CAN low and CAN high are two crucial signals within the OBD2 diagnostic system of modern vehicles. Understanding these signals is fundamental for anyone working with car diagnostics. This article delves into the specifics of CAN high and CAN low within the context of OBD2, explaining their function and importance in vehicle communication and troubleshooting.
Decoding CAN High and CAN Low Signals
The Controller Area Network (CAN) bus system is a robust communication network used in vehicles to allow various electronic control units (ECUs) to communicate with each other. This communication relies on two dedicated wires: CAN high and CAN low. These wires form a differential signaling system, meaning the difference in voltage between the two lines determines the data being transmitted.
When the CAN bus is idle, both CAN high and CAN low rest at a nominal voltage of 2.5V. However, when data is transmitted:
- CAN high: Transitions to a dominant state of approximately 3.75V.
- CAN low: Transitions to a recessive state of approximately 1.25V.
This voltage difference of 2.5V creates a robust signal resistant to electrical noise and interference, common in the harsh environment of a vehicle. The use of twisted-pair wiring further enhances this noise immunity. Each end of the CAN bus network must be terminated with a 120-ohm resistor to prevent signal reflections.
Image: The OSI Model, showcasing the layers of network communication. CAN operates at the physical and data link layers.
CAN Bus and Data Transmission in Vehicles
CAN operates on a principle of arbitration, allowing multiple ECUs to share the bus without a central master controller. Each ECU is assigned a unique identifier (ID). When an ECU wants to transmit data, it checks if the bus is idle. If so, it begins transmitting its message, including its ID. If multiple ECUs try to transmit simultaneously, the ECU with the lower ID wins arbitration and continues transmitting, while the others back off.
Data is transmitted in frames, with a defined structure including:
- Start and Stop Bits: Delineate the beginning and end of a frame.
- CAN ID: Identifies the transmitting ECU and message priority.
- Data: The actual information being transmitted, up to 8 bytes in standard CAN.
- Check Bits: Ensure data integrity.
Image: Structure of a CAN data frame, illustrating the key components for data transmission.
CAN Bus and OBD2: A Crucial Connection
OBD2, or On-Board Diagnostics II, is a standardized system that allows external devices to access diagnostic information from a vehicle’s ECUs. While OBD2 defines the connector and communication protocols, one of these protocols is CAN.
Within the OBD2 16-pin connector, pins 6 and 14 are dedicated to CAN high and CAN low, respectively. Connecting a diagnostic scan tool to these pins allows access to the CAN bus network and the wealth of diagnostic data it carries.
| Pin | Signal |
|---|---|
| 6 | ISO 15765 CAN High |
| 14 | ISO 15765 CAN Low |
Utilizing CAN Low and CAN High for Diagnostics
By monitoring the voltage levels on CAN high and CAN low, diagnostic tools can interpret the data being transmitted between ECUs. This data includes information about engine performance, emissions, transmission operation, and various other vehicle systems. Analyzing this data can pinpoint malfunctions, identify faulty sensors, and aid in troubleshooting vehicle issues. Understanding the role of CAN low and CAN high is crucial for effective vehicle diagnostics and repair. Anomalies in these signals, such as shorts, opens, or improper voltage levels, can indicate problems with the CAN bus network itself.
