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2.2 CAN Higher Layer Protocols*

* This chapter is an excerpt from "A Comprehensible Guide to Controller Area Network" by Wilfried Voss

Even though extremely effective in automobiles and small applications, CAN alone is not suitable for machine automation, since its communication between devices is limited to only 8 bytes. As a consequence, higher layer protocols such as CANopen for machine control, DeviceNet for factory automation and J1939 for vehicles were designed to provide a real networking technology that support messages of unlimited length and allow a master/slave configuration.

In order to explain higher layer protocol we must refer again to the ISO/OSI Reference Model as shown in the picture below.

 

 

The standard CAN implementation bypasses the connection between the Data Link Layer and the Application Layer. The layers above the Data Link Layer are covered by additional software, which represents per definition a higher layer protocol.


To emphasize it yet again, whenever you attempt to add software functions between the CAN Data Link Layer and the Application Layer, you will be adding functionalities that are already covered by off-the-shelf available higher layer protocols such as CANopen and DeviceNet.

 

To put it in a nut shell, higher layer protocols are necessary, because

  • They enable data transport of more than 8 bytes per message

  • Embedded Systems may require an appropriate communication model based on Master/Slave configuration

  • They provide Network Management (Network Start-Up, Node Monitoring, Node Synchronization, etc.)

CANopen

  •  Is suited for embedded applications

  • Was originally designed for motion control

  • Was developed and is maintained by the CAN-in-Automation User Group

Like CAN, the CANopen standard is the responsibility of CiA (CAN-in-Automation). For further information, refer to http://www.can-cia.org. 

DeviceNet

  • Is suited for industrial applications (floor automation)

  • Was developed by Allen Bradley/Rockwell

  • Is maintained by Open DeviceNet Association (ODVA)

The DeviceNet Specification, consisting of two volumes: Volume One - Common Industrial Protocol (CIP) and Volume Three- DeviceNet Adaptation of CIP, is available only for ODVA (Open DeviceNet Vendor Association) members.

For further information, refer to http://www.odva.org.

SAE J1939

  • Defines communication for vehicle networks (trucks, buses, etc.)

  • Is a standard developed by the Society of Automotive Engineers (SAE)

The SAE J1939 Standards Collection can be found exclusively on the Web at http://www.sae.org.

The Society of Automotive Engineers  (SAE ) Truck and Bus Control and Communications Subcommittee has developed a family of standards concerning the design and use of devices that transmit electronic signals and control information among vehicle components. SAE J1939 and its companion documents have quickly become the accepted industry standard and the Controller Area Network (CAN) of choice for off-highway machines in applications such as construction, material handling, and forestry machines.