NMT Overview [LEGACY]

NMT Overview [LEGACY]#

Warning

Legacy documentation. This page documents the CANopen implementation shipped with MD firmware 2.5.x. It is kept for users who still run that firmware and is no longer updated.

The CANopen stack has been rewritten for the current firmware and is documented in CANopen. The two object dictionaries are not compatible, so do not read values across from that section.

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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