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MBus format and physical interface...

 
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st2000
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Joined: 26 Jul 2006
Posts: 15

PostPosted: Sat Jul 29, 2006 8:27 am    Post subject: MBus format and physical interface... Reply with quote

Hi...

I am confused. I would like to understand the interface on my ML320 '98 Mercedes' Head Unit once and for all. But a search of the internet brings up conflicting results...

I see where an Alpine MBus project based around an Atmel processor only has one half duplex communication bus to / from the Head Unit. But I have also found, what I am lead to believe, is the pin out of an M-Class Head Unit which shows no less than 3 data lines. And somewhere I remember reading a description of separate data and sync lines for the MBus. Can anyone clear this up?

Is this the correct Panasonic '98 ML320 Head Unit MBus and audio pin out?:
1 gray - aux stereo in right
2 white - aux stereo in left
3 black common coax shield for 1 & 2
4 red - power constant 12v
5 orange - power switched 12v
6 black - common coax shield for data lines 8, 9 & 10
7 pink - D2B wake up (<<=== what is this????)
8 blue - CDC data (<<=== what type of data????)
9 yellow - CDC data (<<=== same question????)
10 green - CDC data (<<=== again, what type of data????)

I would be very interested in finding out the purpose of pins 8, 9 and 10.

And, as far as the protocol, are all MBus's half duplex? Do they (and more important to me - does mine) follow the 8 bit address, 8 bit command, n x 8 bit data and 8 bit check sum format?

...thanks in advance for any help...
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jbors
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Joined: 12 Nov 2004
Posts: 1675

PostPosted: Sat Jul 29, 2006 8:47 am    Post subject: Reply with quote

MBus is half duplex, you're right.
I think ML320 has not MBus but rather D2B which is somewhat completelly different. We have no information on D2B at the moment.
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st2000
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Joined: 26 Jul 2006
Posts: 15

PostPosted: Sun Jul 30, 2006 12:37 pm    Post subject: Reply with quote

Hi...

Thanks for your response jbors. I broke out the test equipment and probed the end of the cable that connects to the CD-Changer. In my `98 ML320 I find the pin out to be different from what I expected but the cable colors appeared to be correct. This is probably because the pin out of the head unit is different than the CD-Changer.

I don't think I have a "D2B" type of communication link with the head unit in this car. Namely because that position on the installed cable connector is missing that electrical connection. All the other 9 pins have wires. I was able to verify power ground, 12 volts constant, and 12 volts switched. Based on that, it was easy to verify the balance of the pins according to the cable color.

However, I am confused by the 3 data pins. I have them down as pins 8 (Blue cable), 9 (Yellow cable) & 10 (Green cable). When I probed them, 9 & 10 did not show any activity. I powered on the car, then powered on the radio, then cycled through the radio modes to the CD mode. Later with the car powered off, I used a volt meter and saw no voltage and about 3 to 4 meg ohms from pins 9 & 10 to the data ground at pin 6. This would suggest a really high impedance input or no connection at all. Pin 10, on the other hand, has 5 volts on it - even when the car is powered off?!? When the car is powered on this drops to ground. I probed this pin with an oscilloscope and couldn't make out any attempt on the head unit's part to communicate. The level simply drops to ground after powering on the car.

All this time, as expected, the head unit reported that there was no CD-Changer available.

I still think I have an MBus type link to the head unit. But I expected the head unit to periodically check for a CD-Changer by sending out a 3 digit hex value: "0x01 0x08 0x0A". But I see nothing and that confuses me.

Can you verify that the MBus half duplex link is actually on pin 10 (Green cable)? And when the head unit will be sending out data looking for a CD-Changer?

...thanks
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jbors
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Joined: 12 Nov 2004
Posts: 1675

PostPosted: Sun Jul 30, 2006 9:25 pm    Post subject: Reply with quote

I cannt validate what kind of signal your ML has. The only thing is the pinout for the 94-98 MB below. See yourself if it works for you...
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st2000
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Joined: 26 Jul 2006
Posts: 15

PostPosted: Mon Jul 31, 2006 6:06 am    Post subject: Reply with quote

Thanks again jbors.

Wow, except for the power, that is different than what I found on the internet and during my testing. I have 2 actions to take. 1) I need to extract the head unit to see if it matches what I have or your pin out. 2) I want to try a pull up resistor on the CD-Changer cable. There is a slight possibility that the head unit is open collector and needs a terminating pull up resistor. Even if I am wrong, a 2000 ohm resistor should present a less than 10mA load.
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st2000
Newbie


Joined: 26 Jul 2006
Posts: 15

PostPosted: Thu Aug 03, 2006 12:07 pm    Post subject: Reply with quote

I finally tested the 3 pins I think are data by scoping them again
with a 2.2K pull up to +12volts. I found no activity for any of the
following 3 events: Power on the Car, Power on the Radio and Cycle
through CD mode of radio head unit. Can anyone comment on the MBus
protocol? Does the head unit initiate the transaction between the
radio head unit and the cd-changer?
As a last hope, here is a URL to an image of the radio head unit that
is installed in my 1998 ML320. If anyone can identify it as a
Panasonic or Alpine and if it will work with the indashpc.org Mercedes
adaptor I would appreciate it.
http://www.whnet.com/4x4/radio/99mercedes_ml320_rsilver_sm.jpg
...thanks
Also, are the Mercedes indashpc.org to radio head unit cables back in stock?
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jbors
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Joined: 12 Nov 2004
Posts: 1675

PostPosted: Thu Aug 03, 2006 1:16 pm    Post subject: Reply with quote

This unit looks like Matsushita and seems to be compatible with MBus adapter.
Can you post a pic of your stereo ? May be this will help.
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