C5 headunit to rear CD changer protocol
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From: Frisco, TX
C5 headunit to rear CD changer protocol
Anyone know the communication tricks between the C5 headunit and the rear CD changer or can point me to a resource that details it.
I ask because I recently bought a GM10-AUX that's supposed to connect into the CD harness and provide RCA inputs that will then be workable as the AUX input, but the problem is you then lose your CD functionality b/c the GM10-AUX is in its place.
So, I figured I was clever (big mistake) and I bought some relays and attempted to wire things so with the relay in one position, I'd have the CD changer connected to the head unit, and with the relay in the other position, I'd have the GM10-AUX box hooked up.
Well, it only kind of works, and so far it's erratic enough that I don't know the many ways it's not working, but it certainly is not working. One time all seemed fine and another it did not, so I am guessing there might be some start up protocol when power is first applied, but that's a big, fat guess.
Any ideas?
Thanks.
I ask because I recently bought a GM10-AUX that's supposed to connect into the CD harness and provide RCA inputs that will then be workable as the AUX input, but the problem is you then lose your CD functionality b/c the GM10-AUX is in its place.
So, I figured I was clever (big mistake) and I bought some relays and attempted to wire things so with the relay in one position, I'd have the CD changer connected to the head unit, and with the relay in the other position, I'd have the GM10-AUX box hooked up.
Well, it only kind of works, and so far it's erratic enough that I don't know the many ways it's not working, but it certainly is not working. One time all seemed fine and another it did not, so I am guessing there might be some start up protocol when power is first applied, but that's a big, fat guess.
Any ideas?
Thanks.
Racer
Joined: Feb 2004
Posts: 316
Likes: 140
From: Metro Atlanta GA
See this post:
http://forums.corvetteforum.com/showthread.php?t=891451
and look for the response by powerdoc. Drop him an email and he'll send you a schematic that will let you have both your CD player and your AUX. If he's busy, let me know and I'll send you the schematic (assuming he doesn't mind).
http://forums.corvetteforum.com/showthread.php?t=891451
and look for the response by powerdoc. Drop him an email and he'll send you a schematic that will let you have both your CD player and your AUX. If he's busy, let me know and I'll send you the schematic (assuming he doesn't mind).
Thread Starter
Team Owner
Joined: May 2000
Posts: 25,338
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From: Frisco, TX
Thanks for your time and response. I already pestered powerdoc and he was kind enough to respond with the schematic (very nicely drawn, too!). However, his setup is a bit different than what I was trying to achieve in that his keeps the CD player running when he's listening to his MP3. That's no big deal and indeed it may be the way I am ultimately going to go, but I had hoped my way of doing it would be to fully switch between the 2 devices so that the CD player does not run when I'm listening to the XM radio.
Racer
Joined: Feb 2004
Posts: 316
Likes: 140
From: Metro Atlanta GA
Well, good luck. The GM10-AUX unit presumably generates a bogus control signal that the HU interprets as "CD 1, Track 1", or something like that. If it did not generate this signal, the HU would not recognize the auxiliary input, and you could not play your iPod or XM radio through the HU.
So you actually now have two control signal generators, the CD changer and the GM10 device. And I'm guessing your switching circuitry is trying to switch from one to the other. If you have access to a data analyzer you could monitor the real vs. the GM10 signal and see if there is any reason (handshaking, etc.) why your set-up is not working. You would think that the PIE folks would have incorporated a passthrough and switch in their unit if they could have easily, since most people adding XM would also like their CD changer too.
Or you could do what I am doing, replacing the whole darn thing, HU, speakers, and all. When I'm done I'll have a CD changer, iPod and XM, with switching between them at the HU.
So you actually now have two control signal generators, the CD changer and the GM10 device. And I'm guessing your switching circuitry is trying to switch from one to the other. If you have access to a data analyzer you could monitor the real vs. the GM10 signal and see if there is any reason (handshaking, etc.) why your set-up is not working. You would think that the PIE folks would have incorporated a passthrough and switch in their unit if they could have easily, since most people adding XM would also like their CD changer too.
Or you could do what I am doing, replacing the whole darn thing, HU, speakers, and all. When I'm done I'll have a CD changer, iPod and XM, with switching between them at the HU.
Team Owner
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From: S.W. Ohio. . . . . . NRA Life Member
One of the conductors in the rear CD changer harness is a "serial data" wire. The protocol is proprietary. Control of the CD player is accomplished with the Head Unit and CD changer sending serial data back and forth to each other on this signal line.
The L/R audio wires in said harness... are ignored by the head unit unless there is data present on the Serial Data line. And; it has to be data that follows the unique "protocol".
What the PIE adapter does is generate some simple serial data (as described above by an earlier poster) that the head unit recognizes; fooling it into believing the factory changer is installed and operating. Thus, allowing the L/R AUX inputs of the head unit to become active.
I have never seen the specs of the proprietary GM interface published.
There is another adapter mfgr that supposedly makes a multi-input device that allows both the factory CD changer and an aux input device to be installed at the same time.
Sadly; I can't find the link.
The L/R audio wires in said harness... are ignored by the head unit unless there is data present on the Serial Data line. And; it has to be data that follows the unique "protocol".
What the PIE adapter does is generate some simple serial data (as described above by an earlier poster) that the head unit recognizes; fooling it into believing the factory changer is installed and operating. Thus, allowing the L/R AUX inputs of the head unit to become active.
I have never seen the specs of the proprietary GM interface published.
There is another adapter mfgr that supposedly makes a multi-input device that allows both the factory CD changer and an aux input device to be installed at the same time.
Sadly; I can't find the link.
Last edited by Mike Mercury; Sep 9, 2004 at 11:03 PM.
Thread Starter
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Joined: May 2000
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From: Frisco, TX
Thanks for all the responses, and I've fumbled around on this the hard way now. I wish there were a document somewhere of that protocol -- I think it stinks that it's not readily available. We're not talking rocket science or huge innovation, after all.
Anyhow, Vate is definitely correct about the GM10-AUX generating a bogus control signal. The problem is, once it does it, it stays alive in the head unit memory. So, once you switch to the GM10-AUX and all seems swell, when you switch back to the CD player, the head unit is still stuck in the mode that the G10-AUX caused. So far, my brute force experiment was to then "re-boot" the CD player, that is, simply disconnect and re-connect its power. The good news is this clears the state of the head unit, but the bad news is then the CD player has to re-catalog all of its disks, and that's a fairly lengthy process. So, if I'm driving along and I want to "flip" between XM and a CD, I'll be waiting quite some time to hear a CD.
I think I've now figured out why forum member powerdoc just chose to leave the stinking CD player running all the time.
I haven't given up yet, but I'm close.
Thanks again for the wisdom.
Anyhow, Vate is definitely correct about the GM10-AUX generating a bogus control signal. The problem is, once it does it, it stays alive in the head unit memory. So, once you switch to the GM10-AUX and all seems swell, when you switch back to the CD player, the head unit is still stuck in the mode that the G10-AUX caused. So far, my brute force experiment was to then "re-boot" the CD player, that is, simply disconnect and re-connect its power. The good news is this clears the state of the head unit, but the bad news is then the CD player has to re-catalog all of its disks, and that's a fairly lengthy process. So, if I'm driving along and I want to "flip" between XM and a CD, I'll be waiting quite some time to hear a CD.
I think I've now figured out why forum member powerdoc just chose to leave the stinking CD player running all the time.
I haven't given up yet, but I'm close.
Thanks again for the wisdom.
