D-Bus Topology

D-BUS Topology

The D-bus (TXD) is connected to various control units that are diagnosed using DIS or MoDiC. Earlier vehicles also used a second diagnosis line called RXD to allow the test equipment to establish communication. RXD is not a bus line but a one way communication link used to wake up the diagnosis of the connected control unit.

On vehicles produced up to model year 2001 and use the 20 pin under-hood diagnostic connector, the locations of the two links are:
- RXD-Pin 15
- TXD-Pin 20

Later control modules (from 1997) no longer required the separate RXD to establish communication, (DS2 protocol) so Pin 15 was removed from the Diagnostic socket of most vehicles.

To satisfy the requirements of OBD II, in 1995 a standardized connector was installed inside of all vehicles. This connector has to provide access to all powertrain modules via an aftermarket scan tool. TXD II (pin 17) was introduced as a separate communication line exclusive to DME (ECM), AGS (TCM) and EML. TXD II is technically identical to the D bus (TXD).

On vehicles that use only the 16 pin OBD connector in the vehicle, TXD is installed in pin 8. TXD II remains in pin 7.

The term D bus was actually coined with the introduction of the E38 and the expanded use of bus systems in the vehicle. On vehicles from E38 on (except Z3), the D-bus is directly wired to:
- ASC/DSC
- EDC (if equipped)
- LEW
- IKE/KOMBI




The IKE/KOMBI serves as the gateway for the D-bus that converts the telegram format of the I/K bus to the format of the D-bus.

The wire color of the D-bus is uniform throughout the vehicle, it is a single WS/VI wire.