M-Bus Troubleshooting

M-bus Troubleshooting

The failure of communication on the M-bus can be caused by several sources:
- Failure of the bus ribbon, e.g. open or shorted.
- Failure of one of the stepper motors attached to the bus, e.g. shorted to B+ or B-.
- Failure of the voltage or ground supply to the IHK control unit.




The M-bus is active at any time following KLR on. The IHK module provides the voltage necessary to support communication. The voltage level of the M-bus is 5V, but because status communication occurs at an average 50% duty cycle the observed voltage is approximately 2.5V. The presence of 2.5V means that communication is occurring.

Checking the M-bus ribbon is carried out just like any other wiring. Perform continuity tests between the connections of the stepper motors (all motors disconnected) and the control unit without forgetting to make sure that the data line has not shorted to ground or power.

It is recommended to use the "Wire Test" in "Preset Measurements" which is more sensitive than just a resistance check.

If Voltage level and the wire test are O.K, then looking at the communication signal may be useful.

The following is an example of a scope pattern that may be observed when checking the M-bus. Notice the very high frequency of the signal at approximately 20 kHz.




Example of correctly operating M-bus
Communication on the M-bus occurs continuously with an average Period duration of 50 [micro]s. When a command is issued by the IHK control unit the pattern will briefly change in period length and then return to the constant signal.

Failure of the Bus ribbon
The following faults can occur to the M-bus wiring:
- Short Circuit to B+
- Short Circuit to B-
- Bus line down (open)
- Defective plug connections (damaged, corroded, or improperly crimped)

The IHK control module checks the M-bus for continuous position feedback from the stepper motors. If the M-bus is interrupted then the control unit will store a fault for every stepper motor on the bus.

In order to determine if a stepper motor is at fault for the lack of communication, disconnect one stepper motor at a time while monitoring the M-bus signal line with a voltmeter or oscilloscope. The pattern or voltage should return to normal when the defective stepper is found. As a confirmation that communication is restored, change the setting on the IHK panel, if the remaining connected flap motors assume the selected position communication is OK.

Diagnosis of the M-bus is carried out by the DISplus/MoDiC via the IHK module. Available in the Diagnosis Program are:




- Fault/symptom driven test modules
- Diagnosis request (flap position)
- Component activation (flap activation)