Definitions
Bus Line
A bus line is a group signal line that transmits serial data in both directions. It may consist of one wire or two. All control units are connected in parallel in current bus systems, this means that the information sent can be heard by all of the connected units.
Bus Subscriber
Any control module connected to a bus line. e.g. DME, IKE, GM, etc.
Gateway
A Gateway module provides a link between different bus lines to provide a means of sending information from a subscriber of one bus line to the subscriber of another. The Gateway module recognizes from the receiver address whether a message is to be routed through the gateway or not. e.g. IKE, KOMBI.
Master controller
A Master Controller of a bus system provides the operating voltage and wake up signals to the subscriber modules. This task may also be performed by several Standby Masters within a bus system, e.g. GM, LCM.
Serial Data
Serial means one event at a time. In data transmission, the technique of time division is used to separate bits of data sent. The messages sent over a bus are configured serially. Each message consists of
- Transmitter address.
- Length of data.
- Receiver address.
- Command or Information.
- Detailed description of message (data).
- Summary of transmitted information (check sum).
All of the connected control units will receive the information but only the unit in the address will accept and react to the data.
Note. This message format is not used for the CAN Bus.
Topology
In the context of communication networks, Topology describes the configuration or arrangement of a network.
CAN Bus Topology
The CAN bus consists of two twisted copper wires. Each wire contains an opposing signal with the exact same information (CAN-High, CAN-Low). The opposing signals transmitted through the twisted wire serve to suppress any electrical interference. Early CAN bus wiring included a grounded shield around the two wires, later vehicles discarded the shield in favor of the unshielded twisted pair wiring.
Due to the linear structure of the network, the CAN bus is available for other modules in the event of a disconnected or failed control unit. This is referred to as a "Tree" structure with each control unit occupying a branch.
Scheme 19
Example Of Tree Structure 95-97 E38 750iL
As previously mentioned, the CAN bus initially was used as a high speed communication link between the DME and AGS control units.
With the introduction of the E38 750iL (95 M.Y.), the CAN bus was expanded to include the EML and DSC control modules. The 750iL made exclusive use of the "star coupler" to link the individual CAN bus ends to a common connector.
The 1998 model year introduced new users of the CAN bus. The instrument cluster and the steering angle sensor were linked to expand the signal sharing capabilities of the vehicle.
The 1999 750iL was the last vehicle to use the shielded cable, after which the entire CAN bus went to twisted pair wiring.
Always refer to the appropriate article to determine the exact wiring configuration for a specific model.
Scheme 20
On most current models the CAN bus provides data exchange between the following modules
- DME
- EML (750iL)
- AGS/EGS
- ASC/DSC
- IKE/KOMBI
- LEW
On models that use twisted pair, the wire color of the CAN bus is uniform throughout the vehicle with: CAN-Low GE/BR and CAN-High GE/SW or GE/RT. Shielded wiring is easily identified by the black sheath surrounding the CAN bus.
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 after-market 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
Scheme 21
- 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.
P-bus Topology
The extent of the P-bus depends on the special equipment of the vehicle.
Scheme 22
The P-bus consists of a single copper wire. The wire color of the bus is uniform throughout the vehicle: BL/RT.
Due to the linear structure of the network, the P-bus is available for other modules in the event of a disconnected or failed control unit. The P-bus provides the diagnostic connection to the P-modules.
Always refer to the appropriate article to determine the exact wiring configuration for a specific model.