CiA 402 State Machine Explained
Apr 11, 2026
The CiA 402 standard (also known as IEC 61800-7-201) defines the state machine for servo drives and motion controllers. It's used in EtherCAT, CANopen, and POWERLINK networks. If you work with PLCs and servo drives, understanding this state machine is critical for commissioning and troubleshooting.
The States
The CiA 402 state machine has 7 states:
| State | Description |
|---|---|
| Not Ready to Switch On | Drive is initializing. Power stage off. |
| Switch On Disabled | Drive has initialized. Waiting for enable command. |
| Ready to Switch On | Drive is ready. Power stage still off. |
| Switched On | Power stage is on. Motor is held (no motion). |
| Operation Enabled | Drive is active. Motion commands accepted. |
| Quick Stop Active | Emergency deceleration in progress. |
| Fault | An error occurred. Must be acknowledged. |
The Statusword
The drive reports its current state through a 16-bit statusword (object 0x6041). Each bit has a specific meaning:
| Bit | Name | Meaning |
|---|---|---|
| 0 | Ready to Switch On | Drive is initialized |
| 1 | Switched On | Power stage is on |
| 2 | Operation Enabled | Drive is executing motion |
| 3 | Fault | An error has occurred |
| 4 | Voltage Enabled | DC bus voltage is present |
| 5 | Quick Stop | Quick stop is NOT active (active low) |
| 6 | Switch On Disabled | Drive is disabled |
| 7 | Warning | A warning condition exists |
State Identification from Statusword
You can identify the current state by masking specific bits:
| State | Bit pattern (bits 6,5,4,3,2,1,0) |
|---|---|
| Not Ready to Switch On | x0x 0000 |
| Switch On Disabled | x1x 0000 |
| Ready to Switch On | x01 0001 |
| Switched On | x01 0011 |
| Operation Enabled | x01 0111 |
| Quick Stop Active | x00 0111 |
| Fault | x0x 1000 |
The Controlword
The PLC sends commands to the drive through a 16-bit controlword (object 0x6040):
| Bit | Name | Purpose |
|---|---|---|
| 0 | Switch On | Request power stage on |
| 1 | Enable Voltage | Enable DC bus |
| 2 | Quick Stop | Active low — set to 0 for quick stop |
| 3 | Enable Operation | Start motion execution |
| 7 | Fault Reset | Rising edge clears fault |
Common Controlword Commands
| Transition | Controlword value | Description |
|---|---|---|
| Shutdown | 0x0006 |
Go to Ready to Switch On |
| Switch On | 0x0007 |
Enable power stage |
| Enable Operation | 0x000F |
Start motion |
| Disable Voltage | 0x0000 |
Emergency off |
| Quick Stop | 0x0002 |
Decelerate and stop |
| Fault Reset | 0x0080 |
Clear fault condition |
Typical Startup Sequence
To bring a drive from disabled to running:
- Write
0x0006(Shutdown) — transitions to Ready to Switch On - Write
0x0007(Switch On) — transitions to Switched On - Write
0x000F(Enable Operation) — transitions to Operation Enabled - Drive is now ready to accept motion commands
Troubleshooting
Common issues: - Stuck in Switch On Disabled: Check emergency stop circuit, safety inputs - Fault state: Read fault code from object 0x603F, fix cause, then send fault reset - Quick Stop Active: Quick stop input is triggered. Release and re-enable.
Try It Yourself
Use MotionConvert's Statusword Calculator to decode statusword and controlword values. Enter a hex or decimal value to see exactly which bits are set and what state the drive is in.