Diagnostic Connector: Notes
There have been numerous changes to the diagnostic connector since it's introduction in 1988. Early vehicles until the 2001 model year used the 20-pin diagnostic connector located in the engine compartment. On vehicles equipped with the 20-pin connector, diagnostic communication is carried out through the TXD/RXD interface (D-Bus).
RXD is a 12 volt one-way digital signal which is sent to the module subject to diagnosis. This signal was used to wake-up the control module and initiate diagnostic communication. RXD was gradually phased out starting in 1997 (until 2001) and TXD (D-Bus) is now used for all diagnostic communication.
The TXD signal line is bi-directional and allows information to be retrieved (such as fault codes) and commands to be carried out (such as component activation). On vehicles equipped with the 20-pin diagnostic connector, TXD is in pin 20 and RXD in pin 15.
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. Pin 15 (RXD) was still used in the E38 until the end of production in 2001.
In 1995, to satisfy the requirements of OBD II, a standardized 16-pin connector was installed inside of all vehicles. Up until 2001, the 16-pin OBD II connector was not used by BMW diagnostic equipment to access diagnosis, it was reserved for aftermarket scan tool usage. The 20-pin connector was eliminated from all BMW vehicles from 2001 and the 16-pin OBD connector is now used exclusively.
TXD II (pin 17) was introduced as a communication line exclusive to DME (ECM), AGS (TCM) and EML. Pin 2 provided a connection to the 16 pin OBD connector via a bridge in the cap of the 20-pin connector. TXD II is technically identical to the D-Bus (TXD).
Beginning with the introduction of the E65, TXD has been omitted and TXD II is now used exclusively for diagnostic communication. TXD II is in pin 7 of the 16 pin OBD connector.