Siemens SINUMERIK 840D sl / ONE alarm 25000: Axis hardware fault of active encoder
Alarm 25000 “Axis %1 hardware fault of active encoder” on a SINUMERIK 840D sl means the signals of the encoder currently used for position control are missing, do not have the same phase, or show a ground fault or short circuit. The NC switches the axis to follow-up mode, the channel is not ready, the axes must be re-referenced, and it is a power-on alarm: the control must be switched off and on.
Diagnose this with AI
Try Free CNC Alarm Diagnostic Upload a keypad photo, the fault history or a parameter export from your Siemens SINUMERIK 840D sl / ONE. InduQT compares the evidence and suggests the next useful checks.
When it happens
- At power-up
- During motion when a cable or connector loses contact
- After encoder, motor or cable work
Likely causes and checks
1Encoder connector not making contact
- Points to it
- Alarm after work near the motor or drive; may be intermittent with vibration.
- Check
- Check the measuring-circuit connectors at the motor and at the drive or encoder module for correct contact.
2Encoder cable damaged (open, shorted or grounded)
- Points to it
- Alarm at a particular axis position or when the cable chain flexes.
- Check
- With power off, check the encoder cable for continuity, short circuits and grounding.
3Encoder faulty
- Points to it
- Connectors and cable test good; alarm repeats.
- Check
- Check the encoder signals and replace the encoder if faults are found.
4Drive or encoder-module evaluation fault
- Points to it
- Accompanied by SINAMICS encoder faults (2xxxxx) on the same drive.
- Check
- Evaluate the SINAMICS alarms in the SINAMICS S120 List Manual.
Telling it apart
- 25001 is the same fault on the passive encoder (the one not used for position control); it does not stop the axis the same way.
- 25010 is a contamination signal from the measuring system (only encoders that provide one), a reset alarm rather than a power-on alarm.
- 25020 is zero-mark monitoring: pulses counted between zero marks are implausible, pointing to transmission errors or noise.
- Siemens notes MD36310 can switch the monitoring off; do not do this to clear the alarm, because the encoder signal is genuinely missing or corrupt.
Relevant parameters
| Parameter | Name | Why it matters |
|---|---|---|
| MD36310 $MA_ENC_ZERO_MONITORING | Zero mark / encoder monitoring | For PROFIdrive, values above 100 turn power-on alarm 25000 into reset alarm 25010. Machine-builder setting; not a fix. |
| MD11412 $MN_ALARM_REACTION_CHAN_NOREADY | Alarm reaction “channel not ready” | Can reprogram the reaction to this alarm. |
Evidence to collect
- Axis name in the alarm text
- SINAMICS alarms on the same drive
- Connector and cable inspection results
- Whether the alarm returns after power-off/on
Work on fuel trains, burners, pressure vessels and electrical equipment must be done by qualified personnel following the manufacturer’s documentation and local codes. Never bypass a safety device or interlock.