CAN Physical Layer Standards: High-Speed vs. Low-Speed/Fault-Tolerant CAN

Updated May 18, 2018

Reported In

Driver

  • NI-CAN
  • NI-XNET

Issue Details

  • What are the three CAN layers?
  • How are the layers defined?
  • What are the National Instruments physical layer standards? (High-Speed vs. Low-Speed/Fault-Tolerant CAN)

Solution

What are the three CAN layers?
The CAN specification (ISO 11898) developed by Bosch divides CAN into three layers: object layer, transfer layer, and the physical layer.

How are the layers defined?
The object and transfer layers are fully defined in the CAN specification. The physical layer was intentionally left out of the specification, allowing system designers to optimize the voltage levels and transmission medium for their application. As the CAN specification grew in popularity, CAN physical layer standards were created to facilitate the development of CAN networks and devices, and allow interconnectivity. 

What are the National Instruments physical layer standards?
National Instruments CAN interfaces implement two physical layer standards. Although the standards are differentiated using the high speed and low speed terms, they also differ in other areas, such as cabling, fault tolerance and termination: 
  • High Speed CAN offers baud rates from 40 Kbit/s to 1 Mbit/sec, depending on cable length. This is the most popular standard for the physical layer, since it allows for simple cable connection between devices. This is the physical standard used in the DeviceNet and CANopen specifications. High speed CAN networks are terminated with 120 ohm resistors on each end of the network. 
  • Low Speed/Fault Tolerant CAN offers baud rates from 40 Kbit/s to 125 Kbits/sec. This standard allows CAN bus communication to continue in case of a wiring failure on the CAN bus lines. In low speed/fault tolerant CAN networks, each device has its own termination. 

Additional Information

Due to the different types of termination, you cannot use high speed and low speed/fault tolerant CAN devices in the same network.

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