What is the CAN-bus in the car
Abundance of electronics in modern cars is no surprise any more. More and more components and systems are becoming “smart” as they are connected to electronic devices which work using complex algorithms.
For communications between the connection points and the base it is necessary to use some kind of wiring. To get rid of the excessive web of wires, special rails help. In addition to the automotive industry, they can be used for other purposes.
What is a CAN bus in a car
Those motorists who want to know what it is, it is not necessary to refer to the catalogs of winter or summer rubber. We recommend deepening your knowledge of electronics.
The CAN bus of a car is a part of the electronic system of the car designed for rapid monitoring of the technical condition of the vehicle as a whole and its individual elements and systems.
The CAN-bus is mounted in the car to connect sensors and processors into a single information network that helps synchronize commands and information processing. It collects and exchanges data instantaneously. The parallel connection makes it possible to correct the signals sent to systems or units via sensors on the fly.
CAN is an acronym and stands for Controller Area Network, which can be translated as “network of controllers”. In fact, the bus is concerned with receiving information from devices around it and sending data to those devices. The standard was developed and first implemented over three decades ago.
Bus structure and where it is located
CAN is visually a block of plastic with printed circuit boards inside. On the outside of the bus there is most often a universal plug designed for the connection of different cables.
The digital interface is provided by built-in conductors, commonly referred to as CAN. Connection is made through a special cable.
In different makes of cars, the tires may be located in different places. The specific area where the unit is embedded can be found in the car’s instruction manual. Most often, such electronics are located in the cabin of the vehicle, hiding it with a control panel. In some brands, it is customary to install an electronic unit in the under-hood area.
The speed of interaction between the elements of the system, as well as the quality of communication between them depends on the operating parameters. Most often, the characteristics of most modern buses have the following values:
- information over the wired on-board network is transmitted at a speed of approximately 1 MB/s;
- during data exchange between individual control units, the speed is reduced to 500 kb/s;
- Information is transmitted in the interfaces, similar to “Comfort”, with low speed – close to 100 kb/s.
The newest model of the car, the more advanced is its wiring. In case of possible failures you will have to look for analogues.
Each element in the system has a special identifier, as the electronics work according to the principle of coded messages transmission. In this way it is possible to recognize different kinds of information, e.g. “car is moving at 60 km/h” or “crankshaft speed 2000 rpm”. This is checked by an indicator. If the information in the message relates to the scope of a particular unit, it is processed, otherwise the data is ignored.
The standard identifier length is limited to 11 or 29 bits. The design of each information transmitter is also designed to read the parameters sent over the interface. Nodes with low priority are characterized by freeing the data flow from the dominant devices. In this way the latter type does not distort the transmission process.
Data packets from the priority devices are routed with high priority, unaffected by the distortion. If there is a loss of network communication with any transmitter, the electronics automatically perform a recovery.
Bus interface in the car, connected to the autostart module or to the alarm system, is able to start and work in different modes:
- Background mode. In some sources it is called a sleep or autonomous mode. When it is started, the other systems of the car are disabled. Although there are no visible obvious signs of how it works, but power is sent from the onboard network to the digital interface. In this case, the voltage value will be minimal, as such CAN bus will quickly drain the battery in the car.
- Start or wake up. Starting is done after the motorist inserts the key into the keyhole and activates the ignition in a similar manner. When the car has a start/stop button, the mode starts after the button is pressed. A voltage stabilization takes place. The controllers and sensor are energized.
- Activation. After entering this mode, the actuator starts communicating with the built-in controllers. The circuit voltage rises noticeably, as the CAN bus interface operates with a consumption of up to 80-85 mA.
- Deactivation. The system enters the sleep mode after the engine is stopped. Also at this time the nodes and systems included in the network to the bus stop working and exchanging data. They are disconnected from the network.
It takes only a fraction of a second to enter the operating mode and stop the operation. Everything happens automatically and without direct user intervention.
Advantages and disadvantages of embedded buses
Like every system, CAN bus has its positive and negative characteristics. The main pluses are such factors:
- due to the high speed of the devices are able to communicate with packet data almost instantly;
- cable installations can withstand the effects of electromagnetic interference;
- the electronics are endowed with a multi-level control system, which helps minimize the occurrence of errors during the reception/transmission of data packets;
- the bus independently distributes the speed to CANalas due to automatics, having a positive effect on the operation of electronic systems as a whole;
- the manufacturers have taken care of a sufficient degree of security of the digital interface, so that unauthorized external connections will be instantly blocked;
- The use of digital interface in the design allows you to easily install alarm or other security systems with minimal interaction with the on-board regular system.
It is important to know the disadvantages of bus installation:
- Certain interface models are designed for a limited amount of packet data, which is unacceptable for modern cars, studded with a lot of electronics. Adding more data sources to the bus will have a negative impact on the load and will also significantly increase the response time of the equipment;
- The data transmitted through the communication channels has an exclusive purpose. Useful information takes up a minimum of traffic;
- standardization can be disconnected due to the introduction of a higher-level protocol.
Latest generation interfaces are more stable. It is preferable to choose machines with these buses.
Labels and varieties
With their task of communicating additional devices with the ECU, most buses successfully cope, working smoothly. In this situation, the CAN protocol ISO 15765-4 is used.
It is accepted conventionally to divide CAN into groups:
- CAN2, 0B. The marking is characteristic for the equipment used in the 11-bit format. It should be taken into account that information on potential errors will be sent to the microprocessors during the detection of 29-bit identifiers.
- CAN2, 0A. This marking is used for devices with 11-bit data exchange formats. The variety does not have the potential to detect errors from 29-bit modules.
Users distinguish three categories of interfaces:
- The first category is for the automotive engine. After connecting these types of interfaces, communication with the control system via an additional channel is improved. Due to this bus it is possible to synchronize the ECU with other nodes.
- The second category is a kind of “Comfort”. The interface is required to provide a connection with heated seats, control mirrors, comfort adjustments, etc.
- The third category belongs to the category of command-information varieties. It is used for connection to the service systems, for example, to connect the navigation or smartphone.
Protocols are used, according to which there are different types of commands sent via CAN.
Since the bus is in communication with a large number of elements, incorrect operation can lead to negative operating results. This can affect not only the performance of the car, but also the safety of driving.
The system may inform about the malfunction by indirect signs. Drivers should pay attention to it in the following cases:
- Check Engine indicator lights up;
- several icons on the dashboard light up almost simultaneously, for example, parking brake, airbags, high grease pressure, etc.
- It has become impossible to read information on the dashboard, as no information is displayed about the cooling temperature, fuel level, etc.
It is necessary to engage in mandatory more accurate testing. Qualified it can be carried out at the service station, because car service stations are equipped with professional equipment.
The presence of electronics in the car forces the manufacturers to introduce different interfaces. The reliable operation of the tires ensures the stable operation of the vehicle.
Oh these three letters… Calm down, everything is decent, I mean CAN.
In a CAN network, all ECUs are connected to the bus in parallel. Data exchange is done in short packets – messages.
CAN message Each message contains an identifier, which is unique in the network (e.g. “Engine temperature 100 deg” or “Vehicle speed 50 km/h”). When transmitting, all ECUs in the network receive the message and each of them checks the identifier. If the message is relevant to a given ECU, it is processed, otherwise it is ignored. The identifier can be 11 bits or 29 bits long.
Arbitration On the CAN bus, bits 0 and 1 have another name: recessive level and dominant level, respectively. If two different transmitters transmit recessive and dominant levels at the same time, the dominant level will suppress the recessive level. This suppression mechanism ensures arbitration on the bus. Each transmitter simultaneously reads what it sends to the bus. A transmitter with a lower priority is forced to let go of the bus because someone else’s dominant level with a higher priority has distorted its transmission. At the same time, the higher-level packet remains unchanged. The transmitter that lost arbitration can retry after some time.
Physical level Several types of CAN bus can be used in a vehicle.
High speed CAN is mainly used in engine management and chassis management networks. Where high-speed response is needed. Communication speed on this bus is 500 or 250 kbit/sec.
Diagram of ECU connection to high speed CAN bus
Low speed CAN is used in body control network. Communication speed of this bus is, as a rule, 125 kbit/sec.
Diagram of the ECU connection to a low-speed CAN bus
1-wire CAN is cheaper version of Low speed CAN, mainly used by GM. Used for communication between car body ECUs. Operates at 33.3 kbit/sec.
Diagram of ECU connection to single line CAN bus
Reliability Two-wire bus maintains its operation if one of the wires is cut or shorted (for two-wire bus).
CAN has been in use in cars for a very long time. Initially CAN bus was used in simple configurations. For example, for reliable and fast communication between the engine ECU and the automatic transmission ECU. In this configuration, the bus was used only for data transfer. The ECU was started with a line of power and a line from the ignition switch, the diagnosis was made on separate K-line, coming from each ECU.
In more modern cars, the CAN bus transmits not only control, but also diagnostic information. In addition, the CAN bus began to control the ECU power system. In this configuration, all ECUs are connected to a common power supply and CAN bus. The ignition switch is an electronic control unit and information about switching on the ignition is transmitted from it on the CAN bus.
It is possible to distinguish four main phases of the bus operation:
Sleep mode In this mode, all ECUs, except the lock ECU, are in the off state. The CAN driver is powered on. The driver is also in the sleep state. At the same time, its power consumption is about 0.3 mA. Waking up When the ignition key is inserted or the door is opened, the lock gives a dominant state to the CAN bus. This causes the CAN drivers in the dormant ECUs to wake up. The drivers turn on the power stabilizers in their ECUs when they detect activity on the bus. Active mode In active mode, the ECUs are constantly exchanging information. The power consumption of each transmitter at dominant levels can reach 80 mA. Sleep When the ignition is turned off, a CAN bus command is issued to shut down, after which each ECU de-energizes itself and enters sleep mode. Note: For a single line CAN bus wakeup signal is 12 V, the normal exchange is 0-4 V.
A little digression from the original source. The biggest plus, is the very high signal interference immunity. What’s the catch? There are two redundant signals going at the same time, only one high, the other low. They catch the interference. The interference affects both signals equally. And we have the same level at the output. The two signal levels compensate for the interference. You can see it like this: