Introduction to Bus Systems: Multiplexing

Multiplexing relies on the use of digital communication between control units. A digital signal consists of a series of high and low voltage signals which represent "bits" of information.

Using the example of morse code for explanation, the letters SOS are represented in morse code as three dots - three dashes - three dots. Expressed as an electrical signal SOS would be represented as three short pulses - three long pulses - three short pulses.

The basis for digital communication is binary code. Binary code uses only 2 digits - 0 and 1. Electrically, 1 is represented by a voltage pulse and 0 is represented by a low voltage signal usually 0 volts.

In digital communication, each pulse represents a "bit" of data. Eight "bits" of information in a series of pulses makes up one "byte". A byte represents a character in a line of information (data).

In order to illustrate multiplexing in a vehicle application, an example of a K-Bus circuit will be used. The K-Bus (Body Bus) was introduced in the E38 as a low speed data transfer bus. One of the benefits of multiplexing is sensor sharing. The outside (ambient) temperature circuit is an example of "sharing" sensor information.

In the illustration shown below, the ambient temperature sensor is an analog input to the instrument cluster. The temperature information is used by the cluster for the outside temperature display for the driver. The outside temperature information is also needed by the climate control system (IHKA) for temperature control and additional functions.

In previous models (before bus systems), the IHKA required an additional dedicated outside temperature sensor. Using multiplexing principles, the K-Bus can transfer the temperature information (as well as additional data) from the cluster to the climate control system which eliminates the need for an additional sensor.