Basic CAN Recap

Facts about the basic CAN

• ISO11898 OSI-stack level 1+2 - Physical & DataLink Layer
• Typically 2-3 wires (differential + common ground) - e.g. RS-485 - 4 wires when counting power as well.
• Typically Linear network with 120 Ohm termination at both ends (at 1 Mbps)
• Max 1 Mbps up to 40m, half the speed when doubling the length etc.
• Data Frame is 44-108 bits + bitstuffing for synch
  • Message ID in 11 (CAN 2.0A) or 29 bit (CAN 2.0B)
  • Data: 0-8 Bytes per message (fixed usage per ID on a given system by designer)
  • CRC 15 bits (Hamming Distance is 6) - Ethernet has 31
• Bus Arbitration: CSMA/CD with a twist: Most consecutive zeros win (low ID). This means that two CAN devices can start transmitting simultaneously, and the one with the lowest ID will win and continue to transmit a legal message, while the other one silently "backs off". Thus you can prioritize message-types by designing them with a low ID.

Comparing CAN with Ethernet

• Ethernet may take advantage of existing installations (cables, switches)
• Ethernet simply throws away erroneous frames - needs TCP (or user) to retransmit end-to-end
• CAN requires ALL nodes to ACK during transmission! (but ”parks” a faulty node)
• CAN supports priorities by Message ID (lowest ID wins)
• CAN’s shorter messages means shorter waiting time (for a given transmission rate)
• CAN has HW Error-Detection and retransmit on Layer 2
• Missing ACK, Collision, Bus Off

CANOpen

CANOpen is an extension of CAN for the factory floor. The designers realized that CAN is not only cheap, but also very robust in noisy surroundings - like a factory floor. A factory network consists of components from many vendors. For this reason a higher level of standardization than what is offered by CAN was needed.

The figure below shows a typical CANOpen device from ICPDAS. It has 4 analog out 14-bit channels. It is easily mounted in the factory where analog voltages are needed. Note the dials - they are used to set the ID of the device (explained below). One dial sets the tens in the address and the other the ones. There is a similar dial to set the baud-rate. Note also that there is a built-in 120 Ohm termination. This can be disabled if you have external termination. All you need to do, is to attach the twisted pair differential CAN-wires, and the two power-wires in screw-terminals - as well as the wire-pairs for the analog out channels. This device supports voltage out as well as current out (from separate terminals). 4-20 mA current-loop has historically been very popular. All the tools you need to setup the device near the factory-station is a single screwdriver

Facts about CANOpen

• OSI-Stack levels 3-7 (Somewhat simplified)
• Standardizes
  • A number of messages and datatypes as well as byte-ordering etc.
  • Fragmentation to allow longer messages than the usual max 8 bytes
  • Addressing and Token management for shared resources
  • Network Management
  • Concepts like Events on changes
  • Simple mapping of any input to any output
• Overall Concept: Producers & Subscribers
• Introduces an ”Object Directory” - a table of parameters
• Heartbeat - All nodes broadcast their status at given intervals. (Somewhat like the ”Beacon” in WiFi)
• ”Open” because:
  • May run on same network as ordinary CAN (if you avoid reuse of CAN Msg ID's)
  • Can be used by anyone
  • May be extended as you please

CANOpen Coding of the Message ID

• Using the 11-bit CAN Msg ID in CiA ( CAN In Automation) DS301
  • 7 Bits for Node ID 1-127 (0 in ”Broadcasts”)
  • 4 Bit for Function Code
• Control Bits Not Recommended for use
  • No Polling (RTR)
  • No 29 Bit CAN Messages (IDE)
• An implementation may e.g. sacrifice some Node ID's to get more PDO's

CANOpen Object Directory

• All setups and process values (data) are kept in a 16-bit indexed table - the Object Directory
• Each Index has up to 256 sub-indices (if more than 1, the first is the number): [Index, SubIndex]
• Example: A manufacturer implements a Real-Time-Clock at index 2000H
  • [2000H, 0H]: 3 (The number of entries)
  • [2000H, 1H]: Hours (UNSIGNED16)
  • [2000H, 2H]: Minutes (UNSIGNED8)
  • [2000H, 3H]: Seconds (UNSIGNED8)
• EDS - Electronic Data Sheet - ”ini-file” format or xml
  • Applies to multiple nodes - describing the system
  • Used by many tools: Analyzers, Configuration, Standard Test etc
  • Puts name on Objects = Easier in tools - does not go into OD
• DCF - Device Configuration File
  • Almost same format as EDS
  • Applies to a single node
  • Essentially allows store/restore the configuration of a device
  • Includes Data for the Object Directory, as well as min/max/default values
• Tools use SDOs to communicate with the nodes to maintain the above (SDOs are explained below)

Transmit and Receive Process Data Objects

• TPDO = Transmit Process Data Object.
• RPDO = Receive Process Data Object.
• Transmit and Receive direction is related to the CAN-bus - Example:
  • Process Data from a thermo-coupler in the Process Environment is read via the node's A/D and written into the Object Directory - and then transmitted on the CAN-bus as a TPDO - possibly with other data as we have up to 8 bytes in this basic CAN message
  • All nodes receive this and may update their Object Directory (actuator, Display, LED) if an RPDO has a matching COB ID
  • This is PDO linking

CANOpen Message Types

• Network Management (If system has a master) The first thing to come on line after power-up manages the CAN-nodes. We have the following states:
  • Bootup
  • Pre-Operational (configuration)
  • Operational
  • Stopped (by error or operator)
• SDO - Service Data Objects Reads & Writes the parameters in the Object Dictionary on other nodes Typically this is done at startup in Configuration Mode. It may be done once by an external PC with a CAN-interface - or dynamically by a master Fragmentation Supported here - Messages build on several CAN packages
• PDO - Process Data Object ”Shortcuts” to parameters in the nodes
  • TPDO for Transmit. Describes what data to measure from input-terminals and transmit on bus
  • RPDO for Receive. Describes what data to receive from the bus and output on terminals
• Emergency Objects - used from nodes to the master in case of problems

CANOpen variants

• The ”standard” CANOpen is standardized by CiA ”CAN in Automation” as DS301.
• In principle DS301 does not support ”slaves” without a master going operational - but nevertheless this is often supported anyway
• MicroCANOpen is a minimal derivate which as a default has no master. Free Source.
• You can make your own...