PRIORITIZING
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 have to be prioritized. In addition the time delays which occur in case of queue situations must be held within reasonable limits. This is so that the customer does not experience the system as "sluggish" or slow.
To solve problems with conflicts and time delays there is a priority order of messages to ensure good functionality.
Prioritization of messages is determined by the number of zeroes at the beginning of a message, the more zeroes the higher the priority.
Prioritization occurs as follows
- When the network is available, all the control modules that have "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 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 acknowledgment is not sent (a reply is only sent to a direct question from another control module).
However, 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 generate a diagnostic trouble code.
Diagnostic trouble codes (DTCs) can also be stored if the receiving control module does not "understand" the message it has received. Examples of this may include intermittent short-circuits between one of the CAN wires and voltage or earth.
QUALITY FACTOR
In a distributed system where information (signals) is sent between different control modules in a network, it is usually the control module that generates/creates information (the signal) that knows the quality of the information that is sent. To be able to inform receiving control modules about the quality of information, a quality factor (Q-factor) is sent over the network at the same time as the information (signal). The quality factor describes status of the information (signal). For example, if the signal is defective due to a faulty sensor, a receiving control module can use the quality factor to decide if the control module shall take any action (e. g., reconfiguration).
The quality factor can be of the following status
- OK The value indicates that the signal is normal and shows a credible status and accuracy.
- Outside specified range Even though the value indicates that the signal is normal and may be usable, some fault has been detected that indicates that accuracy has been reduced so that performance may be affected or instability can occur.
- No data found The value indicates that the signal has its initial start-up value and has not yet had time to receive measured or calculated value. This may occur when the control module's start-up process is not completely finished. As soon as the control module has started, the signal shall have a correct value. Receiving control module has a built-in delay to wait for correct signal.
- No data exists The value indicates that the signal is faulty. This prevents the signal from being used by receiving control modules.
| Sending control module | Receiving control module and its function | |
|---|---|---|
| Status | "Common" control module | Control module with high demand for accuracy |
| OK | Normal function | Normal function |
| Outside specified range * | Normal function | Controlled limitation of performance may occur * |
| No data found | Normal function | Normal function |
| No data exists * | Limited or lost performance/lost function * | Limited or lost performance/lost function * |
SUMMARY OF QUALITY FACTOR THAT MAY APPLY
* Diagnostic trouble code may be stored.
COMPATIBILITY
The units must "speak" the same language and must be compatible with each other. A standardized communication protocol is used for this.
Signal configuration (sfg) contains the language between the modules. If any module has a signal configuration which does not tally with other modules, this module is unable to communicate with other modules. This means that all modules must have compatible signal configuration. The signal configuration is sometimes changed as new messages are added and old ones are deleted.
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 (CEM), and others.)
- Which control module should do what (for example "you are the control module for the driver's door - you are the control module for the passenger 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.
In the central electronic module (CEM), information is stored on which control modules are installed in this particular vehicle. This information is stored and retrieved from Volvo's central database when software is downloaded. The central electronic module (CEM) also contains information on which primary signal configuration is to be used in the vehicle. Information on the vehicle's signal configuration must be stored in all control modules connected to the CAN network so that they can communicate in the first place.
The signal configuration is a part of the software downloaded after a control module is fitted or replaced. When software is loaded to a control module, the information in the control module indicating, for example, that a new accessory has been connected is updated automatically.
Once software has been downloaded, some values may be changed according to the customer's requirements, for example whether lamp indication upon locking or unlocking the vehicle should be active, or the dip degree of the interior rear view mirror. These settings may disappear when new software is downloaded.
Software is downloaded via the Software Manager.
Note. Even if two vehicles appear identical they may behave differently due to differing configurations, a parameter may have been modified by, e. g., the customer or workshop.
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
High-speed net HS CAN (High Speed), example
Scheme 1500
Low-speed net LS CAN (Low Speed), example
Scheme 1501
To prevent electrical reflections and interference in the CAN network, there are terminating resistors which bridge CAN H (High) and CAN L (Low), one at each end (main line) on both the high and low speed sides of the network. Each terminating resistor has a resistance of 120 ohm
- 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).
Resistance measurement of parallel circuit with terminating resistor
Both the terminating resistors create a parallel circuit The following resistors can be measured under specified conditions
- When the CAN network is intact, the resistance on the network is approx. 60 ohm.
- In case of open circuits on the cables, so that 1 terminating resistor is not included in the circuit, the resistance in the net is approx. 120 ohm (only one main line is measured)
- An open circuit on the cables to control modules located "outside" the measured circuit cannot be detected with this measurement.
- In case of short-circuiting between the communication cables, the resistance is approx. 0 ohm between the cables, regardless of where the short-circuiting is located.
Scheme 1502
Scheme 1503
Scheme 1504
The number of control modules in the CAN network varies depending on the car model, equipment level and structure week, The structure week and construction week are not necessarily the same week. The construction week is the week in which the vehicle is built in the factory. The content of the vehicle is steered towards structure week. A structure week may extend over a number of construction weeks. Structure weeks are specified in the format YYYYWW.
For example, structure week 199815 indicates that the structure week of the vehicle is week 15, year 1998.
Note. To troubleshoot respective CAN-net, see appropriate diagnostic service information for CAN-related diagnostic trouble codes for Central electronic module (CEM). For example, choose any of CEM-DF01 - DF17 depending on CAN-net.
| Explanation | ||
|---|---|---|
| LS CAN | = | CAN Low Speed Section (125 kbit/s) |
| HS CAN | = | CAN High Speed Section (500 kbit/s) |
| Serial | = | Serial communication (Volcano Lite) between the slave control module and the CAN control module (10.4 kbit/s). LIN (9.6 kbit/s). |
| Slave control module | = | A control module with low computing power which is connected to a main "control module" and which only operates on commands from the main control module. |
| Option | = | Accessories installed at the factory to order, when purchasing a new car. |
| Accessories | = | Aftermarket installation. |
| Control module | Name/ Function | Network | Miscellaneous |
|---|---|---|---|
| AEM | Accessory electronic module (AEM) Controls certain accessory functions for the vehicle, This control module communicates with other control modules via the low speed side of the network. | LS CAN | Accessories |
| Audio control module | Audio module (AUM) Controls the radio / audio equipment and communicates with the other modules via the network low speed sector. | LS CAN | Standard/ accessory/option. |
| Brake control module | Brake control module (BCM) Controls the ABS/EBD/DSTC/STC functions of the brake system and communicates with the other modules via the network high speed sector. Contains a terminating resistor for the network. | HS CAN | |
| Climate control module | Climate control module (CCM) Controls the climate control system functions and communicates with the other modules via the network low speed sector. | LS CAN | STD/ MCC/ ECC |
| Central electronic module | Central electronic module (CEM) This is the network's main control module and the interface between the high and low speed sides. The control module has a comprehensive function in the electrical system and handles a large number of functions. | HS CAN/LS CAN | |
| DDM | Driver door module (DDM) Controls the drivers door functions and communicates with the other modules via the network low speed side. | LS CAN | |
| DEM | Differential electronic module (DEM) Controls four wheel drive engagement and communicates with the other modules via the network high speed side. | HS CAN | Option. |
| Driver information module | Driver information module (DIM) Controls the combined instrument panel functions and communicates with the other modules via the network low speed side. | LS CAN | |
| ECM | Engine control module (ECM) Controls the engine's functions and communicates with other control modules via the high speed side of the network. Contains a terminating resistor for the high speed side of the network. | HS CAN | Different versions depending on the engine alternative. |
| ICM | Infotainment control module (ICM) Controls the radio, navigation and telephone functions. This is the primary unit for communication with the units in the MOST network. | LS CAN | XC90 only. |
| PDM | Passenger door module (PDM) Controls the functions in the passenger side door and communicates with the other modules via the network low speed side. | LS CAN | |
| Carphone module | Phone module (PHM) Controls the functions of the carphone and communicates with the other modules via the network low speed side. | LS CAN | Option /accessory. |
| Power seat module | Power seat module (PSM) Controls the positions and functions of the driver's seat. Communicates with other control modules via the low speed side on the network. | LS CAN | Standard/ option NOTE: The power seat module (PSM) does not check seat heating and SIPS (side impact protection system) airbag function. |
| REM | Rear electronic module (REM) Controls the electrical functions in the rear section of the vehicle, and communicates with the other modules via the low speed side on the network. Contains a terminating resistor for the network. | LS CAN | |
| RTI | Road traffic information module (RTI) Controls the road traffic information functions and communicates with the other modules via the low speed side on the network. | LS CAN | Option /accessory |
| Steering angle sensor | Steering wheel angle sensor module (SAS) Collects information about the steering angle and communicates with other control modules using the high speed side on the network. | HS CAN | Option. Used in the DSTC system. |
| SRS | Supplemental restraint system module (SRS) Controls the vehicle's crash safety system and communicates with the other modules via the low speed side on the network. | LS CAN | |
| SUM | Suspension module (SUM) This control module handles the Four-C (Continuously Controlled Chassis Concept) function, i. e. constantly regulates damping in the shock absorbers. Communicates with other control modules via the high speed side on the network. | HS CAN | Option. Not XC90. |
| TCM | Transmission control module (TCM) Controls the automatic gearbox and communicates with the other modules via the high speed side on the network. | HS CAN | Vehicles with automatic transmission only. |
| Upper electronic module | Upper electronic module (UEM) Controls the electrical functions in the upper part of the passenger compartment. Also includes remote control receivers. Communicates with other control modules via the low speed side on the network. Contains a terminating resistor for the network. | LS CAN | Included in the interior rear view mirror. NOTE: There are two types of upper electronic module (UEM): one with an automatic dip function, and one without. |
| NOTE |
|---|
| The power seat module (PSM) does not check seat heating and SIPS (side impact protection system) airbag function. |
| NOTE |
|---|
| There are two types of upper electronic module (UEM): one with an automatic dip function, and one without. |
ELECTRICAL FAULTS
When there is a fault in the signal levels on the communication wire to the central electronic module (CEM), a diagnostic trouble code (DTC) is stored in the central electronic module (CEM). Each individual communication wire to the central electronic module (CEM), CAN L and CAN H on the low speed side of the network and CAN L and CAN H on the high speed side of the network, is checked. There are two different levels detected on each wire: short-circuit to voltage or short-circuit to earth. As the central electronic module (CEM) can also determine when a short-circuit on one or more wires takes place, the control module can also detect a short-circuit between the two CAN wires in the same part of the network.
This gives eight diagnostic trouble codes for detecting electrical faults.
HINT: The central electronic module (CEM) is unable to store diagnostic trouble codes (DTCs) for open-circuits on the CAN wires. Other diagnostic trouble codes (DTCs) may, however, be stored on account of this.
NO COMMUNICATION FROM THE CONTROL MODULE
The central electronic module (CEM) knows which control modules should be present in the CAN network and checks at regular intervals to ensure that all control modules are communicating. If any control module in the CAN network is not communicating, a diagnostic trouble code (DTC) is stored in the central electronic module (CEM). There is a specific diagnostic trouble code (DTC) for every control module in the CAN network.
FAULTY COMMUNICATION
Every control module, apart from the central electronic module (CEM), has two diagnostic trouble code (DTC) types which relate to faulty communication (the central electronic module (CEM) has only one diagnostic trouble code (DTC) type).
These are
- Faulty messages
- Configuration error (not present on the central electronic module (CEM)).
Diagnostic trouble code (DTC) for faulty configuration is missing from the Central electronic module (CEM) since the control module is always the main control module in the network.
Faulty messages
When a control module transmits faulty signals in its messages a number of times, the control module shuts down communication. This is referred to as the control module going to "Bus-off". A diagnostic trouble code (DTC) is stored in the control module that transmitted faulty messages, and the control module stops communicating on the CAN network. This is done so that the control module does not interfere with other communication on the CAN network. If the fault is permanent, it is not possible to communicate with the control module and there is no chance of reading diagnostic trouble codes (DTCs) from the control module that has shut down communication as long as the control module continues to be powered. If the control module which stored a diagnostic trouble code (DTC) is powered down (for example by switching off the ignition or if the fuse to the power feed is removed), the control module will attempt again to communicate when the control module is powered up again. When troubleshooting such faults, therefore, it is important to know what kind of power feed is used for the control module in question.
Configuration fault
The central electronic module (CEM) transmits at regular intervals a control message to all control modules on the CAN network, This message includes, among other things, the ID number of the signal configuration currently being used. The reason for this is that all control modules on the CAN network must have the same signal configuration if they are to be able to communicate.
If the ID number of a control module's signal configuration does not tally with the ID number transmitted by the central electronic module (CEM), a diagnostic trouble code (DTC) is stored in the control module with the faulty signal configuration.
The control module which stores the diagnostic trouble code (DTC) will stop communicating on the CAN network but will still monitor traffic and respond to diagnostics communication.
ADVANTAGES OF A NETWORK
Easier to add further functions and install accessories.
Because the control modules in the network are already connected to each other and are easy to add more information to, all that is required is
- to connect the sensors to the nearest control module
- to connect the controlled component to the nearest control module
- to download software to alter the configuration and programming of the network.
The length of the wiring and the number of components which are introduced with the vehicle are less than previously.
An example of this is the addition of cruise control for the car.
Before the introduction of the network the installation of control modules, switches, vacuum pumps, vacuum servos, hoses and cable harnesses was necessary.
With the network only the installation of a switch and the downloading of software which alters the configuration of the vehicle is required.
Easier to introduce logical functions.
Logical functions can be explained as "If this occurs then the following corrective action must be carried out". For example, the system is programmed so that if a tail light is broken, a message is transmitted via the control area network (CAN) to the driver information module (DIM) to warn the driver.
The only thing you need to do to introduce a logical function is alter the programming of control modules affected - the rear electronic module (REM) and the driver information module (DIM) in the example above.
Introduction of logic functions does not increase the number of components or cables.
Easy to adapt the system to customer and market requirements.
The functions can be altered depending on the requirements/demands of the customer and market. An example of this could be fog tail lamps. Certain markets use two fog tail lamps, others only use one on the driver's side. Previously it was required that different replacement parts were stored for different markets. Now the same replacement part can be used for all markets, by changing the programming depending on the market.
A similar basic system can be used for a whole model program.
Similar networks (hardware) can be used for a large number of different vehicles.
The only thing that differentiates the vehicles is
- the components (control modules, sensors controlled components etc.) which are connected to the system
- Which components do what.
- Which components/functions are standard /optional/accessories
- Configuration/programming of the system