CAN Bus

Basics

• CAN Formats:
  • CAN 2.0 A - Standard Format (11 Bit identifier)
  • CAN 2.0 B - Extended Format (29 Bit identifier)
• ISO 11898:
  • ISO 11898-2 High speed applications up to 1Mbps
  • ISO 11898-3 Low speed applications up to 125Kbps
  
  
• CAN defines the OSI Layers 1 and 2
• Layers above are typically defined by DeviceNet, CANOpen, ISO 1192 (e.g. for heavy trucks), SAE J1939 (automotive), etc.
  
  
• All messages are Broadcast message. Each receive decides itself whether to process the message or ignore it.
• The messages have an ID which defines the source or the content
• 1Mbps -> 40 meters ... 125Kbps -> 500 meters
• The number of nodes is not specified by the CAN Standard, they are limited by electrical loading - but up to 64 nodes are normal.
• Bit stuffing after 5 bits (on error - e.g. 6 equal bits in a row - all nodes reset)

• Data Frame - used to transmit data (used for mainly all data transmitted)
• Remote Frame - used to request data transmission (only infrequently used)
• Error Frame - sent by a node that detects an error
• Overload Frame - send by a node to request a delay in transmission

Implementations

• libsocketcan
• socketcand

DeviceNet

DeviceNet is a CAN based Layer 7 protocol, which was originally developed by Allen Bradley. Operation of DeviceNet is based on an object-oriented communications model. DeviceNet is maintained by the "Open DeviceNet Vendor Association (ODVA)" and can be implemented free of license. (www.odva.org) DeviceNet is widely used in US factory automation.

CANOpen

• Object Directory
  • Index - Physical Address of the information
  • Object Type - VAR, ARRAY, STRING, etc
  • Name - descriptional
  • Data Type - UNSIGNED 32, etc
  • Attribute - rw, ro, wo, const
  • Default Value
• COB-ID - Communication Object Idenfier (4 bit function code, 7 bit node number, node=0 for broadcast)
  • NMT - Network Management
    for setting operating states: Initialized, Pre-Operational, Operational, Prepared, COB-ID=zero
    Node number and commands is stored in content
  • SYNC - Broadcast a "heart beat" by the master
    e.g. configure devices to send data every 2nd SYNC
  • EMCY - emergency message (for sending error codes)
  • PDO - Process Data Object (regular operation to access device's data, contents and trigger can be customized)
  • SDO - Service Data Object (register based access)
  • NODE GUARD - NMT Error Control (test to seed whether the node is alive)
  • ...

• CANOpen is a Master-Slave system
• "CANOpen ready" devices support a predefined connection set
• ...

• The Object Dictionary is stored in an EDS-File (electronic data sheet, example) in the INI file format (according to CiA 306)
• In CiA 311 (since 2007) the Object Dictionary can be stored as XML
• The EDS file (object dictionary) can be converted to an DCF file (device configuration file). The DCF is a modified EDS file with settings for node ID and baud rate
• Except of NMT and SDO the other commands can be customized for special purposes.
• CANOpen devices are required to make the transition from the state "initializing" to "pre-operations" automatically during bootup.
• The "predefined connection set" defines 4 PDOs (4x rec, 4x send); with configuration 512 are possible
• ...

CAN/CANOpen Implementations

• CANfestival (Manual)
• CANopenNode
• CANopenSocket
• SocketCAN (PDF)
• CANopen for Python
• port - ANSI-C CANopen Library (commercial) (pdf)
• Lely Core
• openCANopen

J1939

J1939 Implementations

• J1939 on googlesource.com (doc)
• J1939 on elinux.org

ISO-TP (Transport Layer) - ISO 15765-2

ISO-TP Implementations

CAN Kingdom