Automotive Intranet Network Technology
Controller Area Network (BUS) was introduced by Bosch in 1986 at the Society of Automotive Engineers (SAE) Congress in Detroit. Intel manufactured the CAN ships which were shipped one year later than 1986 and the whole world of automotive was changed. It is supported by international standard under ISO 11898. The one of the most prominent features of CAN bus is replacing heavy cables with lightweight 2 wire. In order to sent information back and for the from engine control units, you need a CAN Bus which is a set of electrical wires (CAN Low and CAN High). Every single engine control unit in the car has its own controller area network. The data is sent and received by node which are the devices on a CAN Bus. Each node consists of a CPU, CAN controller and a transceiver. The data with CAN is sent in frames of four types such as data frames, remote frames, error frames, overload frames.
There are other variations of CAN BUS such as low speed CAN, high speed CAN and Flexible Data Rate CAN. The low speed CAN is mostly used for automotive diagnostics such as dashboard controls and displays, power windows etc. It can transfer data up to 125 kbps and the wiring is considered more economical than high speed CAN. High speed CAN is used for communications between critical subsystems such as anti-lock braking system, electronic stability control, engine control units etc. It can transfer data with range of 1 kbit to 1 Mbit per second. CAN FD on the other hand have maximum data rare up to 1 Mbps to 8 Mbps. It is specified in ISO 11898–1. It can select larger or smaller message sizes, allows an engine control unit to dynamically change their transmission rates. It supports special protocols such as J1939 and CAN 2.0.
Theory of Operation
Normally, the two wires CAN High and CAN Low are at 2.5V determined by the two transistors. The voltage is determined by driver control applied to the CAN High and CAN Low wires. When both transistors are conducting, the voltage drop across the first transistor and the diode is 1.5 V. This will make CAN High wire pulls up to 3.5V. So, when the voltage drop across the second transistor and the diode is 1V, it will make CAN Low wire pull down to 1.5 V. The difference between the two wires will always be zero. The purpose of diodes is to protect the CAN Transceiver from high voltage. When two inputs of CAN High and CAN Low are same, it will give an output of 1 but if two inputs are different, it will give an output of 0.
The TXD block is used for ground fault protection and prevents the bus lines being driven to a permanent dominant state.
The temperature protection protects the CAN Bus Transceiver from damage by switching off the transmitter if the junction temperature exceeds a value of approximately 165 degree centigrade. It is also needed when a bus line short circuits.
CAN Bus Frame
It uses non return to zero bit encoding. A standard CAN Frame has 11 bits frame which is typically used in most cars while 29 bit identifier is sued for heavy duty vehicles. For multiple nodes to initiate messages at the same time, bitwise arbitration is used. In case of messages, it can be one of four types such as Data Frame, Remote Transmission Request Frame, Error Frame, and Overload Frame. To extract a CAN signal, you carve out the relevant bits, take the decimal value and perform a linear scaling:
Physical_value = offset + scale * raw_value_decimal
CAN BUS Cabling
If you ever worked with CAN Bus or FD network, you should know that 120 ohm termination is required at each end of the bus.
So, why use termination? Well, the answer is very simple, improve signal quality and get a recessive level defining a logical 1 in the communication. To improve signal quality, use twisted pair wire by protecting the signal from external electric fields when it experiences a common mode voltage noise level. The twist wire is also resistant to magnetic field disturbance. As you are aware that CAN Bus transmit binary data 0 or 1 and 1 is being considered recessive. So, a resistive load is placed between the two twisted pair wires to achieve certain load to this recessive level. The resistor in this case can be pull up or pull down, so in case of single wire CAN, it will be a pull up while for low speed CAN, you can use a combination of both pull up and pull down resistors. So, by placing two 120 ohm resistors on the cable at two locations will match the characteristic impedance of the line to prevent signal reflections and prevent data loss.
How to Test CAN Bus?
Set the digital multimeter to ohms, disconnect the CAN D-sub 9 pin from the network and place the probes of digital multimeter between pins 2 and 7, if it is terminated correctly, it should display approximately 60 ohms, if it reads different value then make sure that there is a 10 kilo Ohm between CAN High and the ground and then repeat the same step for CAN Low without any CAN communication.
If the problem still persists then connect oscilloscope to CAN High signal which should a signal level of 2.5 volts during the idle phase, you should also see the shape of bits nice and square without any ringing on the rising and falling edges. Now, if the idle level differs from 2.5 volt then this could result from bad common ground. Use a differential probe to obtain the voltage on CAN High relative to CAN Low. If you observe ringing it is due to impedance mismatch.
Now connect oscilloscope to the CAN Low signal which should show voltage decreasing to 1 Volt for the dominant bits and rising back to 2.5 volt for the recessive bits. Now if you observe the ground noise, then use differential voltage probe which will display the signal difference between CAN High and CAN Low and show 0 volt for the recessive bit. Now, if both CAN High and CAN Low are shorted then this will result in a lower signal level on the dominant amplitude.
The Future of CAN Bus
A rapid growth in vehicle telematics and IoT CAN loggers will lead to development of technology in connected vehicles (V2X) and cloud.
The telematics can integrate with existing applications such as vehicle tracking, trailer and asset tracking, maintenance improvements safety tracking and insurance risk management. It will work with a vehicle tracking device installed in a vehicle which will allow sending, receiving or storing of telemetry data. It will connect by using a CAN Bus port with a sim card and enable communication through a wireless network. Once the device collects the GPS data, it will transmit via general packet radio service, cellular network to a centralized server of T-mobile, Verizon, or sprint. This is where Data Scientists will come in handy to interpret the data and display it for end users via secure websites and apps programmed in android or swift software language. This data will include location, speed, braking, fuel consumption, vehicle faults etc. So, in nutshell, it will help achieve operational improvements such as decreased fuel costs, improved safety, elevated productivity and better payroll management.
On the other hand, V2X (vehicle for everything) will pave the way to fully autonomous driving by taking information from sensors, and other sources traveling via high band width, low latency. It includes several components such as V2V (vehicle to vehicle), vehicle to infrastructure (V2I), vehicle to pedestrian (V2P), vehicle to cloud (V2C) and vehicle to network communications (V2N).
V2C: Supports security firmware, updates, information and entertainment such as sharing update road conditions, maintaining up to date firmware.
V2V: protects the vehicle from blind instances in driving and allows the car to be more predictive like blind intersections, non line of sight scenarios
V2I: communicates to and from infrastructure possible hazards and road conditions to vehicles such as road construction, stopped traffic, other hazards on the road.
V2P: helps deter possible accidents by connecting cars to pedestrians, bicyclists through apps on smartphones and wearable such as pedestrians crossing the road, changing lanes.
There is a ongoing battle between dedicated short range communication (DSRC) which is using Wifi IEEE 802.11p and CV2X who is using Cellular LTE standard driven by 5GAA (5G automotive association). The advantages of DSRC permits low latency communication, complements LiDAR and radar in ADAS and provides interoperability to V2I and V2V systems while C-V2X provide low latency by exceeding 1 mile without network connection, able to use all features provided by LTE network, connect car to anything such as V2I, V2V, V2P and suited to systems around the globe.
It will be based on Wi-Fi offshoot, IEEE 802.11p (part of wireless access for vehicular environments program) running in the unlicensed 5.9GHz frequency band. It will go beyond line of sight limited sensors such as cameras, LIDAR, speed limit alerts, electronic parking, toll payments and will deliver performance immunity to extreme weather conditions such as rain, fog, snow etc. The upcoming technologies such as vehicular visible light communication, high bandwidth mmWave and high frequency 60 GHz will be incorporated in the 5G V2X access network. It will offer direct communications between vehicles and infrastructure as well as other road users. It will use transmission mode such as network connectivity to allow a vehicle to get information of road conditions, traffic and weather such as allowing vehicles to see around corners to avoid pedestrians and obstacles. Ford will actually deploy C-V2X in all vehicles beginning in 2022 using Qualcomm’s 9150 C-V2X chipset while DSRC is stand in Cadillac starting in 2017.
Automotive Automated Software Testing: After recognizing the requirement of the component/system you are developing and have modeled in simulation level then you have to verify it by taking several steps such as
Hardware Loop in the test: It will validate embedded controllers to save time and improve test coverage by testing vehicle components and embedded control systems.
Software Loop in the test: Without any hardware it is tested within a simulated environment and allows verifying the code coverage.
Model Loop in the test: This testing is done at early stages of the development cycle and simulated in the modeling framework without any physical hardware component.
Additional CAN Protocols
SAE J1939: It is used by diesel engine makers all over the world and considered high level protocol running on the CAN physical layer. It was developed originally to be used by heavy trucks and tractor trail rigs. The bus speed is either 250 or 500 kbps.
OBD II: This port is found in all cars and it is located within 2 feet of the steering wheel. It allows vehicle owners to diagnose vehicle problems and carry dozens of channels of real time data such as vehicle speed, coolant temperature, RPM and much more.
XCC/CPP: To connect ECUs to calibration systems, this protocol was developed. Its successor CAN Calibration Protocol was developed in the 1990s. The universal measurement and calibration protocol (XCP) can run on top of CAN bus, CAN FD, Flex Ray, Ethernet.
CANopen: This protocol was established in Germany in 1992. It is used for embedded applications and heavily used in motion control, motor control applications, robotics. The three basic communication models for CANopen are Master/Salve, Client/Server, and Producer/Consumer.
Other Automotive Communication Buses
There are other communication buses that are used for automotive applications such as
Media Oriented Systems Support (MOST): It was developed by partnership of car automakers called MOST Cooperation used to interconnect vehicle entertainment and information systems. You can connect up to 64 devices to a MOST ring network and transfer data at rates of 25, 50, and 150 Mbps. It is facing increasing competition from Automotive Ethernet. It is refereed as IEEE 802.3bw-2015 aka 100Base-T1. This technology has been adapted by some car makers for infotainment systems, driver assistance and even ADAS applications. It also transfer rates needed for LIDAR and other sensors, raw camera data, GPS, map data, flat screen displays.
Single Edge Nibble Transmission (SENT) SAE-J2716: This protocol was designed for sensors to send their data to engine control units. It transmits encoded data using pulse code modulation on a single wire. A typical data sent by this protocol is 32 bits (8 nibbles).
FlexRay: This protocol has been standardized under ISO 17458–1 to 17458–5. It is mainly used for power train, safety and chassis control applications. The data is transmitted over unshielded, twisted pair cables. Various network topologies such as Bus, Star, and Hybrid are all supported by this protocol. While CAN Bus uses arbitration bit to determine which data gets priority, FlexRay on the other hand uses Time Division Multiple Access (TDMA) method. It is more expensive to implement this protocol compared to CAN Bus.
Local Interconnect Network (LIN): This single wire network was defined by ISO 9141. It is serial unidirectional messaging system limited to one master and 15 salve nodes. Normally, used for low bandwidth applications such as electric windows, door locks, electric mirrors, power seats and similar. The data rate on LIN is limited to 19.2 kbps or 20 kbps.
Comparison of Vehicle Communication Buses
Conclusion:
So, CAN Bus and various other automotive communication buses are great for embedded applications. CAN Bus is widely used for automotive, industrial, aerospace applications. Also, the system design for CAN Bus system makes it ideal for companies investing in IoT connected cars. Although it still suffers from security issues such as lack of encryption and authentication.