XC70 L5-2.5L Turbo VIN 59 B5254T2 (2003)
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
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.
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.
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
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.
Control Modules in the Control area Network (CAN)
Control modules in the Control area network (CAN)