I write software for a living and I have some computer engineering knowledge). The big problem is that I don't know enough about the protocol to design anything to interface to it. I don't even know for sure what voltage levels the signal uses.

wizardPC · 04-13-2005, 07:42 AM

Quote: Originally Posted by Mask of Destiny

I'd like to avoid unneccessary clutter

So the reason I asked if you had a screen is because I have one for sale. How's this for no unneccessary clutter?

bosskong · 04-16-2005, 05:53 PM

It looks like my 2001 Chevy Tahoe uses this E&C protocol also. I'd like an interface for my carputer that would allow me to press some of the buttons on my head unit (Track ->, Track <-, etc) to change tracks on the computer. Sorry, I can't help provide any information, but if you have links to any webpages with information on this E&C protocol, could you please share them?

I have a PIE GM9-AUX auxiliary input adapter. I took it apart and found that it has a PIC chip (PIC16C622A) in it. I was going to read the chip to see if I can figure out anything about how it communicates, but my PIC reader has a different size socket. I may end up reading a copy of it in the future though, so If I do, I'll make another post.

Chairboy · 04-16-2005, 07:14 PM

The Aux2Car interface will do what you want.

bosskong · 04-18-2005, 03:14 AM

Quote: Originally Posted by Chairboy

The Aux2Car interface will do what you want.

This looks like it does the same thing as the PIE adapter that I mentioned... But I can't see how this would help me to use my head unit to control tracks on my computer. Am I missing something here?

Chairboy · 04-18-2005, 09:50 AM

Ah, sorry, missed that part. Nope.