The figure below illustrates the CANopen NMT (Network Management) global device state machine, which defines the complete state transition process of a CANopen slave station (servo motor / I/O module) during power-on, configuration, operation and shutdown. It serves as the foundation of motion control safety procedures.
(Image Source: Microchip)
I. Classification of Five Core States
1. Initialization (Dotted Box, Automatic Power-On Process)
This phase consists of two automatically progressive sub-states. The device directly enters the Reset Application state upon power-on without any external commands.
Reset Application
Behavior: Clear application-layer data, motor closed-loop variables, I/O signals, DS402 motion states and fault flags; restore the object dictionary to factory default settings.
Automatic Transition: Automatically switch to Reset Communication via the dark-colored arrow without external signals.
Reset Communication
Behavior: Reset CAN communication hardware, PDO/SDO/SYNC configurations, COB-ID, heartbeat parameters and bus filters.
Automatic Transition: Automatically enter the Preoperational state after the completion of initialization.
Tech Tips : Unmarked dark arrows indicate automatic state transitions, which require no NMT commands issued by the master station.
2. Preoperational State (Parameter Configuration State)
The device defaults to this state after power-on initialization. All motion control power-on configuration procedures are completed in this state.
Available Functions: SDO read/write access to the object dictionary (configuring motor parameters, PDO mapping, acceleration/deceleration, limit switches and DS402 operating modes); normal operation of heartbeat and EMCY emergency messages.
Disabled Functions: No transmission or reception of real-time PDO messages; inability to issue position/speed motion commands; motor standby status (no operation).
Application Scenario: The master station configures all motion parameters of the driver via SDO after device power-on.
3. Operational State (Core State for Real-Time Motion Control)
This state is activated after the master station issues the Start Remote Node NMT command, and it is the only state that supports real-time motor motion execution.
Full Functions Enabled: Normal operation of SDO, PDO, SYNC synchronization, heartbeat and EMCY fault messages.
Motion Logic: RPDO receives target position/control words from the master station, while TPDO uploads encoder feedback data. Multi-axis synchronous closed-loop control is realized with the support of SYNC signals.
Safety Rule: Once the device exits the Operational state, PDO transmission stops immediately. The motor loses real-time commands and triggers shutdown protection.
4. Stopped State (Bus Sleep State)
This state is entered when the master station issues theStop Remote Node command, with all service communications completely disabled.
Retained Functions: Only NMT network management messages and heartbeat signals remain active.
Disabled Functions: SDO, PDO, SYNC and EMCY functions are all invalid; parameter read/write and motor control are unavailable.
Application Scenario: Equipment maintenance, temporary shutdown of all axes, with the bus only maintaining node online status.
II. State Transition Commands (NMT Master Station Commands)
All state transition trigger conditions marked in the state machine diagram are implemented via NMT messages sent by the master station:
Start Remote Node
Preoperational → Operational (Enable real-time motion control)
Stopped → Preoperational
Stop Remote Node
Operational → Stopped
Preoperational → Stopped
Enter Preoperational
Operational → Preoperational (Stop PDO transmission and reconfigure parameters via SDO)
Stopped → Preoperational
Reset Application
Any State → Reset Application (Clear motor/application data and automatically complete the initialization process to return to the Preoperational state)
Reset Communication
Any State → Reset Communication (Only reset bus communication configurations and automatically return to the Preoperational state)
III. Standard Power-On Procedure for Motion Control (State Machine Execution Flow)
1. Device power-on & start → Automatically enter Reset Application state → Automatically switch to Reset Communication state → Automatically enter Preoperational state;
2. The master station configures DS402 motion parameters in batches via SDO, including operating mode (position mode), acceleration/deceleration, PDO mapping and limit parameters;
3. The master station sends the Start Remote Node NMT command to enter the Operational state;
4. The master station periodically sends SYNC signals and RPDO messages to execute real-time motor position closed-loop motion control;
5. Motion parameter modification: Issue the Enter Preoperational command to stop PDO transmission, then modify parameters via SDO;
6. Equipment shutdown for maintenance: Issue the Stop Remote Node command to enter the Stopped state;
7. Factory parameter restoration: Issue the Reset Application command to restart the full initialization process.
More Resources:
Microchip Application Notes
Extended Technical Topics
- Why does a simple industrial control project require Modbus, CAN, and EtherCAT?
- Why CAN alone is insufficient, and a complete CANopen architecture is mandatory for motion control?
- Fundamentals of CANopen Motion Control Safety Procedures
- How Does EtherCAT Achieve Low Latency?
- EtherCAT Low-Latency Communication: Physical Layer Reference Circuit
- Modbus RTU Working Principle: Master-Slave Architecture and Polling Mechanism
- Modbus RTU Communication Process