CANopen

Last updated

CANopen is a communication protocol stack and device profile specification for embedded systems used in automation. In terms of the OSI model, CANopen implements the layers above and including the network layer. The CANopen standard consists of an addressing scheme, several small communication protocols and an application layer defined by a device profile. The communication protocols have support for network management, device monitoring and communication between nodes, including a simple transport layer for message segmentation/desegmentation. The lower level protocol implementing the data link and physical layers is usually Controller Area Network (CAN), although devices using some other means of communication (such as Ethernet Powerlink, EtherCAT) can also implement the CANopen device profile.

Contents

The basic CANopen device and communication profiles are given in the CiA 301 specification released by CAN in Automation. Profiles for more specialized devices are built on top of this basic profile, and are specified in numerous other standards released by CAN in Automation, such as CiA 401 for I/O-modules and CiA 402 for motion control.

Device model

Every CANopen device has to implement certain standard features in its controlling software.

Object dictionary

CANopen devices must have an object dictionary, which is used for configuration and communication with the device. An entry in the object dictionary is defined by:

The basic datatypes for object dictionary values such as booleans, integers and floats are defined in the standard (their size in bits is optionally stored in the related type definition, index range 0x0001–0x001F), as well as composite datatypes such as strings, arrays and records (defined in index range 0x0040–0x025F). The composite datatypes can be subindexed with an 8-bit index; the value in subindex 0 of an array or record indicates the number of elements in the data structure, and is of type UNSIGNED8.

For example, the device communication parameters, standardized in the basic device profile CiA 301 are mapped in the index range 0x1000–0x1FFF ("communication profile area"). The first few entries in this area are as follows:

IndexObject nameNameTypeAttributeM/O
0x1000VARdevice typeUNSIGNED32roM
0x1001VARerror registerUNSIGNED8roM
...
0x1008VARmanufacturer device nameVis-StringconstO
...

Given suitable tools, the content of the object dictionary of a device, based on an electronic data sheet (EDS), can be customized to a device configuration file (DCF) to integrate the device into a specific CANopen network. According to CiA 306 , the format of the EDS-file is the INI file format. There is an upcoming XML-style format, that is described in CiA 311 .

Communication

Communication objects

CAN bus, the data link layer of CANopen, can only transmit short packages consisting of an 11-bit id, a remote transmission request (RTR) bit and 0 to 8 bytes of data. The CANopen standard divides the 11-bit CAN frame id into a 4-bit function code and 7-bit CANopen node ID. This limits the number of devices in a CANopen network to 127 (0 being reserved for broadcast). An extension to the CAN bus standard (CAN 2.0 B) allows extended frame ids of 29 bits, but in practice CANopen networks big enough to need the extended id range are rarely seen.

In CANopen the 11-bit id of a CAN-frame is known as communication object identifier, or COB-ID. In case of a transmission collision, the bus arbitration used in the CAN bus allows the frame with the smallest id to be transmitted first and without a delay. Using a low code number for time critical functions ensures the lowest possible delay.

Contents of a CANopen frame:

CAN-IDRTRData lengthData
Length11 bits1 bit4 bits0-8 bytes

The data frame with an 11-bit identifier is also called "base frame format".

The default CAN-ID mapping sorts frames by attributing a function code (NMT, SYNC, EMCY, PDO, SDO...) to the first 4 bits, so that critical functions are given priority. This mapping can however be customized for special purposes (except for NMT and SDO, required for basic communication).

Function codeNode ID
Length4 bits7 bits

The standard reserves certain CAN-IDs to network management and SDO transfers. Some function codes and CAN-IDs have to be mapped to standard functionality after device initialization, but can be configured for other uses later.

Predefined Connection Set

For simple network structures, CANopen supports a predefined allocation of message identifiers.

The transmit and receive directions are from the device's point of view. So a query to a device on the network would send a 0x600+nodeid and get back a 0x580+nodeid. [1]

Communication objectCOB-ID(s) hexSlave nodesSpecification
NMT node control000Receive onlyCiA 301
Global failsafe command001 ?CiA 304
Flying master071 to 076 ?CiA 302-2
Indicate active interface07F ?CiA 302-6
Sync080Receive onlyCiA 301
Emergency080 + NodeIDTransmitCiA 301
TimeStamp100Receive onlyCiA 301
Safety-relevant data objects101 to 180 ?CiA 301
PDO180 + NodeID
200 + NodeID
280 + NodeID
300 + NodeID
380 + NodeID
400 + NodeID
480 + NodeID
500 + NodeID
1. Transmit PDO
1. Receive PDO
2. Transmit PDO
2. Receive PDO
3. Transmit PDO
3. Receive PDO
4. Transmit PDO
4. Receive PDO
CiA 301
SDO580 + NodeID
600 + NodeID
Transmit
Receive
CiA 301
Dynamic SDO request6E0 ?CiA 302-5
Node claiming procedure6E1 to 6E3 ?CiA 416-1
Node claiming procedure6F0 to 6FF ?CiA 416-1
NMT node monitoring (node guarding/heartbeat)700 + NodeIDTransmitCiA 301
LSS7E4
7E5
Transmit
Receive
CiA 305

Communication models

Different kinds of communication models are used in the messaging between CANopen nodes.

In a master/slave relationship, one CANopen node is designated as the master, which sends or requests data from the slaves. The NMT protocol is an example of a master/slave communication model.

A client/server relationship is implemented in the SDO protocol, where the SDO client sends data (the object dictionary index and subindex) to an SDO server, which replies with one or more SDO packages containing the requested data (the contents of the object dictionary at the given index).

A producer/consumer model is used in the Heartbeat and Node Guarding protocols. In the push-model of producer/consumer, the producer sends data to the consumer without a specific request, whereas in the pull model, the consumer has to request the data from the producer.

Protocols

Network management (NMT) protocols

The NMT protocols are used to issue state machine change commands (e.g. to start and stop the devices), detect remote device bootups and error conditions.

The Module control protocol is used by the NMT master to change the state of the devices. The CAN-frame COB-ID of this protocol is always 0, meaning that it has a function code 0 and node ID 0, which means that every node in the network will process this message. The actual node ID, to which the command is meant to, is given in the data part of the message (at the second byte). This can also be 0, meaning that all the devices on the bus should go to the indicated state.

COB-IDData Byte 0Data Byte 1
0x000Requested stateAddressed node
NMT command codeMeaning
0x01Go to 'operational'
0x02Go to 'stopped'
0x80Go to 'pre-operational'
0x81Go to 'reset node'
0x82Go to 'reset communication'

The Heartbeat protocol is used to monitor the nodes in the network and verify that they are alive. A heartbeat producer (usually a slave device) periodically sends a message with the binary function code of 1110 and its node ID (COB-ID19 = 0x700 + node ID). The data part of the frame contains a byte indicating the node status. The heartbeat consumer reads these messages. If the messages fail to arrive within a certain time limit (defined in the object dictionary of the devices) the consumer can take action to, for example, reset the device or indicate an error. The frame format is:

COB-IDData Byte 0
0x700 + node IDState
NMT state codeRepresented state
0x00Boot up (Initialising)
0x04Stopped
0x05Operational
0x7fPre-operational

CANopen devices are required to make the transition from the state Initializing to Pre-operational automatically during bootup. When this transition is made, a single heartbeat message is sent to the bus. This is the bootup protocol.

A response/reply-style (pull model) protocol, called node guarding, exists for slave monitoring.

Service Data Object (SDO) protocol

The SDO protocol is used for setting and for reading values from the object dictionary of a remote device. The device whose object dictionary is accessed is the SDO server and the device accessing the remote device is the SDO client. The communication is always initiated by the SDO client. In CANopen terminology, communication is viewed from the SDO server, so that a read from an object dictionary results in an SDO upload and a write to a dictionary entry is an SDO download.

Because the object dictionary values can be larger than the eight bytes limit of a CAN frame, the SDO protocol implements segmentation and desegmentation of longer messages. Actually, there are two of these protocols: SDO download/upload and SDO Block download/upload. The SDO block transfer is a newer addition to standard, which allows large amounts of data to be transferred with slightly less protocol overhead.

The COB-IDs of the respective SDO transfer messages from client to server and server to client can be set in the object dictionary. Up to 128 SDO servers can be set up in the object dictionary at addresses 0x1200 - 0x127F. Similarly, the SDO client connections of the device can be configured with variables at 0x1280 - 0x12FF. However the pre-defined connection set defines an SDO channel which can be used even just after bootup (in the Pre-operational state) to configure the device. The COB-IDs of this channel are 0x600 + node ID for receiving and 0x580 + node ID for transmitting.

To initiate a download, the SDO client sends the following data in a CAN message with the 'receive' COB-ID of the SDO channel.

Byte Nr:Byte 0Byte 1-2Byte 3Byte 4-7
Length:3 bits1 bit2 bits1 bit1 bit2 bytes1 byte4 bytes
Meaning:ccs=1reserved(=0)nesindexsubindexdata
  • ccs is the client command specifier of the SDO transfer, this is 0 for SDO segment download, 1 for initiating download, 2 for initiating upload, 3 for SDO segment upload, 4 for aborting an SDO transfer, 5 for SDO block upload and 6 for SDO block download
  • n is the number of bytes in the data part of the message which do not contain data, only valid if e and s are set
  • e, if set, indicates an expedited transfer, i.e. all data exchanged are contained within the message. If this bit is cleared then the message is a segmented transfer where the data does not fit into one message and multiple messages are used.
  • s, if set, indicates that the data size is specified in n (if e is set) or in the data part of the message
  • index is the object dictionary index of the data to be accessed, encoded in little endian
  • subindex is the subindex of the object dictionary variable
  • data contains the data to be uploaded in the case of an expedited transfer (e is set), or the size of the data to be uploaded (s is set, e is not set), often encoded in little endian

Process Data Object (PDO) protocol

The Process Data Object protocol is used to process real time data among various nodes. You can transfer up to 8 bytes (64 bits) of data per one PDO either from or to the device. One PDO can contain multiple object dictionary entries and the objects within one PDO are configurable using the mapping and parameter object dictionary entries.

There are two kinds of PDOs: transmit and receive PDOs (TPDO and RPDO). The former is for data coming from the device (the device is a data producer) and the latter is for data going to the device (the device is a data consumer); that is, with RPDO you can send data to the device and with TPDO you can read data from the device. In the pre-defined connection set there are identifiers for four TPDOs and four RPDOs available. With configuration, 512 PDOs are possible.

PDOs can be sent synchronously or asynchronously. Synchronous PDOs are sent after the SYNC message whereas asynchronous messages are sent after internal or external trigger. For example, you can make a request to a device to transmit TPDO that contains data you need by sending an empty TPDO with the RTR flag (if the device is configured to accept TPDO requests).

With RPDOs you can, for example, start two devices simultaneously. You only need to map the same RPDO into two or more different devices and make sure those RPDOs are mapped with the same COB-ID.

Synchronization Object (SYNC) protocol

The Sync-Producer provides the synchronization-signal for the Sync-Consumer. When the Sync-Consumer receive the signal they start carrying out their synchronous tasks.

In general, the fixing of the transmission time of synchronous PDO messages coupled with the periodicity of transmission of the Sync Object guarantees that sensor devices may arrange to sample process variables and that actuator devices may apply their actuation in a coordinated fashion.

The identifier of the Sync Object is available at index 1005h.

Time Stamp Object (TIME) protocol

Usually the Time-Stamp object represents a time as a 6-byte field: a count of milliseconds after midnight (at most 27 bits, stored in a 32-bit field), and an unsigned 16-bit number of days since January 1, 1984. (This will overflow on 7 June 2163.)

Some time critical applications especially in large networks with reduced transmission rates require very accurate synchronization; it may be necessary to synchronize the local clocks with an accuracy in the order of microseconds. This is achieved by using the optional high resolution synchronization protocol which employs a special form of timestamp message to adjust the inevitable drift of the local clocks.

The high-resolution timestamp is encoded as unsigned32 with a resolution of 1 microsecond which means that the time counter restarts every 72 minutes. It is configured by mapping the high resolution time-stamp (object 1013h) into a PDO.

Emergency Object (EMCY) protocol

Emergency messages are triggered by the occurrence of a device internal fatal error situation and are transmitted from the concerned application device to the other devices with high priority. This makes them suitable for interrupt type error alerts. An Emergency Telegram may be sent only once per ‘error event’, i.e. the emergency messages must not be repeated. As long as no new errors occur on a device no further emergency message must be sent. By means of CANopen Communication Profile defined emergency error codes, the error register and device specific additional information are specified in the device profiles.

Initialization

Sample trace of communications between a master and two pressure transducer slaves configured for id 1 and node ID 2.

CAN IDDATA LENGTHDATADescription
0x0201 00Master puts all devices on the bus into operational mode
0x800Master sends a SYNC message, which triggers devices to send data
0x1814CD 82 01 00Node at ID 1 (CID-0x180), reading pressure of 0x0182CD (99021) pascals
0x1824E5 83 01 00Node at ID 2 (CID-0x180), reading pressure of 0x0183E5 (99301) pascals

Electronic Data Sheet

Electronic Data Sheet (EDS) is a file format, defined in CiA306, that describes the communication behaviour and the object dictionary entries of a device. This allows tools such as service tools, configuration tools, development tools, and others to handle the devices properly.

Those EDS files are mandatory for passing the CiA CANopen conformance test.

Since end of 2007 a new XML based format called XDD is defined in CiA311. XDD is conformant to ISO standard 15745.

Glossary of CANopen terms

See also

Related Research Articles

In computer networking, the maximum transmission unit (MTU) is the size of the largest protocol data unit (PDU) that can be communicated in a single network layer transaction. The MTU relates to, but is not identical to the maximum frame size that can be transported on the data link layer, e.g., Ethernet frame.

<span class="mw-page-title-main">Synchronous optical networking</span> Standardized protocol

Synchronous Optical Networking (SONET) and Synchronous Digital Hierarchy (SDH) are standardized protocols that transfer multiple digital bit streams synchronously over optical fiber using lasers or highly coherent light from light-emitting diodes (LEDs). At low transmission rates data can also be transferred via an electrical interface. The method was developed to replace the plesiochronous digital hierarchy (PDH) system for transporting large amounts of telephone calls and data traffic over the same fiber without the problems of synchronization.

Simple Network Management Protocol (SNMP) is an Internet Standard protocol for collecting and organizing information about managed devices on IP networks and for modifying that information to change device behavior. Devices that typically support SNMP include cable modems, routers, network switches, servers, workstations, printers, and more.

The Spanning Tree Protocol (STP) is a network protocol that builds a loop-free logical topology for Ethernet networks. The basic function of STP is to prevent bridge loops and the broadcast radiation that results from them. Spanning tree also allows a network design to include backup links providing fault tolerance if an active link fails.

The data link layer, or layer 2, is the second layer of the seven-layer OSI model of computer networking. This layer is the protocol layer that transfers data between nodes on a network segment across the physical layer. The data link layer provides the functional and procedural means to transfer data between network entities and may also provide the means to detect and possibly correct errors that can occur in the physical layer.

<span class="mw-page-title-main">CAN bus</span> Standard for serial communication between devices without host computer

A controller area network (CAN) is a vehicle bus standard designed to enable efficient communication primarily between electronic control units (ECUs). Originally developed to reduce the complexity and cost of electrical wiring in automobiles through multiplexing, the CAN bus protocol has since been adopted in various other contexts. This broadcast-based, message-oriented protocol ensures data integrity and prioritization through a process called arbitration, allowing the highest priority device to continue transmitting if multiple devices attempt to send data simultaneously, while others back off. Its reliability is enhanced by differential signaling, which mitigates electrical noise. Common versions of the CAN protocol include CAN 2.0, CAN FD, and CAN XL which vary in their data rate capabilities and maximum data payload sizes.

Serial Peripheral Interface (SPI) is a de facto standard for synchronous serial communication, used primarily in embedded systems for short-distance wired communication between integrated circuits.

<span class="mw-page-title-main">Modbus</span> Serial communications protocol

Modbus or MODBUS is a client/server data communications protocol in the application layer. It was originally designed for use with programmable logic controllers (PLCs), but has become a de facto standard communication protocol for communication between industrial electronic devices in a wide range of buses and networks.

LIN is a network protocol used for communication between components in modern vehicles. It is a low-cost single-wire serial protocol that supports communications up to 19.2 Kbit/s with a maximum bus length of 40 metres (131.2 ft).

The Time-Triggered Protocol (TTP) is an open computer network protocol for control systems. It was designed as a time-triggered fieldbus for vehicles and industrial applications. and standardized in 2011 as SAE AS6003. TTP controllers have accumulated over 500 million flight hours in commercial DAL A aviation application, in power generation, environmental and flight controls. TTP is used in FADEC and modular aerospace controls, and flight computers. In addition, TTP devices have accumulated over 1 billion operational hours in SIL4 railway signalling applications.

In telecommunications and computer networking, connection-oriented communication is a communication protocol where a communication session or a semi-permanent connection is established before any useful data can be transferred. The established connection ensures that data is delivered in the correct order to the upper communication layer. The alternative is called connectionless communication, such as the datagram mode communication used by Internet Protocol (IP) and User Datagram Protocol (UDP), where data may be delivered out of order, since different network packets are routed independently and may be delivered over different paths.

Real-Time Messaging Protocol (RTMP) is a communication protocol for streaming audio, video, and data over the Internet. Originally developed as a proprietary protocol by Macromedia for streaming between Flash Player and the Flash Communication Server, Adobe has released an incomplete version of the specification of the protocol for public use.

EtherCAT is an Ethernet-based fieldbus system developed by Beckhoff Automation. The protocol is standardized in IEC 61158 and is suitable for both hard and soft real-time computing requirements in automation technology.

DeviceNet is a network protocol used in the automation industry to interconnect control devices for data exchange. It utilizes the Common Industrial Protocol over a Controller Area Network media layer and defines an application layer to cover a range of device profiles. Typical applications include information exchange, safety devices, and large I/O control networks.

In computer networks, a syncword, sync character, sync sequence or preamble is used to synchronize a data transmission by indicating the end of header information and the start of data. The syncword is a known sequence of data used to identify the start of a frame, and is also called reference signal or midamble in wireless communications.

<span class="mw-page-title-main">TTP/A</span>

TTP/A is a time-triggered real-time fieldbus protocol used for the interconnection of low-cost smart transducer nodes. In contrast to the Time-Triggered Protocol (TTP), which is a high-speed, fault-tolerant communication network intended for safety critical applications, TTP/A provides a low-speed, low-cost protocol suitable for non-critical applications like car body electronics.

In mobile-telephone technology, the UniPro protocol stack follows the architecture of the classical OSI Reference Model. In UniPro, the OSI Physical Layer is split into two sublayers: Layer 1 and Layer 1.5 which abstracts from differences between alternative Layer 1 technologies. The actual physical layer is a separate specification as the various PHY options are reused in other MIPI Alliance specifications.

Constrained Application Protocol (CoAP) is a specialized UDP-based Internet application protocol for constrained devices, as defined in RFC 7252. It enables those constrained devices called "nodes" to communicate with the wider Internet using similar protocols. CoAP is designed for use between devices on the same constrained network, between devices and general nodes on the Internet, and between devices on different constrained networks both joined by an internet. CoAP is also being used via other mechanisms, such as SMS on mobile communication networks.

Time-Sensitive Networking (TSN) is a set of standards under development by the Time-Sensitive Networking task group of the IEEE 802.1 working group. The TSN task group was formed in November 2012 by renaming the existing Audio Video Bridging Task Group and continuing its work. The name changed as a result of the extension of the working area of the standardization group. The standards define mechanisms for the time-sensitive transmission of data over deterministic Ethernet networks.

CAN FD is a data-communication protocol used for broadcasting sensor data and control information on 2 wire interconnections between different parts of electronic instrumentation and control system. This protocol is used in modern high performance vehicles.

References

  1. ^ CiA 301 CANopen application layer specification, free downloadable from CAN in Automation
  2. ^ CiA 306 CANopen Electronic Data Sheet (EDS) specification
  3. ^ CiA 311 CANopen XML-EDS specification
  4. ^ Predefined Connection Set from CANopen Basics
  5. ^ CiA 401 CANopen device profile specification for generic I/O modules, free downloadable from CAN in Automation
  6. ^ CiA 402 CANopen device profile for motion controllers and drives (same as IEC 61800-7-201/301)
  1. "SDO - Service Data Objects - CanOpen". ByteMe. Retrieved 7 June 2023.