CANopen FD and the OSI reference model

The Open System Interconnection (OSI) reference model by ISO specifies seven layers. The CANopen FD application layer and communication profile as specified in CiA 1301 covers mainly the network, the transport, presentation, and the application layers. The session layer is not used. The data link layer complies with ISO 11898-1 and uses data frames in FBFF (Classical Base Frame Format) by default and optionally data frames in FEFF (Classical Extended Frame Format). Remote frames are allowed, but not recommended at all.

CANopen uses the CAN physical layer as defined in ISO 11898-1:2015 (physical signaling sub-layer) and in ISO 11898-2:2016 (physical media access sub-layer). This enables bit-rates up to 1 Mbit/s in the arbitration phase and up to 2 Mbit/s in the data phase. In order to improve interoperability, CiA 1301 recommends the bit-rates to the following values and sample-point locations:

If other bit-times are required, the CiA 601-3 recommendation should be consulted. It is recommended to use daisy-chained line topologies or line topologies with very short stubs. Bus ends of the network cable needs to be terminated by resistors (nominally 120 Ohm each). The physical layer network design at bit-rates above 1 Mbit/s requires additional attention.

The network length at a given bit-rate depends not only on the configured sample, but for example also on the used cables and not terminated stubs. In 1-Mbit/s networks you can achieve about 25 m. Using 500 kbit/s network length of up to 125 m are possible. In 250-m networks bit-rates of up to 250 kbit/s can be reached. Setting the bit-rate to 125 kbit/s allows up to 500 m and at 50 kbit/s the maximum length is 1 km.

The network designer must assign uniquely the CANopen node-ID to each connected CANopen device. Additionally, all CANopen nodes must use the very same bit-timing settings in all nodes.

The CANopen profile specifications defining the process data, configuration parameters, and the diagnostic information are above the OSI reference model. This includes also the mapping of process data into PDOs.

CiA 302-7 specifies a network layer. The described protocols allow accessing CANopen devices from another CANopen network segment (remote SDO services). This could be used to configure from a single point a complex CANopen system comprising several segments. Of course, this network layer can be also used for diagnosis purposes (remote EMCY services).

Application level User program(s)
Data level CiA 4XX: Device and application profiles

OSI layers

Application layer CiA 301: NMT, Heartbeat, SDO, PDO, SYNC, EMCY, TIME
Presentation layer CiA 301: Data types and encoding rules
Session layer Not applicable
Transport layer CiA 301: Segmented SDO
Network layer (CiA 302-7: SDO and EMCY routing, PDO bridging)*
Data link layer ISO 11898-1
Physical layer ISO 11898-2, CiA 301 (bit-timing), CiA 303-1 (cable and connectors)

* only necessary in multiple CANopen network architectures