1. What is CAN Bus
CAN Bus (Controller Area Network) is a widely used communication system in vehicles and industrial equipment, designed for reliable data transmission between various Electronic Control Units (ECUs). For example, in automobiles, the CAN Bus enables quick and secure information exchange between the braking system and the engine management system.
A vivid analogy is to compare the CAN Bus to the human nervous system: the CAN Bus acts like a neural network, responsible for signal transmission; each Electronic Control Unit (also known as a “CAN node”) is akin to different organs in the human body, interconnected and coordinated through this network. Any information perceived by one component can be shared with other components via this network.
All ECUs are connected via a pair of twisted wires: CAN-H (high-level line) and CAN-L (low-level line), which are typically distinguished by different colors. In practical applications, retrofitted devices such as electric pedals, electric tailgates, and voice-controlled electric tailgates need to connect to the vehicle’s CAN Bus to obtain relevant signals.
2. The Role of Electronic Control Units (ECUs)
Electronic Control Units (ECUs) are embedded systems that control specific functions, such as engine control units, steering control systems, temperature control systems, and pedal controllers. Modern vehicles may contain dozens of ECUs, which are connected in parallel on the CAN Bus, continuously exchanging information with each other.
The CAN Bus employs a broadcast communication mechanism: any ECU can prepare and send data (such as sensor readings), which will be received by all other ECUs on the network. Each ECU checks the received data and decides whether to process or ignore the information.
3. Types of CAN Bus
Common types of CAN Bus in vehicles include:
– Low-speed CAN (Fault-tolerant CAN)
– High-speed CAN
– CAN FD (Flexible Data Rate)
– CAN XL
Aftermarket devices (such as electric pedals and electric tailgates) typically support high-speed CAN and need to connect to the vehicle’s high-speed CAN network.
Additionally, the CAN network in vehicles is divided by functional domains. For example, in Volkswagen vehicles:
– Comfort CAN network: controls comfort functions such as doors, windows, and air conditioning
– Power CAN network: manages power-related functions such as the engine, braking system, and ABS
– Infotainment CAN network: responsible for entertainment systems such as radios, phones, and navigation
Aftermarket devices like electric pedals and electric tailgates typically need to sense the door status, so they should connect to the comfort CAN network.
4. How to Distinguish Between CAN-H and CAN-L
When installing aftermarket devices, it is essential to correctly connect the controller’s CAN harness to the vehicle’s CAN network. The connection should be made according to the device manual and the color coding of the vehicle’s harness:
– Volkswagen: CAN-H is usually green, and CAN-L is orange-brown
– Cadillac: CAN-H is usually blue, and CAN-L is white
Voltage Characteristics:
– When signal transmission occurs:
– CAN-H: high level 3.5V, low level 2.5V
– CAN-L: high level 2.5V, low level 1.5V
– When no signal is present: both CAN-H and CAN-L voltages are 2.5V
Practical Measurement Method:
When measuring with a multimeter, the average voltage is measured, with voltage values approximately as follows:
– CAN-H to ground voltage is about 2.8V
– CAN-L to ground voltage is about 2.2V
(Note: Actual voltages may vary slightly due to data transmission content)