CAN Bus Communication: Notes
The CAN bus is a serial communications bus in which all connected control units can send as well as receive information. Data over the CAN bus operates at a rate of up to 1Mb/s (megabits per second).
The CAN protocol was developed by Intel and Bosch in 1988 for use in the automotive industry to provide a standardized, reliable and cost-effective communications bus to combat the increasing size of wiring harnesses.
The CAN bus was originally introduced on BMW automobiles in the 1993 E32 740i/IL as a data link between the TCM (EGS) and the ECM (DME).
On earlier EGS systems, various signals were transmitted on individual signal wires. This reduced reliability and increased the amount of wiring needed. The CAN bus allows faster signal transmission and increased versatility. For example, the signals listed in the chart below were previously transmitted on individual wires, now these signals are all on the CAN bus. This chart represents only some of the signals on the CAN bus, there are many more signals transmitted between the TCM and ECM.
| Sender | Information Item | Receiver | Signal Use |
|---|---|---|---|
| ECM | Engine Temperature | TCM | Shift Point Calculation |
| ECM | Engine Load (tL) | TCM | Shift Point Calculation |
| ECM | Engine RPM (TD) | TCM | TCC Slippage |
| ECM | Throttle Position (DKV) | TCM | Shift Point Calculation |
| ECM | A/C Compressor ON | TCM | Fine tune shift points to compensate for increased engine load. |
| TCM | Transmission Range | ECM | Engine Idle Speed Control |
| TCM | Torque Reduction Signal (ME) | ECM | Timing Retard during shifts. |
| TCM | TCC Lockup Status | ECM | Engine Timing Map adjustment. |