Date registered: Dec 2005
Vehicle: 222.132 & 221.122 & 212.025 & 906.633
Mentioned: 3 Post(s)
Quoted: 37 Post(s)
The multiplexer is actually pretty simple by hardware. It only has a few very simple logic circuits, RS232 converter, voltage regulator and a big relay matrix. In addition to this very basic HW it has a micro controller, like 80C51. I'm not familiar with the micro controller tasks but the communications to the SDS computer goes via this chip and it does include security features that makes copy devices difficult to build (specifically to be future proof).
The main function is the capability to connect individual diagnostic lines from the old diagnostics connector. This is from the time before CAN bus (used in parallel with OBD-II in some "transient" cars) where basically each control unit had its own diagnostics line (photo attached shows this old connector, note that not all pins are explained). The relay matrix is able to pick up the particular line that the test equipment wants to talk to.
New cars use the ODB-II connector for diagnosis purposes. The W220 ODB-II connector has some other diagnostics pins in addition to the CAN but one like on the W221 does not use anything else than the CAN, power and ground.
In practice the complex test scenarios are done by a series of different commands to different control units. On a CAN bus this is easy as the BUS data rate is high, actually the laptop RS232 interface is limiting the data transfer rate.
On modern MBs all testing could be done without any multiplexer if MB so decided. A relatively simple USB to CAN bus interface would be enough (there are even interface devices that read the ODB-II data on line and send it over bluetooth to an application on a smart phone. As far as I know, VW group is doing this.
Anyway, building all the diagnostics features and DAS SW is a huge work and MB would need to get that paid by service shops that make use of all this. If there was no security built in the SW and HW, copy products would be far more common and those using the genuine devices would have to pay even more than today, if there are less buyers sharing the bill.
I could post a circuit diagram of the multiplexer but I don't think it is needed and a good idea anyway. You can probably find an example from those who make copy products.
Last edited by Diesel Benz; 01-23-2008 at 10:46 PM.