CANopen NMT Overview

CANopen NMT Overview#

1. Introduction#

Network Management (NMT) in CANopen is a service that manages the state of nodes (slaves) on the network and ensures smooth operation. The NMT acts like a conductor, coordinating the transitions of nodes between different operating states. During the boot-up, if the motor have correct parameter, the motor should enter automatically into the Operational mode.

2. NMT States#

A CANopen node can be in one of the following states:

  • Initialization – The node starts up and initializes itself.

  • Pre-Operational – The node can communicate via SDO and receive NMT commands, but PDOs are not active.

  • Operational – Full communication is enabled (PDO, SDO, SYNC, etc.).

  • Stopped – The node stops PDO/SDO communication but stays present on the network and can still receive NMT commands.

    ../_images/NMT_state_machine.jpg

2.1. Allowed Communication per State#

State

PDO Communication

SDO Communication

NMT Commands Accepted

Initialization

❌

❌

None

Pre-Operational

❌

✅

Start, Stop, Reset

Operational

✅

✅

Stop, Reset

Stopped

❌

❌

Start, Reset

3. NMT Message Structure#

An NMT message always uses CAN-ID 0x000 and has a fixed length of 2 bytes:

Byte

Content

0

NMT Command (Start, Stop, Reset, etc.)

1

Node-ID (0x00 = broadcast to all nodes)

3.1. NMT Commands#

Command

Hex Value

Effect

Start Remote Node

0x01

Switches node to Operational

Stop Remote Node

0x02

Switches node to Stopped

Enter Pre-Operational

0x80

Switches node to Pre-Operational

Reset Node

0x81

Reboots the node and reinitializes application variables

Reset Communication

0x82

Reboots only the communication stack

4. Heartbeat#

The Heartbeat is a monitoring mechanism that ensures nodes are alive and functioning.
Each node periodically sends a message containing its current NMT state.

4.1. Heartbeat Message Structure#

Field Content

CAN-ID 0x700 + Node-ID Data 1 byte representing the NMT state of the node

Example:
If node 0x05 is in Operational state, it will send:
CAN-ID = 0x705, Data = 0x05

4.2. Purpose of Heartbeat#

  • Detect nodes that are offline or unresponsive

  • Allow the master to monitor network health

  • Trigger safety actions when communication is lost

5. Conclusion#

NMT services are crucial for:
✅ Setting nodes to the desired state
✅ Monitoring their presence with Heartbeat
✅ Reacting quickly to network failures

Together, NMT and Heartbeat provide reliability and safety in CANopen-based communication systems.

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