It has been important to have different styles of communications for many years. Perhaps one of the earliest forms of this would be to communicate with flags. This allowed communications over long distances. However, this would require that the two individuals would need to be in visible site of one another. Technology soon helped this problem with the telegraph. The telegraph enabled communication over vast distances. It accomplished this with messages that were comprised of on-off electrical pulses against time. This on-off coding was referred to as Morse code. These on-off pulses made up letters that would enable words and complete messages to be created. This communication style of on-off electrical pulses is still used in modern vehicles to send and receive messages.
One such communication language is the Controller Area Network (CAN). The CAN protocol was developed by Bosch in the early 1980s for automotive in-vehicle networks. The first vehicle in which the CAN language was used was the 1991 Mercedes S class. The CAN protocol has emerged over the last 25 years as the dominant automotive programming language. In 2008, regulations required that all vehicles sold in the U.S. must use the CAN protocol to communicate with emissions-related microprocessors. This CAN standard is ISO15765. It is important to be aware, though, that a vehicle platform can have several different protocols and several different network topologies in use on the same vehicle. Even CAN-based networks on the same vehicle can be different, as we shall see.
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Understanding topology and protocol
It will be imperative to know which system topology (the way the individual modules are interconnected) and which protocol speeds are being used on a vehicle before you start to diagnose it. The network topology can be laid out in three distinct configurations: the ring, the star and the bus (Figure 1). Each of these topologies has advantages and disadvantages in their use. In the ring topology, for instance, if the communications wire becomes shorted or broken on dual ring networks, all the modules or nodes can still communicate. This is a closed architecture network, making it difficult to add nodes in the field. In the star topology, if the communication wire becomes shorted or broken, only one node will be lost. The problem with the star topology is the center node is the master and the other nodes are referred to as slaves. If the master node goes down, the entire network loses communication. The CAN network uses a bus topology and, if the communication wire becomes shorted or broken in the network, it will maintain communication as long as a dual wire bus is used.
The CAN bus topology uses a multi-master system where each node (also known as an ECU or module) is self-sufficient.