CAN bus in modern cars. What is a CAN bus? CAN bus signal test CAN bus not working

Diagnostics and repair: CAN bus

21.02.2006

This is what (mostly) the same "tire" looks like CAN ', which in recent years we will have to deal with more and more often:

photo 1

This is an ordinary two-wire cable called Twisted Pair. .
The above photo 1 shows CAN High and CAN Low wires power unit.
These wires are used to exchange data between control units, they can carry information about vehicle speed, rotation speed crankshaft, ignition timing and so on.
Please note that one of the wires is additionally marked with a black stripe. This is how the wire is marked and visually determined. CAN High (orange-black).
Wire color CAN Low - orange-brown.
For the main color of the tire CAN adopted orange color.

In the drawings and drawings, it is customary to depict the colors of the bus wires CAN other colors, namely:

photo 2

CAN High - yellow color
CAN Low - green color

There are several types of tires CAN determined by the functions they perform:
Powertrain CAN bus(fast channel) .
It allows transmit information at a speed of 500 kbps and is used for communication between control units (engine - transmission)
Comfort CAN bus(slow channel) .
It allows transmit information at a speed of 100 kbps and is used for communication between control units that are part of the Comfort system.
Infotainment CAN data bus(slow channel), which allows you to transfer data at a speed of 100 kBit / s. Provides communication between different service systems (for example, telephone and navigation systems).

New car models are becoming more and more like airplanes - in terms of the number of declared features for safety, comfort and environmental friendliness. There are more and more control units and it is unrealistic to "pull" from each bunch of wires.
Therefore, in addition to the tire CAN there are already other tires that have received the names:
– LIN bus (single-wire bus)
– MOST bus (optical fiber bus)
– Bluetooth wireless bus

But let's not "blur the thought along the tree", let's focus our attention for now on one particular tire: CAN (according to the corporation Bosch).

Using the CAN bus as an example power unit, you can see the waveform:

Photo 3

When on High CAN bus dominant state, then the voltage of the wire rises to 3.5 volts.
In the recessive state, the voltage on both wires is 2.5 volts.
When on the wire low dominant state, the voltage drops to 1.5 volts.
("Dominant" - a phenomenon that dominates, dominates or dominates in any area - from dictionaries).

To improve the reliability of data transmission, the bus CAN a differential method is used to transmit signals over two wires, called Twisted Pair . And the wires that form this pair are called CAN High and CAN Low .
In the initial state of the bus on both wires, constant pressure at a certain (basic) level. for bus CAN power unit, it is approximately equal to 2.5 volts.
This initial state is called "resting state" or "recessive".

How signals are transmitted and converted CAN bus?

Each control unit is connected to CAN bus through a separate device called a transceiver, which has a signal receiver, which is a differential amplifier installed at the signal input:

photo 4

Coming by wire high and low signals are fed into the differential amplifier, processed and fed to the input of the control unit.
These signals represent the voltage at the output of the differential amplifier.
The differential amplifier generates this output voltage as the difference between the voltages on the High and low tires CAN.
This eliminates the influence of the base voltage value (for the CAN bus of the power unit it is 2.5 V) or any voltage caused, for example, by external interference.

By the way, about interference. As they say, "tire CAN quite resistant to interference, which is why it has found such wide application.
Let's try to figure this out.

CAN bus wires power unit are located in engine compartment and they can be affected by interference of various kinds, for example, interference from the ignition system.

Since the CAN bus consists of two wires that are twisted together, then the interference simultaneously affects two wires:

From the above figure, you can see what happens next: in the differential amplifier, the voltage on the Low wire (1.5 V - " pp ") is subtracted from the voltage
on the High wire (3.5 V - " pp ") and there is no interference in the processed signal (" Pp" - interference).

Note: Due to the availability of time, the article may have a continuation - much remains "behind the scenes".

Kucher V.P.
© Legion-Avtodata

Used cable

Multifunction cable

Checking Instructions

Voltage test (oscilloscope): To test the voltage, the battery must be connected and the ignition on.
Resistance measurement: When measuring resistance, the object to be measured must be de-energized before the measurement. To do this, the battery is disconnected. Wait 3 minutes for all capacitors in the system to discharge.

Information on the CAN bus

The CAN bus (Controller Area Network) is a serial communication bus system with the following features:

signal propagation occurs in both directions.
Each message is received by all bus subscribers. Each bus subscriber decides for himself whether he will use the message
Additional bus subscribers are added by a simple parallel connection.
The bus system forms a system with a driver. Each bus user can be a master or slave, depending on whether it is connected as a transmitter or receiver.
A two-wire connection is used as the transmission medium. Wire designations: CAN low and CAN high.
As a rule, each bus user can communicate on the bus with all other bus users. Communication on the bus is governed by access rules. The main difference between the data bus K-CAN (body CAN), PT-CAN (bus Engine CAN and transmission) and the F-CAN bus (chassis CAN bus) is:
- K-CAN: baud rate approx. 100 Kbps. Single wire mode possible.
- PT-CAN: baud rate approx. 500 Kbps. Single wire mode is not possible.
- F-CAN: baud rate approx. 500 Kbps. Single wire mode is not possible.

Master device: the master device is the active communication partner from which the communication initiative comes. The master has priority and controls the communication. It can send messages to the passive bus user (actuator) via the bus system and, upon request, receive its messages.

Executive device: the executive device is a passive participant in the communication. It receives a command to receive and transmit data.

Driver system: in a system with a master device, communication participants can at a certain point in time take on the role of master or executive device.

Oscillography K-CAN, PT-CAN, F-CAN

In order to be more clear whether the CAN bus is working properly, it is necessary to observe the communication on the bus. In this case, it is not necessary to analyze individual bits, but only to make sure that the CAN bus is working. The oscilloscope shows: ”CAN bus appears to be working without disturbance”.

K-CAN:

CAN low to ground: U min = 1 V and U max = 5 V

CAN high against ground: U min = 0 V and U max = 4 V

Oscilloscope settings for measuring on K-CAN bus:

Rice. one: K-CAN measurement: CH1 CAN low, CH2 CAN high

When measuring with an oscilloscope the voltage between the low CAN (or high CAN-High) wire and ground, a rectangular signal is obtained within the voltage range:

PT-CAN and F-CAN

CAN low to ground: U min = 1.5 V and U max = 2.5 V

CAN high against ground: U min = 2.5 V and U max = 3.5 V

These values are approximate and may vary, depending on the bus load, by up to 100 mV.

Oscilloscope settings for PT-CAN (or F-CAN) measurements:

Figure 2: PT-CAN measurement: CH1 CAN low, CH2 CAN high

Procedure for measuring resistance with terminating resistance K-CAN, PT-CAN and F-CAN

Resistance measurement verification process:

The CAN bus must be in a de-energized state
No other measuring devices must be connected (parallel connection measuring instruments)
The measurement is made between CAN low and CAN high wires.
The actual values may differ from the specified values by several ohms.

K-CAN

A separate resistance measurement cannot be carried out on the K-CAN bus, as the resistance varies depending on the switching logic of the ECU!

PT-CAN, F-CAN

To prevent signal reflection, two CAN bus subscribers (with the maximum distance in the PT-CAN network) are terminated with 120 Ω. Both load resistance connected in parallel and form an equivalent resistance of 60 ohms. With the supply voltage off, this equivalent resistance can be measured between data lines. In addition, individual resistances can be measured individually.

Instructions for measuring with 60 ohms: Disconnect an easily accessible control unit from the bus. Measure the resistance at the connector between the low and high CAN wires.

Note!

Not all vehicles have a terminating resistor on the CAN bus. The presence of a built-in terminating resistor on the connected vehicle can be checked using the corresponding wiring diagram.

CAN bus not working

If the K-CAN or PT-CAN data bus is not working, then there may be a short circuit or open circuit in the CAN high or low levels. Or the ECU is faulty.

Disconnect the CAN bus devices one by one until the block causing the fault (= control unit X) is found.
Check wires to ECU X for short circuit or open circuit.
Check computer X if possible.
This sequence of actions leads to success only if the short circuit has a tested wire from the ECU to the CAN bus. If the wire in the CAN bus itself has a short circuit, then you need to check the wiring harness.

We reserve the right to typographical errors, semantic errors and technical changes.

In order to streamline the work of all controllers that facilitate control and increase control of driving a car, a CAN bus is used. You can connect such a device to the car alarm with your own hands.

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What is a CAN bus and how it works

CAN bus is a network of controllers. The device is used to combine all the control modules of the car into one working network with a common wire. This device consists of one pair of cables called CAN. Information transmitted through channels from one module to another is sent in an encoded form.

Scheme for connecting devices to the CAN bus in Mercedes

What functions can the CAN bus perform:

connection to the car on-board network of any devices and devices;
simplification of the connection and operation algorithm auxiliary systems cars;
the unit can simultaneously receive and transmit digital data from different sources;
the use of the bus reduces the impact of external electromagnetic fields on the functioning of the main and auxiliary systems of the machine;
The CAN bus allows you to speed up the procedure for transferring information to certain devices and vehicle components.

This system operates in several modes:

Background. All devices are off, but the bus is powered. The voltage value is too low, so the bus will not be able to discharge the battery.
Launch mode. When the car enthusiast inserts the key into the lock and turns it or presses the Start button, the device is activated. The option to stabilize the power supplied to the controllers and sensors is enabled.
Active mode. In this case, data is exchanged between all controllers and sensors. When operating in active mode, the power consumption parameter can be increased up to 85 mA.
Sleep or shutdown mode. When the power unit is turned off, the KAN controllers cease to function. When the sleep mode is turned on, all machine nodes are disconnected from the on-board network.

Channel Vialon Sushka in his video told about the CAN bus and what you need to know about its operation.

Pros and cons

What are the advantages of CAN-bus:

Easy to install the device in the car. The owner of the machine does not have to spend money on installation, since you can do this task yourself.
Device performance. The device allows you to quickly exchange information between systems.
Immunity to interference.
All tires have a multi-level control system. Its use makes it possible to prevent errors in the transmission and reception of data.
During operation, the bus automatically spreads the speed over different channels. This ensures that all systems work optimally.
High security of the device, if necessary, the system blocks unauthorized access.
Large selection of devices various types from different manufacturers. You can choose an option designed for a specific car model.

What are the disadvantages of the device:

In devices there are restrictions on the amount of transmitted data. AT modern cars many electronic devices are used. Their large number leads to high congestion of the information transmission channel. This causes an increase in response time.
Most of the data sent over the bus has a specific purpose. A small part of the traffic is allocated to useful information.
When using the protocol top level the car owner may face the problem of lack of standardization.

Types and markings

The most popular type of tires are devices designed by Robert Bosch. The device can function sequentially, that is, the signal is transmitted after the signal. Such devices are called Serial BUS. On sale you can also find parallel buses Parallel BUS. In them, data transmission is carried out through several communication channels.

You can learn about the varieties, the principle of operation, as well as the capabilities of the CAN bus from the video shot by the DIYorDIE channel.

Taking into account different types identifiers can be divided into several types of devices:

CH2, 0A Active This is how devices that support 11-bit data exchange are marked. These nodes do not indicate pulse errors of the 29-bit node.
CH2, 0V Active This is how devices operating in 11-bit format are marked. The main difference is that when a 29-bit identifier is found in the system, they will send an error message to the control module.

It must be taken into account that in modern machines these types of devices are not used. This is due to the fact that the operation of the system must be consistent and logical. And in this case it can operate at several pulse rates - at 125 or 250 kbps. More low speed used to control additional devices, such as lighting in the cabin, power windows, wipers, etc. High speed needed to ensure the working condition of the transmission, power unit, ABS systems etc.

Variety of bus functions

Consider what functions exist for various devices.

Device for car engine

When connecting the device, a fast data transmission channel is provided, through which information is distributed at a speed of 500 kbps. The main purpose of the bus is to synchronize the operation of the control module, for example, the gearbox and the motor.

Comfort device

The data transfer rate on this channel is lower and is 100 kbps. The function of such a bus is to connect all devices belonging to this class.

Information and command device

The data transfer rate is the same as in the case of Comfort type devices. The main task of the bus is to provide communication between serving nodes, for example, a mobile device and a navigation system.

Tires from different manufacturers are shown in the photo.

1. Device for automotive internal combustion engine 2. Interface analyzer

Can there be problems with the CAN bus?

AT modern car the digital bus is in constant use. It works simultaneously with several systems, and information is constantly transmitted through its communication channels. Over time, the device may experience problems. As a result, the data analyzer will not function correctly. If problems are found, the car owner must find the cause.

For what reasons there are failures in work:

damage or breakage of electrical circuits of the device;
there was a short circuit in the system to the battery or ground;
could close the CAN-High or CAN-Low systems;
there was damage to the rubberized jumpers;
discharge battery or a decrease in voltage in the on-board network caused by incorrect operation of the generating device;
the ignition coil has failed.

When looking for causes, keep in mind that the malfunction may be incorrect work auxiliary devices installed additionally. For example, the reason may be a malfunction anti-theft system, controllers and devices.

About CAN bus repair dashboard in a Ford Focus 2 car can be found in a video shot by a Brock user - Video Corporation.

The troubleshooting process is as follows:

First, the car owner diagnoses the state of the system. It is advisable to carry out a computer check to identify any problems.
At the next stage, the voltage level and resistance of electrical circuits are diagnosed.
If everything is in order, then the resistance parameter of the rubberized jumpers is checked.

Diagnostics of the CAN bus performance requires certain skills and experience, so it is better to entrust the troubleshooting procedure to specialists.

How to connect an alarm via CAN bus

To connect the CAN bus with your own hands to the car alarm of a car with or without auto start, you need to know where the anti-theft system control unit is located. If the alarm was installed independently, then the search process will not cause difficulties for the car owner. The control module is usually placed under the dashboard near the steering wheel or behind the control panel.

How to make the connection procedure:

The anti-theft system must be installed and connected to all nodes and elements.
Find a thick orange cable, it connects to the digital bus.
The anti-theft system adapter is connected to the contact of the found tire.
The device is mounted in a safe and convenient place, the device is fixed. Everything needs to be isolated. electrical circuits to prevent rubbing and current leakage. Diagnostics of the correctness of the performed task is carried out.
At the final stage, all channels are configured to ensure the working state of the system. You also need to set the functional number of the device.

CAN-bus is a device that facilitates the control of the machine by exchanging information with other auto systems. Data transfer from one car unit to another is carried out through special channels using encryption.

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What is CAN bus

The electronic CAN interface in a car is a network of controllers used to combine all control modules into a single system.

This interface is a block with which blocks can be connected via wires:

anti-theft complex equipped with an autorun function or without it;
machine motor control systems;
anti-blocking unit;
security systems, in particular, pillows;
management automatic transmission gear;
control panel, etc.

Device and where is the bus

Structurally, the CAN bus is a block made in a plastic case, or a connector for connecting cables. The digital interface consists of several conductors called CAN. One cable is used to connect blocks and devices.

The installation location of the device depends on the model. vehicle. Usually this nuance is indicated in the service manual. The CAN bus is installed in the passenger compartment, under the control panel, sometimes it can be located in the engine compartment.

How does it work?

Principle of operation automatic system is to send encrypted messages. Each of them has a special identifier that is unique. For example, "the temperature of the power unit is 100 degrees" or "the speed of the car is 60 km / h." When sending messages, all electronic modules will receive the relevant information, which is checked by identifiers. When the data transmitted between devices is related to a specific block, then they are processed, if not, they are ignored.

The length of the CAN bus identifier can be 11 or 29 bits.

Each information transmitter simultaneously reads the data transmitted to the interface. device with more low priority should let go of the tire, since the dominant level with high rate distorts its transmission. At the same time, the boosted package remains untouched. A transmitter that has lost connection restores it after a certain time.

Interface connected to signaling or module auto start, can function in different modes:

Background, which is called sleeping or standalone. When it is started, all the main systems of the machine are disabled. But at the same time, the digital interface receives power from the mains. The voltage value is minimal, which helps to prevent battery discharge.
Startup or wakeup mode. It begins to function when the driver inserts the key into the lock and turns it to activate the ignition. If the machine is equipped with a Start/Stop button, this happens when it is pressed. The voltage stabilization option is activated. Power is supplied to controllers and sensors.
Active. When this mode is activated, the data exchange procedure is carried out between the regulators and executive devices. The circuit voltage parameter increases as the interface can draw up to 85 mA of current.
Deactivation or sleep. When the power unit stops, all systems and components connected to the CAN bus stop functioning. They are deactivated from electrical network vehicle.

Characteristics

Technical properties of the digital interface:

the total value of the information transfer rate is about 1 Mb / s;
when sending data between control units various systems this figure is reduced to 500 kb / s;
the data transfer rate in the Comfort interface is always 100 kb/s.

The channel "Electrical Engineering and Electronics for Programmers" talked about the principle of sending packet data, as well as the characteristics of digital adapters.

Types of CAN buses

Conventionally, CAN buses can be divided into two types according to the identifiers used:

CH2, 0A. So labeled digital devices, which can function in 11-bit data exchange format. This type of interface, by definition, cannot detect errors on signals from modules operating with 29 bits.
CH2, 0V. This is how digital interfaces operating in 11-bit format are marked. But key feature is that error data will be transmitted to microprocessor devices if a 29-bit identifier is detected.

CAN buses can be divided into three categories according to type:

For the power unit of the car. If you connect this type of interface to it, this will provide fast communication between control systems via an additional channel. The purpose of the bus is to synchronize the operation of the engine ECU with other nodes. For example, a gearbox anti-lock braking system etc.
Comfort devices. This kind of digital interface is used to connect all systems in this category. For example, electronic adjustment mirrors, heated seats, etc.
Information and command interfaces. They have the same data transfer rate. They are used to provide high-quality communication between the nodes necessary for servicing the vehicle. For example, between electronic unit management and navigation system or smartphone.

The channel "Electrical Engineering and Electronics for Programmers" spoke about the principle of operation, as well as about the types of digital interfaces.

Instructions for connecting an alarm via CAN bus

When installing an anti-theft system, a simple option for connecting it to onboard network- to tie security installation with digital interface. But this method is possible if there is a CAN bus in the car.

To install a car alarm and connect it to the CAN interface, you need to know the installation location of the system control unit.

If the signaling was set by specialists, then you need to seek help with this issue at the service station. Typically, the device is located behind the dashboard of the car or under it. Sometimes installers put the microprocessor module in free space behind the glove compartment or car radio.

What will be needed?

To complete the task you will need:

multimeter;
stationery knife;
insulating tape;
screwdriver.

step by step

Connection procedure anti-theft installation to the CAN bus is carried out as follows:

First you need to make sure that all elements security complex installed and working. We are talking about a microprocessor unit, an antenna module, service button, siren, as well as limit switches. If the alarm has an autostart option, you need to make sure that this device is installed correctly. All elements of the anti-theft installation are connected to the microprocessor unit.
A search is made for the main conductor going to the CAN bus. It is thicker and its insulation is usually dyed orange.
The main car alarm unit is connected to this contact. The digital interface connector is used to perform the task.
The control unit is being installed security system if it hasn't been installed. It should be placed in a dry and inaccessible place for prying eyes. After installation, the device must be fixed with high quality, otherwise vibrations will negatively affect it during movement. As a result, this will lead to a rapid breakdown of the module.
The junction of the conductors is carefully insulated, the use of heat shrink tubing is allowed. It is recommended to additionally wrap the wires with electrical tape. This will increase their service life and prevent the erasure of the insulating layer. When the connection is made, a check is made. If there are problems in the transmission of packet data, using a multimeter, you should diagnose the integrity of the electrical circuits.
At the final stage, all communication channels are configured, including additional ones, if any. This will ensure smooth operation security system. Used for setting service book, which is included in the package of anti-theft installation.

The user Sigmax69 spoke about the connection of the security complex with a digital interface using the Hyundai Solaris 2017 car as an example.

Faults

Since the CAN interface is tied to many vehicle systems, if one of the nodes breaks down or malfunctions, it may malfunction. Their presence will affect the functioning of the main units.

Signs and causes

The following “symptoms” can report the occurrence of malfunctions:

on the dashboard, several icons lit up at the same time for no reason - airbags, steering, pressure in the lubrication system, etc.;
appeared luminous Check indicator engine;
there is no information on the control panel about the temperature of the power unit, the fuel level in the tank, speed, etc.

Reasons for malfunctions in the CAN interface:

broken wiring in one of the systems or damage to power lines;
short circuit in the operation of the units to the battery or ground;
damage to the rubber jumpers on the connector;
oxidation of contacts, as a result of which signal transmission between systems is disrupted;
discharge of the car battery or a drop in the voltage value in the electrical network, which is associated with the malfunctioning of the generator set;
closure of CAN-high or CAN-low systems;
the occurrence of malfunctions in the operation of the ignition coil.

The channel "KV Avtoservis" told more about breakdowns of the digital interface and testing using a computer.

Diagnostics

To determine the cause of the problem, you will need a tester, the use of a multimeter is recommended.

Verification process:

Diagnostics begins with a search for a twisted-pair CAN bus conductor. The cable has black or orange-grey insulation. The first is the dominant level, and the second is the secondary.
Using a multimeter, the voltage value on the contact elements is checked. When performing the task, the ignition must be turned on. The test procedure will reveal the voltage in the range from 0 to 11 volts. In practice, this is usually 4.5 V.
The ignition is switched off. The conductor with a negative contact is disconnected from the battery; first, the clamp must be loosened with a wrench.
The resistance parameter between the conductors is measured. You can learn about the closure of contacts if this value tends to zero. When the diagnostics showed that the resistance is infinite, then there is a break in the power line. The problem may lie directly in the contact. It is required to check the connector and all wires in more detail.
In practice, a short circuit usually occurs due to a breakdown of the control devices. To search for a failed module, turn off the power to each unit in turn and check the resistance value.

User Filat Ogorodnikov spoke about CAN bus diagnostics using an oscilloscope.

How to make an analyzer with your own hands?

Do your own assembly this device only a professional in the field of electronics and electrical engineering can.

The main nuances of the procedure:

In accordance with the diagram on the first photo in the gallery, you need to purchase all the elements for developing the analyzer. The components are signed on it. You will need a board with an STM32F103C8T6 controller. You will need an electrical circuit of a stabilized regulatory device and a CAN transceiver MCP2551.
If necessary, a bluetooth module is added to the analyzer. This will allow you to write basic information to the mobile device during the operation of the device.
The programming procedure is carried out using any utility. It is recommended to use the programs KANHacker or Arduino. The first option is more functional and has the option of filtering packet data.
To implement the firmware, you will need a USB-TTL converter device, you will need it for debugging. A simple option is to use ST-Link version 2.
After downloading the program to a computer, the main EXE file must be flashed into the controller using a programmer. After completing the task, a bootloader jumper is placed, and the manufactured device is connected to the PC via the USB output.
You can upload the firmware to the analyzer using software MPHIDFlash.
When the software update is completed, disconnect the wire and remove the jumper. Drivers are being installed. If the device is assembled correctly, then on the computer it will be defined as a COM port, this can be viewed in the task manager.

Photo gallery

Scheme for developing a CAN analyzer Main board for device assembly

Pros and cons of CAN buses

Advantages of the digital interface:

performance. The device can quickly exchange packet data between different systems.
High resistance to electromagnetic interference.
All digital interfaces have a multi-level control system. Thanks to this, it is possible to prevent errors in the transmission of information and its reception.
During operation, the bus itself spreads the speed through the channels in automatic mode. This ensures effective work electronic systems vehicle.
The digital interface is secure. If someone tries to get illegal access to the electronic components and systems of the car, the bus will automatically block this attempt.
The presence of a digital interface allows you to simplify the installation of the security system on the car with minimal interference with the regular on-board network.

Disadvantages of the CAN bus:

Some interfaces have limits on the amount of information that can be transmitted. This disadvantage will be significant for a modern car, "stuffed" with electronics. When additional devices are added, more than high load. Because of this, the response time is reduced.
All packet data that is sent over the bus has a specific purpose. For useful information the minimum amount of traffic is allocated.
If protocol is applied advanced level, this will cause a lack of standardization.

Video "Do-it-yourself CAN-interface repair"

User Roman Brock talked about the procedure for restoring the dashboard tire in a Ford Focus 2 restyling car.