Networking and Multiplexing

Even the most basic vehicles include many electronically controlled systems. If each electronic system had its own ECU, harness, and sensors, the weight of the added components would negate any efficiency the system provided. A vehicle’s multiple electronic systems could require more than 1 mile, or 1.6 kilometers, of insulated wiring, consisting of around 1000 individual wires and many terminals.

One solution to the problem is the use of a system that integrates sensors into a common wiring harness by combining many of the individual systems, where possible, into a multiplexed serial communications network so that they can share information. An added advantage of such a system is that if there is less wire and fewer connections, there is less chance of poor connections causing faults.

FIGURE 36-60
Typical CAN-bus diagram.

One system is referred to as a controlled area network bus (CAN-bus), and it uses two thin wires (called a twisted pair) to connect, or multiplex, many of the control units and their sensors to each other Figure 36-60. The output devices are referred to as nodes.

The advantage of a multiplex network is that it enables a decreased number of dedicated wires for each function; therefore, there is a reduction in the number of wires in the wiring harness, reduced system cost and weight, and improved reliability, serviceability, and installation. In addition, common sensor data, such as vehicle speed and engine temperature, are available on the network, so data can be shared, thereby reducing the number of sensors.

Networking makes it more efficient to add or modify system functionality through the addition of control units and the use of software updates. These updates allow the manufacturer to provide fixes for issues that may crop up after vehicles have been delivered to customers and put into service. Other control units can be added to the system by simply connecting them to the network and the new controlled device. Software can then be updated if necessary to control the new device. A diagnostic scan tool can be connected to the CAN-bus circuit to both extract operational information and command controlled devices to operate, which assists greatly in diagnosis and fault finding.