A modern car is equipped with electronic control units for various systems: engine, anti-lock braking system, bodywork and others. Essentially, these units are microcomputers.
In order to understand what a CAN bus is in a car, imagine that a local network is organized in the car, to which these microcomputers are connected - so that they work in a complex.
This is similar to how office computers are connected into a network so that employees can easily take information from each other, and the boss has the ability to quickly monitor the work of office employees.
The head of the car is the on-board computer and the diagnostic system.
History of development and unification of Controller Area Network
BOSCH, conducting research in the field of automation in the 80s of the last century, proposed a microcontroller communication standard that could be used in the automotive industry.
The CAN standard is applied not only in cars. Currently, it is used in the concept of "smart home", industrial automation, etc.
Applied to automotive engineering the CAN (Controller Area Network) standard is adapted to the bus with the physical layer. It is organized using a twisted pair of conductors along which packets of signals of different polarity go.
This standard received the international classification ISO 11898. A frame (packet) includes an 11-bit information signal (or 29-bit in extended mode).
In general, a CAN bus may not necessarily be implemented with twisted pair conductors. It can be both fiber optic and radio channel.
It can be assumed that with the introduction of unmanned vehicles, the CAN bus is transformed into a mobile interface for transmitting information of one, and possibly a complex of cars.
Vehicle CAN bus: what it is and how it works
The bus is a local area network through which information is exchanged between control units of various vehicle systems. Thus, the control unit, for example, of a car engine, in addition to the main microcontroller serving the engine, assumes the presence of a CAN controller, which generates pulses on two buses: CAN-high and CAN-low (H and L).
These signals are transmitted over wires (twisted pair) by the transceiver. Transceivers, or transceivers, are designed for:
- amplification of signals,
- ensuring noise immunity of transmitted pulses;
- adjusting the bit rate;
- protection of the line in case of damage to the CAN bus.
Now the following types of transceivers are used in automotive technology - High Speed and Fault Toleran. High Speed Transmitter provides a relatively high data transfer rate - up to 1 megabit per second. The second type of transmitter has a lower data transfer rate - up to 120 kilobits per second. But it is less sensitive (tolerant to errors) to the quality of the CAN bus, and allows deviations of its parameters.
Data exchange organization diagram
The structural diagram of connecting various vehicle units to the CAN bus can be depicted as follows:
To match all devices, that is, to organize optimal conditions and speed of reception - transmission, the output impedances of the transmitters should be approximately the same.
In the event of a disconnection or damage to any of the control units of the vehicle systems, the bus resistance changes, impedance matching breaks down, which leads to a significant decrease in the bus speed. Such violations can lead to a complete loss of communication on the CAN bus.
Some vehicles use a separate gateway module to troubleshoot CAN synchronization problems.
Each message transmitted on the CAN bus has its own identifier, for example "coolant temperature" and a code corresponding to its value, such as "98.7 degrees Celsius". These are not necessarily absolute values, in most cases these are relative binary units, which are further converted into control and monitoring signals.
The same data is used by diagnostic tools to monitor and process information about the main systems of the vehicle.
The main modes of operation of the CAN bus:
- active (ignition on);
- sleeping (with the ignition off);
- waking up and falling asleep (when turning the ignition on and off).
During sleep mode, the bus current is at its lowest. However, at the same time, signals about the state of opening of doors and windows and other systems associated with the security functions of the car are transmitted via the bus (with a lower frequency).
Most modern diagnostic devices provide a mode for diagnosing errors via the CAN bus. Technically, this is organized by directly connecting the conductors to the diagnostic connector.
Advantages and disadvantages of using CAN bus in a car
To begin with, if the CAN standard had not been proposed in the 80s of the last century, a different type of interaction between car systems would necessarily take its place.
It is possible, of course, to place all the control units of the vehicle systems in a single superblock, in which the interaction of different systems can be programmatically ensured. Such attempts were made by French manufacturers. However, as functionality and performance increase, the likelihood of failures increases significantly. Malfunctions, such as wipers, can cause the engine to fail to start.
The main advantages of using the CAN bus:
- the ability to conduct operational control and;
- combining information streams in a single anti-jamming channel;
- universality, contributing to the unification of diagnostic processes;
- connectivity security systems via CAN bus (there is no need to pull the wiring to each control element).
Disadvantages of CAN bus:
- low reliability;
- damage to one of the control units can lead to complete inoperability of the CAN connection.
Trouble-shooting
There is no CAN malfunction indicator light on the vehicle dashboard. It is possible to judge that the performance of the CAN bus is impaired by indirect indicators:
- several indicator lamps of malfunctions on the dashboard simultaneously lit up;
- the coolant temperature indicators, fuel levels have disappeared;
First of all, you should perform diagnostics. If it indicates a CAN bus malfunction, you should start troubleshooting the problem.
Sequence of work:
- Find the twisted pair bus conductors. They are often black (high) and orange-brown (low) in color.
- Check with the ignition on using a multimeter for the voltage on the conductors. Levels should not be 0 or more than 11 Volts (usually around 4.5 Volts).
- Switch off the ignition, remove the terminal battery... Measure the resistance between the conductors. If it tends to zero, then there is a short circuit in the bus, if to infinity - an open circuit.
- Proceed to search for an open or short circuit.
- If there is a suspicion that the bus closure is due to the failure of a control unit, you can sequentially disconnect the control units and monitor the resistance and functionality of the bus.
CAN bus malfunction refers to complex malfunctions of the vehicle's electrical equipment. If the car owner does not have the necessary electrical repair skills, then it is better to use the services of a specialist.
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18702
In order to understand the principles of the CAN bus, we decided to write / translate a number of articles on this topic, as usual, based on materials from foreign sources.
One of such sources, which, as it seemed to us, quite aptly illustrates the principles of the CAN bus, was a video presentation of the CANBASIC educational product from Igendi Engineering (http://canbasic.com).
Welcome to the presentation of the new CANBASIC product, a training system (board) dedicated to the functioning of the CAN bus.
We will start with the basics of building a CAN bus network. The diagram shows a car with its lighting system.
Conventional wiring shown in which each lamp is directly connected to some kind of switch or brake pedal contact.
Similar functionality is now shown using CAN bus technology. Front and rear lighting devices connected to control modules. The monitoring modules are connected in parallel with the same bus wires.
This small example demonstrates that the volume of wiring is decreasing. On top of that, the control modules can detect burned out lamps and inform the driver about it.
The car in the indicated view contains four control modules and clearly reflects the construction of the CANBASIC training system (board)
In the above, there are four bus nodes (CAN nodes).
The front module controls the front lights.
The alarm unit monitors the interior of the vehicle.
The main control module connects all systems vehicle for diagnostics.
The rear assembly controls the rear lights.
On the CANBASIC training board, you can see the routing (location) of the three signals, Power, CAN-Hi and Ground, connecting in the control module.
In most vehicles, you need an OBD-USB converter to connect the main control module to a PC using diagnostic software.
The CANBASIC board already contains an OBD-USB converter and can be directly connected to a PC.
The board is powered by the USB interface, so no additional cables are needed.
Bus wires are used to transfer a lot of data. How it works?
How CAN bus works
This data is transmitted sequentially. Here's an example.
The person with the lamp, the transmitter, wants to send some information to the person with the telescope, the receiver (receiver). He wants to transfer data.
In order to do this, they agreed that the receiver monitors the status of the lamp every 10 seconds.
It looks like this:
After 80 seconds:
Now 8 bits of data have been transmitted at 0.1 bits per second (i.e. 1 bit in 10 seconds). This is called serial data transmission.
To use this approach in an automotive application, the time interval is reduced from 10 seconds to 0.000006 seconds. To transmit information by changing the voltage level on the data bus.
An oscilloscope is used to measure the electrical signals of the CAN bus. Two measurement pads on the CANBASIC board allow you to measure this signal.
The oscilloscope resolution is decreased to show the complete CAN message.
As a result, single CAN bits can no longer be recognized. To solve this problem, the CANBASIC module is equipped with a digital storage oscilloscope.
We insert the CANBASIC module into a free USB connector, after which it will be automatically detected. Software CANBASIC can be launched right now.
You can see the view of the software oscilloscope with the bit values attached. The data transferred in the previous example is shown in red.
To explain other parts of the CAN message, we color the CAN frame and attach descriptions to it.
Each colored part of a CAN message corresponds to an input field of the same color. The area marked in red contains user data information, which can be specified in bit, nibble, or hexadecimal format.
The yellow area defines the amount of user data. A unique identifier can be set in the green zone.
The blue area allows you to set the CAN message for the remote request. This means that a response from another CAN node will be expected. (The system developers themselves recommend not to use remote requests for a number of reasons leading to system glitches, but there will be another article about that.)
Many systems with a CAN bus are protected from interference by a second CAN-LO channel for data transmission, which is inverted with respect to the CAN-HI signal (i.e. the same signal, only with the opposite sign).
Six consecutive bits with the same level define the end of the CAN frame.
Coincidentally, other parts of the CAN frame may contain more than five consecutive bits with the same level.
To avoid this bit mark, if five consecutive bits with the same level appear, the opposite bit is inserted at the end of the CAN frame. These bits are called stuff bits (garbage bits). CAN receivers (signal receivers) ignore these bits.
With the help of the input fields, all the data of a CH frame can be specified and therefore every CH message can be sent.
The inserted data is immediately updated in the CAN frame, in this example the data length will be changed from one byte to 8 bytes and shifted back one byte.
The descriptive text indicates that the turn signal will be controlled with the identifier “2C1” and data bits 0 and 1. All data bits are reset to 0.
The identifier is set to "2C1". To activate the turn signal, the data bit must be set from 0 to 1.
In salon mode, you can control the entire module with simple mouse clicks. CAN data is set automatically according to the desired action.
Turn signal lamps can be installed on the low beam to work as a DRL. The brightness will be controlled by pulse width modulation (PWM), in accordance with the capabilities of modern diode technology.
Now we can activate the low beam headlights, fog lights, brake lights and high beam headlights.
When the dipped beam is switched off, the fog lights are also switched off. CANBASIC light system control logic matches cars Volkswagen brands... Ignition and return-to-home features are also included.
With a signal node, you can read the sensor signal after initiating a remote request.
In remote request mode, the second CAN frame will be received and shown below the sent CAN frame.
The CAN data byte now contains the sensor measurement result. By moving your finger closer to the sensor, you can change the measured value.
The pause key freezes the current CAN frame and allows accurate analysis.
As already shown, various parts of the CAN frame can be hidden.
In addition, hiding of each bit in the CH-frame is supported.
This is very useful if you want to use the CAN frame representation in your own documents, such as an exercise sheet.
Automotive electrical circuits have grown in size and complexity with each passing year. On the first cars produced, the ignition worked from the magneto, and there was no battery and generator at all. The headlights used acetylene burners.
In 1975, the length of the wires in the automobile electrical diagram was equal to several hundred meters and was comparable to the electrics of light aircraft.
The desire to simplify the wiring was as follows: you only need one wire, connect all consumers to it and connect a control device to each. Pass through this wire electric current to consumers and device control signals.
Video
By 1991, thanks to a digital breakthrough, Bosch and Intel created a CAN (Controller Area Network) interface for multiprocessor systems. on-board computers... In electronics, such a system is called a "bus".
In a serial bus, data is transmitted pulse by pulse over a twisted pair (two wires), and in a parallel bus, data is transmitted over several wires at the same time.
With higher performance, the parallel bus complicates the wiring of the vehicle. The serial bus transmits information up to 1 Mbit / s.
Different blocks share data, the rule by which this happens is called a protocol. The protocol can send commands to different blocks, request data from one or all of them. In addition to a specific call to the device, the protocol can set the importance and commands. For example, the command to turn on the engine cooling fan will take precedence over the command to lower the side window.
The minimization of modern electronics made it possible to organize the production of cheap control modules and communication systems. In the car network, they can be combined into chains, stars and rings.
Information goes both ways, for example, by turning on the lamp high beam, the signal on the instrument panel will light up - it is on or not.
The engine management system selects the best mode, receiving data from all devices in the circuit, the lighting system will turn on or off the headlights, the navigation system will plot or change the route, and so on.
Thanks to this protocol, diagnostics of the engine and other vehicle devices has been simplified.
The desire to have only one wire in the car did not come true, but the CAN module and the data transfer protocol increased the reliability of the system and simplified the wiring.
Video
CAN bus - what is it?
CAN - bus ("can bus") is a control system for all electrical devices and digital communication in a car, which can receive information from devices, exchange data between them, and also control them. Data on technical condition and control signals go in digital form over twisted pair due to a special protocol. From on-board network of the car, power is supplied to each consumer, but they are all connected in parallel. This option increased the reliability of the entire electrical circuit, reduced the number of wires and simplified installation.
The appearance of digital buses in cars occurred later than electronic components began to be widely introduced into them. At that time, they only needed a digital "output" to "communicate" with diagnostic equipment- low-speed serial interfaces like ISO 9141-2 (K-Line) were enough for this. However, the apparent complication of onboard electronics with the transition to CAN architecture has become its simplification.
Indeed, why have a separate speed sensor if the ABS unit already has information about the speed of rotation of each wheel? It is enough to transmit this information to the dashboard and to the engine control unit. For security systems, this is even more important: for example, the airbag controller is already becoming able to independently turn off the engine in a collision by sending the appropriate command to the engine ECU, and de-energize the maximum on-board circuits by sending a command to the power control unit. Previously, for safety, it was necessary to use unreliable measures such as inertial switches and squibs at the battery terminal ( BMW owners we are already familiar with its "glitches").
However, it was impossible to implement full-fledged "communication" of control units on the basis of the old principles. The amount of data and their importance increased by an order of magnitude, that is, a bus was required that is not only capable of operating at high speed and protected from interference, but also provides minimal transmission delays. For a car moving at high speed, even milliseconds can already play a critical role. A solution to satisfy such demands already existed in the industry - we are talking about CAN BUS (Controller Area Network).
The essence of the CAN bus
Digital CAN bus is not a specific physical protocol. The principle of operation of the CAN bus, developed by Bosch back in the eighties, allows it to be implemented with any type of transmission - even by wire, at least by fiber, at least by radio. The CAN bus works with hardware support for block priorities and the ability for the "more important" to interrupt the transmission of the "less important".
For this, the concept of dominant and recessive bits has been introduced: in simple terms, the CAN protocol will allow any block to communicate at the right time, stopping data transmission from less important systems by simply transmitting a dominant bit while there is a recessive bit on the bus. This happens purely physically - for example, if the "plus" on the wire means "one" (dominant bit), and the absence of a signal means "zero" (recessive bit), then the transmission of "one" will unambiguously suppress the "zero".
Imagine a class at the beginning of a lesson. The students (low priority controllers) talk quietly with each other. But, as soon as the teacher (high priority controller) gives a loud command "Silence in the classroom!" In contrast to the school class, this rule works on a permanent basis in the CAN bus.
What is it for? So that important data is transmitted with a minimum of delays, even at the cost of not transferring unimportant data to the bus (this distinguishes the CAN bus from the familiar to everyone on Ethernet computers). In the event of an accident, the ability of the injection computer to receive information about this from the SRS controller is incomparably more important than the ability of the dashboard to receive the next data packet on the driving speed.
In modern cars, the physical distinction between low and high priorities has become the norm. They use two or even more physical buses of low and high speed - usually it is a “motor” CAN-bus and a “body” one, the data streams between them do not intersect. Only the CAN-bus controller is connected to all at once, which makes it possible to "communicate" with all units through one connector.
For example, Volkswagen technical documentation defines three types of CAN buses used:
- The "fast" bus, operating at a speed of 500 kilobits per second, integrates the engine, ABS, SRS and transmission control units.
- "Slow" operates at a speed of 100 kbps and combines the units of the "Comfort" system (central locking, power windows, and so on).
- The third works at the same speed, but transfers information only between navigation, built-in phone, and so on. On older cars (eg Golf IV), the data bus and the comfort bus were physically combined.
Interesting fact: on Renault Logan the second generation and its "soplatformenniki" also physically have two buses, but the second one connects exclusively the multimedia system with the CAN controller, the second one contains the engine ECU, the ABS controller, the airbags, and the UCH at the same time.
Physically, cars with a CAN bus use it in the form of a twisted differential pair: in it, both wires serve to transmit a single signal, which is defined as the voltage difference on both wires. This is necessary for simple and reliable interference protection. An unshielded wire works like an antenna, that is, a source of radio interference is able to induce an electromotive force in it, sufficient for the interference to be perceived by the controllers as a really transmitted bit of information.
But in a twisted pair on both wires, the EMF value of the interference will be the same, so that the voltage difference will remain unchanged. Therefore, in order to find the CAN bus in the car, look for a twisted pair of wires - the main thing is not to confuse it with the wiring of the ABS sensors, which are also laid inside the car with a twisted pair to protect against interference.
The CAN bus diagnostic connector was not reinvented: the wires were brought out to the free pins of the already standardized one in the pads, in it the CAN bus is located on pins 6 (CAN-H) and 14 (CAN-L).
Since there can be several CAN buses on a car, it is often practiced to use different physical signal levels at each. Again, for an example, refer to Volkswagen documentation... This is how the data transfer in the motor bus looks like:
When no data is transmitted on the bus or a recessive bit is transmitted, on both wires of the twisted pair the voltmeter will show 2.5 V relative to "ground" (the difference in signals is zero). At the moment of transmission of the dominant bit on the CAN-High wire, the voltage rises to 3.5 V, while on the CAN-Low it drops to one and a half. The difference is 2 volts and means "one".
Everything looks different on the Comfort bus:
Here, "zero" is, on the contrary, a 5 volt difference, and the voltage on the Low wire is higher than on the High wire. A "unit" is a change in the voltage difference up to 2.2 V.
Checking the CAN bus at the physical level is carried out using an oscilloscope, which allows you to see the real passage of signals over a twisted pair: it is naturally impossible to "see" the alternation of pulses of such length with an ordinary tester.
"Decoding" of the vehicle CAN-bus is also carried out by a specialized device - an analyzer. It allows data packets to be output from the bus as they are transmitted.
You yourself understand that CAN bus diagnostics at the "amateur" level without the appropriate equipment and knowledge does not make sense, and it is simply impossible. The maximum that can be done by "improvised" means to check the kan-bus is to measure the voltage and resistance on the wires, comparing them with the reference ones for a particular car and a particular bus. This is important - above we specifically gave an example that even on the same car, there can be a serious difference between the tires.
Malfunctions
Although the CAN interface is well protected from interference, electrical problems have become a serious problem for it. The interconnection of the blocks into a single network made it vulnerable. The CAN-interface on cars has become a real nightmare for low-skilled auto electricians for one of its features: strong voltage surges (for example, winter) can not only "hang" a detected CAN bus error, but also fill the memory of controllers with sporadic errors of a random nature.
As a result, a whole "garland" of indicators lights up on the dashboard. And while a newbie is scratching his head in shock: “but what is this?”, A competent diagnostician will first of all put a normal battery.
Purely electrical problems are bus wire breaks, short circuits to ground or plus. The principle of differential transmission in case of breakage of any of the wires or "wrong" signal on it becomes unrealizable. The worst of all is the short circuit of the wire, because it "paralyzes" the entire bus.
Imagine a simple motor bus in the form of a wire on which several blocks "sit in a row" - a motor controller, an ABS controller, dashboard and a diagnostic connector. A break at the connector is not terrible for the car - all units will continue to transmit information to each other in the normal mode, only diagnostics will become impossible. If we break the wire between the ABS controller and the panel, we will be able to see only it with the scanner on the bus, it will not show either the speed or the engine rpm.
But if there is a break between the engine ECU and ABS machine, most likely, it will not start anymore: the unit, without "seeing" the controller it needs (information about the speed is taken into account when calculating the injection time and ignition timing), will go into emergency mode.
If you do not cut the wires, but simply constantly apply “plus” or “ground” to one of them, the car will be knocked out, since none of the blocks will be able to transmit data to the other. That's why Golden Rule an auto electrician, translated into Russian by the censor, sounds like "do not get crooked hands into the bus", and a number of car manufacturers prohibit connecting uncertified to the CAN bus additional devices third-party production (for example, alarm).
Fortunately, connecting the CAN-bus signaling not a connector to a connector, but crashing directly into the car's bus, gives the "curved" installer the opportunity to mix up the wires in places. After that, the car will not just refuse to start - if there is an onboard circuit control controller that distributes power, even the ignition is not a fact that it will turn on.
In order to coherently and harmoniously manage systems, ensure the quality and functionality of data transmission, many automotive companies use modern system known as CAN bus. The principle of its organization deserves a detailed consideration.
general characteristics
Visually, the CAN bus looks like an asynchronous sequence. Its information is transmitted over two twisted conductors, radio channel or fiber optic.
Several devices are capable of controlling the bus at the same time. Their number is not limited, and the information exchange rate is programmed up to 1 Mbit / s.
The CAN bus in modern cars is regulated by the "CAN Sorcjfication version 2.0" specification.
It has two sections. Protocol A describes the transfer of information using an 11-bit data transfer system. Part B performs these functions when using the 29-bit version.
CAN has personal clock nodes. Each of them sends signals to all systems at the same time. Receiving devices attached to the bus determine if the signal is within their competence. Each system has hardware filtering of messages addressed to it.
Varieties and labeling
One of the most famous today is the CAN bus developed by Robert Bosch. CAN BUS (the system is known under this name) is sequential, where the pulse is supplied after the pulse. It's called Serial bus. If information is transmitted over several wires, then this is a parallel bus Parallel bus.
I - control units;
II - system communications.
Based on the varieties of CAN bus identifiers, there are two types of marking.
In the case when the node supports the 11-bit information exchange format and does not indicate errors in the signals of the 29-bit identifier, it is marked "CAN2,0A Active, CAN2,0B Passive".
When these generators use both types of identifiers, the bus is labeled "CAN2,0B Active".
There are nodes that support communication in 11-bit format, and when they see a 29-bit identifier in the system, they give an error message. In modern cars, such CAN buses are not used, because the system must be logical and consistent.
The system operates at two types of signal transmission rates - 125, 250 kbit / s. The former are intended for auxiliary devices (windows, lighting), and the latter provide the main control (automatic transmission, engine, ABS).
Signal transmission
Physically, the CAN bus conductor of a modern car is made of two components. The first is black and is called CAN-High. The second conductor, orange-brown, is called CAN-Low. Thanks to the presented communication structure, the mass of conductors has been removed from the car circuit. In the production of vehicles, this allows the weight of the product to be reduced by up to 50 kg.
The total network load consists of disparate block resistances, which are part of a protocol called CAN bus.
The transmission-reception rates of each system are also different. Therefore, the processing of messages of different types is provided. According to the CAN bus description, this function is performed by a signal converter. It is called an electronic gateway.
This device is located in the design of the control unit, but it can be made in the form of a separate device.
The presented interface is also used to output and input diagnostic signals. For this, the presence of a unified OBD block is provided. This is a special connector for system diagnostics.
Varieties of bus functions
Exists different types presented device.
- KAN-bus of the power unit. It is a fast channel that transmits messages at 500 kbps. Its main task lies in the communication of control units, for example, transmission-engine.
- The "Comfort" system is a slower channel that transmits data at a speed of 100 kbps. It connects all the devices in the Comfort system.
- The bus information command program also transmits signals slowly (100 kbps). Its main purpose is to provide communication between service systems, such as telephone and navigation.
When studying the question of what a CAN bus is, it may seem that in terms of the number of programs it is similar to an aircraft system. However, in order to ensure quality, safety and comfort while driving, no programs will be superfluous.
Bus interference
All control units are connected to the CAN bus by transceivers. They have message receivers, which are selective amplifiers.
The description of the CAN bus stipulates the receipt of messages along the High and Low conductors to the differential amplifier, where it is processed and sent to the control unit.
The amplifier detects this output as the voltage difference between the High and Low wires. This approach eliminates the influence of external interference.
To understand what the KAN-bus and its structure are, one should remember its appearance. These are two conductors twisted together.
Since the noise signal goes to both wires at once, during the processing the value of the Low voltage is subtracted from the High voltage.
This makes the CAN bus a reliable system.
Message types
The protocol provides for the use of four types of commands in the exchange of information via the CAN bus.
I - CAN bus;
II - resistance resistor;
III - interface.
In the process of receiving and transmitting information, a certain time is allotted for one operation. If it exited, an error frame is generated. The Error Frame also lasts for a certain amount of time. The defective unit is automatically disconnected from the bus when a large number of errors have accumulated.
System functionality
To understand what a CAN bus is, you need to understand its functional purpose.
It is designed to transmit frames in real time, which contain information about the value (for example, a change in speed) or about the occurrence of an event from one transmitter node to program receivers.
The command consists of 3 sections: name, event value, variable observation time.
The key value is attached to the indicator variable. If there is no time data in the message, then this message is accepted by the system upon receipt.
When a communication system computer requests a parameter status indicator, it is sent in priority order.
Bus conflict resolution
When signals arriving on the bus arrive at several controllers, the system chooses in which order each will be processed. Two or more devices can start working almost simultaneously. To avoid a conflict, monitoring is performed. The CAN bus of a modern car performs this operation in the process of sending a message.
There is a gradation of messages according to priority and recessive gradation. The information with the lowest numeral expression of the arbitration field wins when a conflict occurs on the bus. The rest of the transmitters will try to send their frames later if nothing changes.
In the process of transferring information, the time indicated in it is not lost even in the presence of a conflict state of the system.
Physical components
The bus device consists, in addition to the cable, of several elements.
Transceiver microcircuits are often found from Philips, as well as Siliconix, Bosch, Infineon.
To understand what a CAN bus is, you should study its components. Maximum length conductor at a speed of 1 Mbit / s reaches 40 m. The CAN bus (also known as CAN-BUS) is endowed with a terminator at the end.
For this, 120 ohm resistors are installed at the end of the conductors. This is to eliminate message reflections at the end of the bus and to ensure that the bus is receiving the correct current levels.
The conductor itself, depending on the design, can be shielded or unshielded. The terminal resistance can deviate from the classic and be in the range from 108 to 132 ohms.
ICAN technology
Consideration should be given to the engine blocking program when considering vehicle tires.
For this, data exchange has been developed via the CAN bus, the iCAN module. It connects to the digital bus and is responsible for the corresponding command.
It has small dimensions and can be attached to any section of the bus. When the car starts moving, iCAN sends a command to the corresponding units, and the engine stalls. The advantage of this program is that there is no break in the signal. There is instruction electronic unit, the message then deactivates the functioning of the corresponding actuators.
This type of blocking is characterized by the highest secrecy and therefore reliability. In this case, errors are not recorded in the ECU memory. The CAN bus provides all information about the speed and movement of the vehicle to this module.
Anti-theft protection
The iCAN module is installed in any node where the harnesses are located, in the place where the bus is installed. Due to the minimal dimensions and a special algorithm of actions, it is almost impossible to identify the blocking by conventional methods when making theft.
Externally, this module is disguised as different monitoring sensors, which also makes it impossible to detect. If desired, it is possible to configure the operation of the device for automatic protection of car windows and mirrors.
If the vehicle has an auto-start engine, iCAN will not interfere with its operation, as it is triggered when the vehicle starts moving.
Having become familiar with the device and the principles of data exchange with which the CAN bus is endowed, it becomes clear why everything modern cars apply these technologies in the development of vehicle control.
The presented technology is rather complex in its structure. However, all the functions incorporated in it will ensure the most efficient, safe and comfortable driving.
Existing developments will help to protect the vehicle even from theft. Thanks to this, as well as a set of other functions, the CAN bus is popular and in demand.
