M-Bus Topology
The M-Bus consists of a three wire ribbon attached to the climate control housing and connecting all of the "smart" stepper motors in the system. The number of stepper motors depends upon the vehicle model and climate control system (IHKA, IHKR etc.).
For example, the E38 (shown) with IHKA uses 9 "smart" stepper motors and one motor that is conventionally controlled. The M-Bus is divided into two circuits due to the large number of stepper motors. Other models such as the E39, E46 and E53 only use one circuit for the M-Bus and less stepper motors.