Modern regular vehicles can have 30-70 ECUs, while sophisticated luxury units can have up to 150. These computers use a standardized mode of communication via the 2-wire CAN bus harness.
The harness interconnects these controllers (CAN nodes or ECUs), transmitting electrical signals in the wires for the nodes to interpret the incoming data. Here’s how this harness operates.
Table of Contents
- What Is a 2-Wire CAN Bus Harness?
- Why Use a 2-Wire CAN Bus Harness?
- 2-Wire CAN Bus Harness ISO Standards
- 2-Wire CAN Bus Harness Electrical Properties
- Applications of a 2-Wire CAN Bus Harness
- Wrap Up
What Is a 2-Wire CAN Bus Harness?
A 2-wire CAN bus harness is a multi-master serial bus that interconnects electronic control units in a vehicle, meaning the CAN bus network must have at least two controllers.
Some of these nodes can also be gateways that allow laptops and other general-purpose computing devices to connect to the controller area network via an ethernet port or USB.
Components of CAN Bus Nodes
CAN bus nodes must have these three components.
- Transceiver (Medium Access Control): Receives and converts the data stream from the physical CAN bus level to a state the controller can understand. It also does the reverse, converting the CAN controller’s data stream to an electrical signal to send to the bus for transmission.
An electronic control unit
- CAN Controller (Data Link Layer): Receives and stores the serial bits until an interrupt triggers the CPU to access the data. Also, it transmits the CPU’s data serially into the bus when free.
- CPU: Interprets the received message and decides what to send to the other nodes. Also, it connects to actuators, sensors, and control devices.
Therefore, the CAN bus forms the physical layer for electrical signal transmission.
Why Use a 2-Wire CAN Bus Harness?
CAN bus cables contain two wires for data transmission, each with a unique function?
The CANH (CAN high) wire transmits or operates at a higher voltage than the CANL (CAN low) wire. CANH usually operates at a range of 2.5V to 3.5V, while CANL runs at 1.5V to 2.5V.
This bus uses differential wired AND signals to transmit the electrical signals. When CANH is greater than CANL, the bus goes into a dominant state.
If a CAN node wants to send a logic 0 data bit, it drives the CAN bus to this dominant state.
A motorcycle ECU plug
But when CANH is less than or equal to CANL, the bus goes into a recessive state. A CAN node drives the bus into this recessive state using the resistor terminations if it wants to transmit a logic 1 data bit.
The two wires are usually a twisted pair with a 120-ohm characteristic impedance.
2-Wire CAN Bus Harness ISO Standards
Two ISO CAN bus industry standards define the transmission speeds of the wires.
ISO 11898-2 (High-Speed CAN)
This standard can hit transmission rates of up to 1Mbps on CAN and 5Mbps on CAN-FD (CAN Flexible Data rate), the second generation of CAN.
The bus running this standard is linear and terminated on each end using 120-ohm resistors.
CAN node signaling using this standard drives the CANH wire towards the higher end (3.5V) and CANL towards the lower end (1.5V) when transmitting in a dominant state (logic 0).
But if none of the nodes is transmitting a 0 data bit, the terminating resistors return the voltage of the wires to a recessive state (logic 1). This condition makes the nominal differential signal voltage equal to zero.
Electrical wires connecting the rear side of an ECU
But there is a slight allowance. Receivers translate any nominal differentiating voltage of less than 0.5V to be a recessive state (logic 1). On the other hand, the dominant nominal differentiating voltage is 2V.
ISO 11898-3 (Low Speed/Fault Tolerant CAN)
This standard can have a linear, star, or multiple-star bus architecture with a node termination being a fraction of the total termination resistance (close to but not less than 100 ohms).
The standard’s signaling is similar to that of the high-speed type but with broader voltage swings and a lower transmission speed that maxes out at 125kbps.
In the dominant state (logic 0), the nodes drive CANH towards the connected device’s power supply voltage, usually 3.3V or 5V. On the CANL side, the nodes push the voltage towards 0V.
But in the recessive state (logic 1), the terminating resistors pull the CANH voltage to 0 and CANL to 5V.
This setup allows the ECUs to have simple receivers, but both bus wires should be able to handle -27 to 40V without failure.
2-Wire CAN Bus Harness Electrical Properties
This 2-wire CAN bus wiring harness has the following electrical properties.
Each CAN bus must have a resistor termination to return the wires to their recessive states and suppress reflections.
High-speed CAN buses are ideal for linking different operating environments in industrial and automotive applications.
They are like the WAN in internet connections to interconnect distant parts using a linear, fast data bit rate link, such as fiber.
Multi-colored wires connecting the engine control unit
But low-speed CAN buses are suitable for interconnecting groups of nodes like in a LAN.
Slower Transition From Dominant to Recessive
In both standards, the nodes actively control the electrical signals in the dominant state (logic 0), but the terminating resistors handle the recessive state.
Therefore, the transition speed is faster when switching from a recessive to a dominant state. However, the transition speed from dominant to recessive depends on the cable length and wire capacitance.
Power and Ground Connections
Terminating biases include power and common reference ground connections to the 2-wire CAN bus harness, making it a 4-wire cable type. This setup ensures each CAN bus has a termination and electrical bias at the end.
The CAN-GND signal wire usually connects to the cable shield cable shield to prevent CAN bus system interference.
A car wire harness with multiple connectors
Maximum and Minimum Voltage
The CAN bus standards only state the minimum and maximum common mode bus voltage but don’t specify how to achieve this voltage control.
Applications of a 2-Wire CAN Bus Harness
The CAN bus wiring harness is synonymous with vehicle module communication, but it is also prevalent in the following areas.
- Aviation and navigation electronic equipment
- Boats and ships
- 3D printers
- Building automation
- Escalators and elevators
- Mechanical control and industrial automation
- Agricultural equipment
- Lighting control systems
- Powered pedaling-assisted bicycles
A modern car’s ECU with its electrical cabling
Although the 2-wire CAN bus has multiple applications, it is more critical in the automotive industry and will play a more significant role as vehicles advance, incorporating more ECUs.
The basic principles of the network, such as its electrical properties, might change over time as car advancements demand new communication standards.
For instance, there might be a need to hasten the switching from dominant to recessive states. Only the future will tell.
That’s it for now. Share your thoughts in the comments below to keep the conversation going.