Construction Of The Network

The Controller Area Network (CAN)

Construction of the network

General
The network is made up of a number of control modules (sometimes called nodes) which are connected to each other serially via two communication cables. Each control module is supplied with power and grounded individually and receives messages / commands via the two communication cables. In both networks (high and low speed sides) the control modules are connected to a number of different spurs. In the event of an open-circuit those control modules beyond the open-circuit will not be able to communicate with other parts of the network. However, a failure in one of the spurs can lead to communication failure with one control module while communication in the rest of the same network is working.

High and low speed sides on the network
The network consists of two parts. A high speed side (HS CAN) transmitting signals/messages between the central electronic module (CEM) and control modules that are mainly in the engine compartment. A low speed side (LS CAN) transmitting signals/messages between the central electronic module (CEM) and control modules that are mainly in the passenger compartment and cargo compartment.

The interface between the high and low speed sides is supplied by the central electronic module (CEM) which converts the transmission speed up or down for communication between the two network sides.

Transmission speeds
The Volvo control area network (CAN) has two transmission speeds.
- The HS CAN (High speed) has a transfer speed of 500 kbit/s
- The LS CAN (Low speed) has a transfer speed of 125 kbit/s.

1 kbit/s = 1024 bits per second (1 byte = 8 bits).

For serial communication (Volcano Lite) between a control module and a slave control module the transmission speed is 9.6 kbit/s.

Terminating resistor
To prevent electrical reflection and interference in the control area network (CAN) there are terminating resistors which bridge the CAN H and CAN L. One at each end on the high and low speed sides of the network. Each terminating resistor has a resistance of 120 Ohms.
- For the HS CAN (High Speed) the terminating resistor is in the steering wheel module (SWM) and the engine control module (ECM)
- For the LS CAN (Low Speed) the terminating resistors are in the driver information module (DIM) and the supplemental restraint system module (SRS).

Both the terminating resistors create a parallel circuit. The following resistances can be measured under specified conditions:
- When the control area network (CAN) is intact the resistance on each side of the network is approximately 60 Ohms.
- If there is a short-circuit between the communication cables the resistance is approximately 0 Ohm.