Reading In Push-Button Positions

Multifunction steering wheel
The voltage codes from the buttons on the multifunction steering wheel are read in by the instrument cluster (A1) over the horn/airbag clock spring contact (A45).
Here it is necessary t0 observe a few peculiarities:
- All buttons on the multifunction steering wheel are read in directly by the instrument cluster over 2 leads only (3rd lead is for control illumination.)
If voltage coding of the various button functions were accomplished only using various resistors in corresponding increments, sufficient stability against interference would no longer be ensured. For this reason, the buttons are read in as follows:
The voltage generated by the instrument cluster (A1) for checking the buttons changes polarity continuously. The buttons on the multifunction steering wheel are subdivided into two groups (left and right button groups (S110, S111) with the associated buttons for the horn).
In each case, a diode is connected in series with these button groups whose polarity is different for each group. The left or right button group is checked depending on the polarity of the test voltage from the instrument cluster (A1). The button check "switches" back and forth between the two switch groups. The actual voltage coding is therefore accomplished within one group of buttons only.

Priorities
If a number of buttons are passed simultaneously within a button group on the multifunction steering wheel, only one function is actuated by the instrument cluster (A1) in each case. This requires different priorities for the various functions within one button group. The button with the lowest resistance has the highest priority.

- Priorities within right button group on multifunction steering wheel (S111):
1. Fanfare horns pushbutton (S111s5)
2. Button for termination of telephone call
3. Button for accepting telephone call
4. - button, for setting specific functions and volume control
5. | button, for setting specific functions and volume control

Example:
If the button for terminating a telephone call is pressed simultaneously with the volume button, only actuation of the button for terminating the telephone call is relayed on!

- Priorities within left button group on multifunction steering wheel (S110):
1. Fanfare horns pushbutton (S110s5)
2. Voice control system button
3. System selection button
4 Page back button
5. Page forward button

Editing of multifunction steering wheel button statuses
The function of the multifunction steering wheel buttons depends partially on the current content of the multifunction display (A1p13). For this reason, the button statuses are interpreted in relation to the current display contents in the instrument cluster (A1).

Example:
If the + button, setting specific functions and volume control (S111s1) on the multifunction steering wheel is pressed, the volume of the audio system is normally increased.
However, if the multifunction display (A1p13) momentarily shows an adjustment, e.g. for the time, the time can be set by pressing the button. In this case, the volume remains the same.

Interlinkage
The multifunction steering wheel button statuses are relayed via the CAN bus. (A1 - A40/3, A2) as a message from the instrument cluster (A1) to the radio (A2) or to the COMMAND operating display and control module (A40/3).