In-vehicle controller area networks (CANs) have been around for some time having originally been developed in the early 1980s. The CAN serial bus system protocol was standardized in 1993 (ISO 11898-2) and is now available in hardware from many manufacturers for a range of applications that address a broad spectrum of industry areas including automotive.
As the electronic content in vehicles has increased dramatically with the inclusion of a multitude of convenience features such as powered mirrors, powered seat positioning, parking sensors, etc. so has the physical length of the CAN network. Also, customer demand and increasingly stringent government legislation has led to the development and implementation of a wide range of electronic safety features such as anti-lock braking, traction control, and multiple airbagsall of which add to the length of the CAN bus.
With separate buses for distinct types of features such as body electronics, interior lighting, and power train functions, the total bus length for high specification luxury vehicles may reach in the region of 3,000 meters (9,900 ft) and incorporate between 60 and 80 individual bus driven modules.
The need to achieve a minimum data ratearound 1 Mbit/sin order for the vehicle's electronic features to work correctly, coupled with the fact that as the bus length increases the data rate falls away, has created the need for CAN repeaters. Repeaters ensure that, in basic terms, the data rate and the quality of the signal on the bus are maintained. Originally repeaters comprising discrete components satisfied the growing needs of an automotive industry moving towards CAN networks. Now fully integrated single-chip repeaters that use advanced mixed-signal technology are lower cost, and more robust and reliable.
CAN networksa new approach just at the right time
The electronic infrastructure of vehicles has changed significantly in recent years with the move away from wiring looms towards CAN networks. Wiring looms are expensive and cumbersome, and unlike most systems on a vehicle have to be assembled in a relatively primitive fashion by feeding wires around a large template in a process that is more akin to weaving fabric than high-tech automotive assembly. A further difficulty with wiring looms is that if a problem is discovered either in the final test of the loom prior to installation into the vehicle, or worse, once the vehicle is sold and in the possession of its proud owner, the process of locating and rectifying the problem can be hugely time consuming and expensive.
Without the introduction of CAN networks, the rapid increase in the amount of electronic content in passenger vehicles would have resulted in such complex, expensive, and unwieldy wiring looms that the feasibility of implementing specific electronic features would most likely have been questioned. The routing of large bundles of wires in restricted spaces such as door panels containing several electronics modules would have been difficult, if not impossible.
CAN is a serial communications protocol which supports real-time control and multiplexing with a high safety level. It provides two communication services: Sending of a message known as data frame transmission; and the requesting of a messageknown as remote transmission request. Other services such as error signaling and automatic re-transmission of erroneous frames are performed automatically and are user-transparent.
Within a CAN system a multi-master hierarchy exists. This architecture allows the building of intelligent and redundant systems so that if one node is defective the network continues to work unimpeded. A further feature is that information sent is transmitted to all devices on the bus, all receiving devices read the message and then decide if it is relevant to them. This along with error detecting mechanisms and faulty message re-transmission guarantees data integrity.
The role of repeatersEnabling the network to cover greater physical distances. With increasing electronic content in passenger vehicles, CAN network lines are getting longer. Also, industrial applications such as elevators can potentially have networks with lines of between 100 and 200 meters (330 to 660 ft) in length.
Maintaining impedance levels for a network. This is critical at the interface with diagnostic devices and other equipment that are regularly connected and disconnected. For example, by connecting diagnostic equipment directly to the CAN-bus and eliminating the controller in-between, communication speed, diagnosis, and operation are improved. Another example can be found in a truck and trailer application where adding the trailer doubles the length of the CAN line. A repeater used at the interface keeps the impedance the same and maintains data rates regardless of whether the trailer is connected or not.
Repeaters are a vital part of a CAN system, they can perform roles that include:
Conventional CAN repeaters
Based on discrete component solutions, CAN repeaters used in many current applications comprise a module containing CAN transmitters and receivers, a microcontroller, and relevant supporting logic. The technology is well established, but in the latest applications such devices have a larger form factor, comparatively high power consumption, and lack of robustness compared to new approaches.