Serial Data Bus

Shared Information over serial data bus

A data bus with a "Class A" speed rating is a relatively slow, low-speed circuit that typically carries less than 10 kilobits (10 Kbps) of information per second. A data bus that operates at Class A speeds is limited to simple command functions like operating power mirrors, power seats, power widows, power door locks, remote trunk releases and lights.

A data bus with a "Class B" rating, by comparison, may operate from 10 Kbps up to 125 Kbps, depending on the operating protocol (SAE J1850 or Europe's ISO 9141-2). This is fast enough to carry more complex information and time-sensitive data. Systems that may share a data bus with a Class B rating include electronic instrumentation, electronic transmission controls, security systems, and climate control.

Class C is currently the fastest data bus rating. Class C systems can operate at speeds up to 1 megabits per second, which is up to 100 times faster than a typical Class B data bus. Many of the vehicles that are currently using a Class C data bus are operating at speeds of around 500 Kbps, which is fast enough for powertrain control modules, air bag modules, and fast-acting antilock brake and stability control systems. Down the road are coming even faster systems, with "class D" ratings of over 1 megabytes per second. And some applications such as onboard entertainment systems require even higher speed audio and video streaming.

One thing to keep in mind about the CAN standard is that CAN as well as other protocols such as SAE J1939, GMLAN, OBD2, SAE J1587 and LIN have more to do with the way information is formatted, transmitted and received than how fast it is sent. This means the automotive engineers who design the onboard electronics for CAN-compliant vehicles are free to choose any operating speed they want (up to one megabits per second) as well as the type of bus conductor (one wire, twisted paired wires or a fibre optic cable). On most cars today, a high-speed data bus is needed to handle the volume of information going back and forth between all the onboard electronics.

In 1995, GM introduced its own "Class 2" data bus to handle communication between modules. The system ran at a speed of 10,400 bits per second (10.4 Kbps), which was more than adequate for vehicles a decade ago. In 2004, GM moved to their next generation data bus system which they called "GMLAN" (GM Local Area Network). Introduced on the Cadillac XLR and Saturn Ion, GMLAN added the capability to operate at two speeds on two separate buses: a low speed (33.33 Kbps) bus and a high speed (500 Kbps) bus.

The low speed side of the GMLAN system operates on a single wire bus to handle body-related control functions, while the high speed bus uses two wires to carry data between the powertrain, transmission and antilock brake modules. A "gateway" node connects the high speed bus and low speed bus, and allows information to be shared back and forth. For example, the radio (which is connected to the low speed bus) may adjust volume based on engine speed and vehicle speed (from the high speed bus) to offset road noise.

Mercedes also uses several different bus speeds on their vehicles. Depending on the application, there may be a high-speed 500 Kbps CAN-C bus for the powertrain, transmission and ABS modules, and a slower-speed 83 Kbps CAN-B bus for the body control functions. On some Mercedes cars, there may be as many as 30 modules on the CAN-B bus. Up to model year 2002, all communication between the CAN-C and CAN-B bus went through the electronic ignition switch (EIS) module. After 2002, a new "gateway" module handles the inter-bus communications as well as onboard diagnostics via a CAN-D bus.

Data is sent as a series of digital bits consisting of "0's" and "1's". If you looked at the data on a scope, you would see a square wave pattern that changes between a high and low voltage reading. The low voltage reading usually corresponds to the "0" while the high voltage reading corresponds to the "1". The actual voltage readings will vary depending on the application and protocols the vehicle manufacturer is using, but most operate in the 5 to 7 volts range.