2111 The Principle Of A Busbased Vehicle System

The controls (switches) for almost every system on a vehicle must be near the driver's seat. With ordinary wiring this means that there is a cable taking electrical power to the switch and another taking the electricity from the switch to the unit being operated.

As the amount of electrically-operated equipment on vehicles has increased a very large number of electrical cables have become concentrated near the driving position. This causes problems such as finding sufficient space for the wires, extra cable connectors that can cause defects etc. Multiplexed, or data bus-based systems overcome some of these problems.

Figure 2.12 shows the basic concept of multiplexed vehicle wiring. In order to keep it as simple as possible fuses etc. have been omitted because at this stage, it is the 'idea' that is the focus of attention.

1 & 2. Electronic switches for side and tail lights.

3. Electronic switch for head lights.

4. Electronic switch for rear window demister.

A, B, C & D. Dash panel switches for lights etc.

1 & 2. Electronic switches for side and tail lights.

3. Electronic switch for head lights.

4. Electronic switch for rear window demister.

A, B, C & D. Dash panel switches for lights etc.

Fig. 2.12 The multiplexed wiring concept

As the legend for the diagram states, the broken line represents the data bus. This is the electrical conductor (wire) which conveys messages along the data bus to the respective remote control units. These messages are composed of digital data (0s and 1s) as described earlier.

The rectangles numbered 1, 2, 3 and 4 represent the electronic interface that permits two-way communication between the ECU and the lamps, or the heated rear window. The dash panel switches are connected to a multiplexer (MUX) which permits binary codes to represent different combinations of switch positions to be transmitted via the ECU onto the data bus. For example, switches for the side and tail lamps on and the other switches off could result in a binary code of 1000, plus the other bits (0s and 1s) required by the protocol, which are placed on the data bus so that the side lamps are energized. Operating other switches, e.g. switching on the heated rear window would result in a different code and this would be transmitted by the ECU to the data bus, in a similar way. As the processor is moving the data bits at a rate of around 10 000 per second it is evident that, to the human eye, any changes appear to occur instantaneously.

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