Construction Of the Network

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 serially connected. 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.

High and low speed sides on the network
The network consists of two parts: a high speed side (HS CAN) which transmits signals/messages between the central electronic module (CEM) and control modules in the engine compartment (generally), and a low speed side (LS CAN) which transmits signals/messages between the central electronic module (CEM) and control modules in the passenger compartment and luggage compartment.
The interface between the high and low speed sides is supplied by the central electronic module (CEM) which speeds up or slows down the 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 10.4 kbit/s.
There is another serial standard known as LIN. This is an international standard which is becoming more commonly used. Its function is almost identical to that of Volcano Lite. The difference is in the communication method and rate. The transfer rate for LIN 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 ohms.
- For HS CAN (high speed), the terminating resistors are located in the brake control module (BCM) and the engine control module (ECM).
- For LS CAN (low speed), the terminating resistors are located in the upper electronic module (UEM) and the rear electronic module (REM).
Both the terminating resistors create a parallel circuit The following resistors can be measured under specified conditions:
- When the CAN network is entire, the resistance on the network is approx. 60 ohms.
- In the event of an open-circuit on the wires, the resistance on the network is approx. 120 ohms.
- In the event of a short-circuit between the communication wires, the resistance is approx. 0 ohms.