# 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 It is kept for users who still run that firmware and is no longer updated.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](md_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.

  ```{figure} images/NMT_state_machine.jpg
  ---
  align: center
  width: 1000px
  ---
  ```

### 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              |

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## 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.