Construction of the Network

Construction of the network

General
The network is made up of a number of control modules (sometimes called nodes) that are connected to each other via two communication cables. Each control module is individually powered and grounded and receives messages/commands via two the 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.

Transfer speeds
The Volvo Controller area network (CAN) has two transfer speeds.
- HS CAN (high speed) has a transfer rate of 500 kbit/s.
- LS CAN (low speed) has a transfer rate 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 transfer rate is 9.6 kbit/s.

Terminating resistor
To prevent electrical reflections and interference in the CAN network, there are terminating resistors which bridge CAN H and CAN L. One at each end on both the high and low speed sides of the network. Each terminating resistor has a resistance of 120 ohm.
- For the HS CAN (High Speed) the terminating resistor is in the Central electronic module (CEM) and the engine control module (ECM).
- For the LS CAN (Low Speed) the terminating resistor is in the Central electronic module (CEM) and Driver information module (DIM).
Both the terminating resistors create a parallel circuit The following resistors 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 ohm.
- In the event of a short-circuit between the communication cables, the resistance is approx. 0 ohm.