Introduction to Bus Systems: M-Bus: Notes
The M-Bus is used exclusively in the climate control systems for the control of the "smart:" stepper motors. These stepper motors are used to control various air distribution flaps. In previous climate control systems, such as E32/E34, the stepper motors were directly controlled by the climate control module.
The M-Bus was introduced on the E38 climate control system (IHKA). The M-Bus was also installed on subsequent models equipped with IHKA and IHKR.
The M-Bus communicates with the "smart" stepper motors which contain a processor capable of transmitting and receiving messages. The stepper motor is then operated by final stage transistors located within the stepper motor electronics.
The M-Bus consists of a three wire ribbon cable containing the following wires:
- Power (B+)
- Ground
- Bidirectional Signal Line
Each stepper motor on the M-Bus has a unique part number to distinguish it's location on the climate control housing. The part number corresponds to a unique electronic address on the M-Bus. Since each stepper motor contains a unique electronic address, the motor will only respond specific commands. A stepper motor installed in the wrong location would result in improper operation.