Further guidance
A CAN bus provides serial communication between control units. For example, a powertrain CAN bus allows an ABS control unit to broadcast a message containing wheel speed data simultaneously to the Engine Control Module (ECM), Transmission Control Module (TCM), Instrument Cluster (IC), and Supplementary Restraint System (SRS).
CAN messages are transmitted digitally as a series of low or high values within a fixed structure known as a frame. The smallest data unit within these binary encoded messages is a bit, logically representing either 0 or 1. A message identifier follows the start of the frame. The identifier assists message arbitration when two or more control units try to broadcast a message at the same time; the lower the identifier’s value, the higher the message’s priority. Various values, including the data payload, and a checksum follow the identifier.
When a control unit receives a message, it calculates a checksum from the data payload and compares it to the value broadcast within the message. If the two are equal, the message is valid. The receiving control unit confirms this by transmitting an acknowledgement during the penultimate bit of the message broadcast. Therefore, the broadcaster will know if a control unit has received an invalid message.
CAN buses are either low or high speed; low speed buses communicate at a fixed rate up to 125 kbit/s, whereas high speed buses communicate at a fixed rate up to 1 Mbit/s. A variant, CAN FD, communicates at variable rates up to 12 Mbit/s. The application determines the bus speed. For example, safety critical powertrain CAN buses require real time communication and are always high speed, typically operating at a rate of 500 kbit/s.
CAN gateways connect buses of different speeds or types. For example, an IC might act as an interface between the powertrain and convenience CAN buses to provide, amongst other things, automatic door locking functionality; e.g. a vehicle speed message from the ABS control unit on the higher speed bus can be transmitted to the convenience control unit on the lower speed bus via the IC. The convenience control module would then know to lock the doors once a certain speed has been reached.
Gateways can also control diagnostic access. When present, diagnostic testers must establish communication with the gateway via the DLC. The gateway then passes diagnostic messages between the tester and the other control units. The tester does not have direct access to the other CAN buses or their messages. Furthermore, it will not be possible to use the DLC as an access point to test CAN bus integrity. Alternative test locations must be identified.
The voltage difference between the CAN-L and CAN-H lines represents the bus’s logical state. Therefore, the lines are referenced to each other rather than to an external potential, such as the chassis ground. This differential arrangement improves noise rejection as interference affects the lines equally and their voltage difference is maintained. Typically, the lines are configured as twisted pairs to reduce interference effects.
On some CAN buses, where the attached control units share a common reference potential (e.g. the chassis ground), the CAN controllers can switch to single line operation to provide fault tolerance in the event of an open circuit on either the CAN-L or CAN-H lines.
High speed CAN buses use terminating resistors to remove transmission reflections within the bus; without the resistors, transmissions can bounce back from the end points and distort the messages. Typically, a 120 ohm resistor is used to connect the CAN-L and CAN-H lines within the two control units at each end of the bus. In this parallel configuration, the total resistance between the CAN-L and CAN-H lines is around 60 ohms. Therefore, measurement of this resistance will indicate the bus’s integrity. Resistance measurements must not be performed on buses without termination resistors, unless all the attached control units have been previously disconnected.
CAN bus faults can cause many symptoms. Typically, they are characterized by a partial or total loss of vehicle or system functionality or a visual or audible warning to the vehicle operator.
CAN buses can be prone to circuit faults, such as:
- Shorts of either the CAN-L or CAN-H lines to B-, B+, or each other;
- Open circuits within the CAN-L and CAN-H lines, termination resistors, or connections;
- Interference from untwisted CAN lines or a degradation in their shielding, as might arise from previous repairs, the use of puncture probes, abrasion, or general wear and tear; and
- Interference from other electrically noisy components.
Similarly, the connected control units can be susceptible to faults with their:
- Power supply or ground circuits;
- CAN controllers and transceivers; or
- Software, as might arise from memory corruption, incorrect programming or coding errors.
