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CAN/CANopen® FAQ

Q: What is the identifier of a node, message and or variable?

A: In CAN and CANopen® there are several identifiers used for different purposes. Beginners tend to mix-up these, so pay close attention to the different meaning of the word "identifier":

  1. On the CAN level (looking at CAN messages on the bus, generated by a CAN controller, no higher-layer protocol involved), the "identifier" is the CAN message identifier. Version CAN 2.0A allows for an 11-bit ID ID (up to 2048 different identifiers), version CAN 2.0B for a 29-bit ID.
  2. Higher-layer protocols such as CANopen® use node identifiers to address a specific node in the network. The node ID is in the range of 1 to 127 in CANopen® and 0 to 63 in DeviceNet. Sometimes, the node-ID is embedded into the CAN ID. The pre-defined connection set of CANopen® places the node ID into the lower 7 bits of the 11-bit CAN ID.
  3. In CANopen®, network variables have their own identifier. All network variables are located in the object dictionary, which is a look-up table using a 16-bit index and a 8-bit subindex. The index and subindex are used to identify one specific network variable in one specific node. A typical access (SDO access, service data object) to such a variable uses a CAN message that contains a node-ID within the CAN-ID and the index and subindex (indexing a variable in the object dictionary) within the data field.

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