All two-wire CAN bus lines terminate in a resistor(s) of a known value. Some schematics may include other information in the boxes with two arrows pointing in opposite directions. In the auto repair world, the term used to describe the design, layout and behavior of a serial data bus configuration is “topology.” Reading the Wiring DiagramĪs a technician in the modern vehicle era, you’re going to need to understand these “bus lines.” The dotted line at the edge of the component, node or module indicates where the CAN bus enters and exits.
What it can tell you is that a module is communicating and the bus is active.īut, the most critical skill for working on serial data buses is learning how to read the wiring diagrams to figure out how modules and sensors are structured on the bus.
You are never going to be able to look at the signals on a scope, decipher a series of 1s and 0s, and say that it is a command to turn on the brake light.
Modern buses are able to do this with better software and with hardware that can interpret the signals with faster processors.įaster speeds are needed so the ABS and PCM modules can communicate quickly if a stability control correction needs to be made that involves closing the throttle and applying the brakes. The faster the switching, the more information can be transmitted in a given amount of time. What separates the earliest serial data bus from a modern CAN bus is how fast the system can toggle between 0 and 5 volts. This is how all serial data buses operate. Some systems use a variable pulse width that not only toggles between on/off, but can transmit additional information by varying the length of time the voltage is either on or off.
Whatever the bus message, it’s comprised of 0s and 1s, or the states of highs and lows. This could be a either a J1850 or CAN-Hi bus. It could represent what the throttle position voltage is, a signal being sent from the airbag module to the BCM reporting the status of a sensor. If a signal was on an equal length of time as it was off, you would have 0, 1, 0, 1, 0, 1 as the binary message being sent out. The extra 2 volts gives the network a safety buffer that may help the vehicle as it ages. This extra voltage is to accommodate resistance in the wires and ground problems that may cause voltage drops. On most automotive serial data buses, the peak voltage level might be 7 volts. This includes laptops, DVD players and PCMs. Zero volts on any serial data bus is translated into binary language as “1,” and when the voltage increases the voltage to a specified level, it equals “0.” Most electronic devices operate on signals toggling between 0 and 5 volts.
This code can communicate commands that allow something as simple as rolling up a window or as complex as stability control correction.
Modules toggle the signal off and on, making the 1s and 0s of digital binary language like Morse code. Serial data buses may seem like a daunting concept to some technicians, but understanding them is now a required skill to work on most modern vehicles.Ī serial data bus uses voltage to communicate. One sensor can share information with multiple modules without having to connect directly to the multiple modules. Serial data buses also help to eliminate multiple sensors and wiring. If each system required its own switch and wiring, the complexity of the wiring harness and switches would be a diagnostic nightmare. On a modern vehicle, the position of the brake pedal is used by the shift interlock, ABS system, cruise control, traction control, brake lights and electric emergency brake. If serial data buses did not exist, a wiring harness would have to be five times its normal size and use twice as many sensors to deliver the same level of functionality and safety we see in the modern vehicle.