Cables, Can H And Can L
The CAN H and CAN L cables must not be confused with HS CAN and MS CAN which are speeds on the control area network. CAN H and CAN L are a way of distributing signals within the multiplex communication network.
Communication occurs through two cables. These two cables are paired together.
The voltage levels for communication occur using differential voltage levels.
The reason for communicating using paired cables and differential voltage levels is that the network becomes less sensitive to interference.
The same message is transferred simultaneously by both cables, but different voltage levels are used.
- Binary 1 = 2.5 V on both CAN H and CAN L.
- Binary 0 = 4 V on CAN H and 1 V on CAN L.
When measured between CAN L and earth, the average voltage level is approx. 2.3 V.
When measured between CAN H and earth, the average voltage level is approx. 2.8 V.
These measurement values generally apply to "normal" traffic on the controller area network (CAN), i. e. if no fault has occurred that causes the control modules in the network to start transmitting error messages.
When measuring between the two CAN-cables, at normal traffic on the network, the average voltage level is approx. 0.55-0.90 V (up to max. 0.6-1.4 V)
Volvo's CAN-net meets ISO 11898-2. Other standard is 11898-3 where the voltage on CAN H changes from approx. 5 V to approx. 1 V, and CAN L from approx. 0 V to approx. 4 V when changing from logic "0" to logic "1". Other standard is SAE J2411 where only one cable is used for CAN-communication.
Certain control modules, such as the Supplemental restraint system module (SRS) and the Brake control module (BCM), also have CAN communication for their own sensors. These networks are called internal controller area networks (CANs) and have nothing to do with communication on the general controller area network (CAN). These controller area networks (CANs) are only used to transmit data between the relevant sensor and the control module that the sensor is connected to.