Debugging CiA 402 Drive States: Not Ready, Fault, and Stuck in Switched On
May 04, 2026
When a CiA 402 drive refuses to enable, the diagnosis almost always comes down to the statusword it is reporting — not the controlword you are sending. This guide walks through the four states engineers most often get stuck in (Not Ready to Switch On, Switch On Disabled, Switched On, Fault), how to tell them apart from a captured statusword, and the vendor-specific gotchas that waste a day when you do not know about them.
TL;DR
Capture the statusword. Paste it into the CiA 402 Decoder. Look at bits 0–3, 4 (Voltage Enabled), 9 (Remote) first. Most "drive won't enable" problems are hardware enables (bit 4), local control (bit 9), or STO / safety inputs that are not asserted — not controlword logic.
How to read the drive's state from a captured statusword
Before changing anything: decode the value you have.
- Bit 3 (Fault): if set, the drive is in Fault or Fault Reaction Active. Read 0x603F for the code.
- Bit 4 (Voltage Enabled): if 0, the DC bus is not present. No amount of controlword tweaking will help.
- Bit 9 (Remote): if 0, the drive is ignoring your controlword. Fix local/keypad mode first.
- Bits 0–3 mask: identifies the DS402 state. See the state machine article for the masks.
The decoder does all four steps in one click.
"Not Ready to Switch On" will not go away
Not Ready to Switch On is the power-up state. It should self-transition to Switch On Disabled within a few hundred milliseconds. If it does not, the drive is stuck in initialisation.
STO and safety circuit
Most modern drives will not leave Not Ready to Switch On until both STO inputs are asserted. If you have a single-channel wired harness, or a safety relay that is de-energised, the drive stays put. Check the drive's STO-status LED or parameter (vendor-specific) before anything else.
DC bus not up
On drives with an external DC bus precharge, the drive stays in Not Ready until the bus is above the undervoltage threshold. Symptom: bit 4 (Voltage Enabled) is 0. Fix: precharge first, or wait longer if your contactor is slow.
Missing hardware enable
Some vendors — AKD, Yaskawa Sigma-7 — require a hardware enable input in addition to the fieldbus commands. No enable input means no state transitions.
"Switch On Disabled" and the voltage-enabled bit
Switch On Disabled is the normal post-init state. If you are sending Shutdown (0x0006) and staying here, the drive is rejecting the transition.
Quick checklist: - Bit 4 (Voltage Enabled) = 0: DC bus is off. Fix that first. - Bit 9 (Remote) = 0: drive is on local control (keypad, USB, engineering tool). Release local mode. - Drive-specific enable input: HWENABLE on AKD, EN on IndraDrive.
Only once bits 4 and 9 are 1 should you suspect the controlword itself.
"Switched On" but motor will not move
The drive accepted Switch On (output stage is closed), but it will not track your position / velocity / torque setpoints.
Enable Operation not set
The most common cause. You are sending 0x0007, not 0x000F. Statusword bit 2 (OE) is 0 — the drive is telling you it is not in Operation Enabled.
Mode of operation wrong
The drive is in Operation Enabled but 0x6060 is set to a mode you are not sending setpoints for. Check 0x6061 (Modes of Operation Display) to confirm.
Target never accepted (PP new-set-point)
In PP mode, you must pulse CW.4 (New Setpoint) after writing 0x607A. Without the rising edge, the drive never latches the target. Statusword bit 12 (Setpoint Ack) tells you whether the handshake is working. See the controlword cookbook for the full handshake.
Fault state and 0x603F
Reading the error code
Object 0x603F (Error Code, UNSIGNED16) holds the most recent fault. Every vendor maps its internal fault numbers into this object. Consult the drive manual for the decoded meaning — CiA does not standardise the codes themselves.
Some vendors also maintain a history in 0x1003 (Pre-defined Error Field). Useful when the first fault cascaded and you only caught the last one.
Clearing: rising edge of bit 7
Fault Reset is edge-triggered on CW bit 7 going 0 → 1. Common failure modes:
- You set CW = 0x0080 once at init and left it there. First fault cleared; every subsequent fault does not.
- You are writing 0x0086 continuously. Same problem — the edge only happens once.
Fix: write 0x0080, wait ≥10 ms, write 0x0000 or 0x0006. Pulse.
If the fault will not clear, the cause is still present. Bit 3 (Fault) stays high until both (a) you send the edge and (b) the drive's internal fault latch agrees the cause is gone.
Quick Stop Active — different from Fault
Quick Stop Active (statusword bits 0–3 = 0111, bit 5 = 0) is not a fault. It is the drive decelerating under control because the master sent CW.2 = 0. Bit 3 (Fault) is 0.
Recovery depends on 0x605A (Quick Stop Option Code): - 0x605A ∈ {1, 2, 3, 4}: drive auto-transitions to Switch On Disabled when stopped. You then need the full Shutdown → Switch On → Enable Operation sequence. - 0x605A ∈ {5, 6, 7, 8}: drive stays in Quick Stop Active until the master sends Enable Operation (0x000F), which takes it back to Operation Enabled. Only some drives support this.
Check 0x605A on your drive before expecting one or the other.
Fault Reaction Active vs. Fault — often missed
Fault Reaction Active is a brief intermediate state between "fault detected" and Fault. The drive is executing 0x605E (Fault Reaction Option Code) — coasting, decelerating, or quick-stopping. You will see statusword 0x020F (bits 0, 1, 2, 3, and sometimes 9 set — bit 3 asserts before bits 0/1/2 clear). Then the drive lands in Fault with statusword around 0x0218.
If your fault handler only looks for bit 3 in a specific mask, you might miss FRA and think the drive flickered. Log both states.
Vendor gotchas
SINAMICS PROFIdrive mask (ZSW1 vs statusword)
Siemens SINAMICS S120 on PROFINET does not expose 0x6041 — it exposes ZSW1 (status word 1) with a PROFIdrive state machine, different bit meanings, and different transition commands (ON/OFF1/OFF2/OFF3 via STW1). If you are debugging a SINAMICS and your decoder says "Unknown state", the problem is that you are looking at PROFIdrive bits through a CiA decoder.
Beckhoff TwinCAT NC axis requires MC_Power
On EL72xx / AX8000 under TwinCAT MC_7xxx, the NC layer wraps the CiA state machine. Your PLC logic calls MC_Power.Enable := TRUE, which NC translates into the Shutdown → Switch On → Enable Operation sequence — but it also enforces bRegulatorOn and bDriveOn inputs. The CiA statusword will sit in Switched On until NC is satisfied both hardware enables are asserted.
Kollmorgen AKD op-mode selection
AKD requires 0x6060 (Mode of Operation) to be set before entering Operation Enabled. Sending 0x000F with 0x6060 = 0 lands you in Operation Enabled but the drive does nothing. Always set 0x6060 first.
Yaskawa Sigma-7 SV_ON vs. enable operation
Sigma-7 has a native "SV_ON" (servo on) bit in manufacturer statusword space that can be cleared independently of Operation Enabled if the SV_ON mapping is wrong. Symptom: CiA bit 2 = 1, but the motor is free. Check Yaskawa's PnXXX parameters for the SV_ON source.
FAQ
What does "servo drive not ready to switch on" mean? The drive is still initialising — STO inputs, the DC bus, or a hardware enable signal is typically not yet asserted. Resolve those first; the drive self-transitions.
How do I clear a CiA 402 fault? Read 0x603F for the code, fix the cause, then pulse controlword bit 7 (value 0x0080) before writing 0x0000 or 0x0006.
Why is my drive stuck in Switched On? Controlword bit 3 (Enable Operation) is not set, or the operation mode in 0x6060 is not what you are sending setpoints for. Check bit 2 of the statusword.
What does Quick Stop Active mean in CiA 402? The master sent CW bit 2 = 0 and the drive is decelerating under control per 0x605A. It is not a fault. Recovery depends on the option code.
Is Not Ready to Switch On always a hardware problem? Usually. STO, DC bus, or a hardware enable signal is not asserted. Fieldbus commands cannot move the drive out of Not Ready.
Try it
Paste your captured statusword before changing anything. Knowing which state you are in is half the fix.