CAN Network
CAN-network (Controller Area Network)
General
CAN (Controller Area Network) is a standardised system for multiplex-communication. Volvo has developed a new standard for multiplex communication - VOLCANO. VOLCANO is a further development of CAN and operates with real time operation and prioritising. The description of prioritising is developed from VOLCANO but is designated CAN.
The standard for Control area network (CAN) specifies:
- That two cables should be used (CAN H and CAN L)
- Which voltage levels should be used
- What a message should look like
- How transfer errors should be handled.
Cables, CAN H and CAN L
The CAN H and CAN L cables must not be confused with HS CAN and LS CAN which are speeds on the Controller 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. CAN H is always white and CAN L is always green.
The cables are made of copper and are easily identified by the green and white colouring.
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.
Average voltage level is 2.5V.
Controller area network (CAN) message
The message consists of the following components:
- Identifier ("flag"), which indicates the message identity and prioritisation
- Data information (value, information etc.)
- Check sum, used to check that the message has arrived correctly
- Stop signal, which indicates that the message has finished.
A complete Controller area message is called a frame.
Prioritising
Conflicts can occur in a network when several control modules wish to send a message at the same time. For example when the driver presses the brake pedal at the same time as the passenger changes the climate control settings and a passenger in the rear seat opens the power window.
For safe function the messages must be prioritised. In addition the time delays which occur in the event of a queue situation must be held with reasonable limits. This is so that the customer does not experience the system as slow.
To solve problems with conflicts and time delays there is a priority order of messages to ensure good functionality.
Prioritisation of messages is determined by the number of zeroes at the beginning of a message, the more zeroes the higher the priority.
Prioritisation occurs as follows:
- When the network is available all the control modules with "something to say" send bit one in their message
- All the control modules detect what has been transmitted on the network
- If a control module has transmitted 0 those that have sent 1 stop and wait until the next time the network is available
- Those that transmitted 0 transmit bit two of the message
- If a control module has transmitted 0 as bit two those that have sent 1 stop and wait until the next time the network is available and so on.
The message with the highest priority (most zeroes at the beginning) "wins" and is sent first.
The end of a message is seven zeroes. The control modules then know that the network is available and a new message can be sent in priority order.
Two types of message
There are two types of message in the system:
- Periodical frames. These messages are sent regularly and give the present status of a parameter. They are used for information which is frequently updated, speed signals for example
- Event frames, which are only sent when predetermined conditions have been met. This type of message is used for things that seldom occur, raising / lowering a window for example.
The message can contain an update bit which states how "fresh" the information is.
The system assumes that the receiver has received the message so an acknowledgement is not sent (a reply is only sent to a direct question from another control module).
But the receiver knows how often it should receive a message about which status applies. If the message is missing the receiver can connect an emergency program and / or set a diagnostic trouble code.
Compatibility
The units must "speak" the same language and must be compatible with each other. A standardised communication protocol is used for this.
Signal configuration (sfg) contains the language between the units. If any module has a signal configuration which does not correspond to other units the module cannot communicate. This means that all the units must have compatible signal configuration. The signal configuration is occasionally modified so that the new messages are added and old messages removed.
Configuration
Instructions for the following are downloaded when a system is configured:
- Which control modules are included in the system (for example central electronic module, and others.)
- Which control module should do what (for example "you are the control module for the passenger door - you are the control module for the driver's door")
- Which functions should be included (for example if the alarm function should be on or off)
- Which components are connected to the control modules (for example whether the inclination sensor is included in the alarm or not)
- Which messages a control module is to transmit and which it is to receive
- Where the different data should be stored.
The configuration must be adapted after installation of accessories and must be downloaded again after any control module is replaced. The configuration is adapted and downloaded via VIDA.
Note! Even if two cars appear identical, they may behave differently because of differing configurations, a parameter may have been modified by, e.g., the customer or workshop.