Introduction

Introduction
The M-bus is used exclusively between a climate control module, e.g. IHKA/IHKR and a set of stepper motors used to move various air distribution doors.







These "smart" stepper motors have an integrated communication I.C. that allows them to listen as well as transmit information over a single bi-directional data link

To distinguish its position in the IHK housing, stepper motor is programmed by the manufacturer with its own electronic address.

Each stepper motor on the M-bus also has a unique part number that is printed on the body of the motor. It is vital that each stepper motor be installed in its correct location for proper operation due to the addressing.




A three wire ribbon connector is used to connect the stepper motor to the, IHK control unit. All of the stepper motors are connected in parallel to each other via the M-bus ribbon cable.

The ribbon contains:
- Power
- Ground
- Bi-directional signal wire

Each stepper motor acts as a "slave", it listens to all data on the bus, but only accepts or responds as long as the message is transmitted without errors and it recognizes it's own address.

M-bus protocol: The M-bus protocol differs from the CAN and the I/K/P busses in that communication takes place on a constant basis within a framework time of 650 microseconds.

When the IHK module is commanding a change in position of one or more stepper motors the sequence of data is:
1. Start bit
- Informs the stepper motors that a command is coming.
2. Synchro bit
- Establishes the message as originating from the IHK control unit.
3. Data field
- The command to move a stepper motor to a particular position.
4. Address field
- The IHK control unit names the stepper motor the command is intended for.

If the message was received by the stepper motor without error (oversampling) the stepper will carry out the command and transmit its acknowledgement:
1. Synchro bit
- Establishes the message as originating from the stepper motor.
2. Data field
- Status information from the actuator (feedback).
3. End of frame
- Closes the communication session.

Communication continues on the M-bus until the GM sends the "go to sleep" command over the K-bus.