What causes Siemens SINUMERIK alarm 25040?

Encoder cable damage or degradation including shield breaks, conductor fatigue at flex points in cable tracks, or connector pin corrosion causing intermittent signal dropout or noise injection that corrupts A/B quadrature channels. Encoder bearing wear or mechanical coupling looseness at the motor shaft allowing radial or axial play that creates position jitter exceeding MD36300 monitoring tolerance, particularly under rapid acceleration or cutting loads. Encoder power supply voltage drop below 4.75V (for 5V encoders) or outside 4.5-5.5V range due to cable resistance, inadequate drive regulator capacity on SMC/SME modules, or voltage sag during regenerative braking events. Contamination of encoder optical disk or magnetic scale from coolant ingress, oil mist, or metallic debris causing erratic pulse output or track signal amplitude degradation visible in SINAMICS encoder diagnostics. Electrical noise coupling into encoder cables from inadequate shield grounding (shield must terminate at drive end only with 360-degree clamp), parallel routing with motor power cables under 100mm separation, or missing ferrite cores on encoder connector. Incorrect encoder type configuration in MD31050 (absolute/incremental) or MD31060 (pulses per revolution) causing the monitoring algorithm to apply wrong deviation thresholds or interpolation errors during position comparison.

How do I fix Siemens SINUMERIK alarm 25040?

1. Step 1: Acknowledge alarm and check SINUMERIK alarm buffer (Diagnostics > Alarms > Alarm Log) for accompanying drive alarms 300501 (encoder fault), 30002 (encoder signal lost), or 31117 (position deviation). Note alarm frequency and whether it occurs during motion, acceleration, or at standstill to narrow diagnostic focus. 2. Step 2: Measure encoder supply voltage at motor connector with multimeter while axis is enabled. Verify 4.9-5.1V for 5V encoders or 11-13V for EnDat/BiSS types. If voltage is below minimum, check cable continuity (should be Encoder > Supply voltage display. 3. Step 3: Inspect encoder cable routing from motor to drive for damage, minimum bend radius violations (must exceed 10x cable diameter), tie-wrap compression points, and verify shield is grounded at drive end only with 360-degree EMC cable gland. Measure shield-to-PE ground resistance: must be Drive > Encoder signals). Monitor real-time track signal amplitude (should be 80-100%), incremental signal quality indicator, and Vpp levels on A/B channels. Values below 70% indicate encoder internal wear or contamination. If available, use oscilloscope on A/B/Z channels looking for 1Vpp differential minimum and symmetrical 90-degree phase shift. 5. Step 5: Manually rotate motor shaft with drive disabled (safe torque off active) and feel for mechanical resistance, binding, or axial/radial play at encoder coupling. Hollow-shaft encoders tolerate <0.05mm axial play; exceeding this indicates bearing failure. Verify encoder mounting screws are torqued to specification (typically 0.8-1.2 Nm for M3 fasteners). 6. Step 6: If hardware checks pass, review MD36300 (encoder monitoring tolerance) and MD36310 (encoder monitoring time) in NCK machine data. Compare values against machine manufacturer specifications. Overly aggressive tolerances (<5 pulses deviation) can cause false alarms on marginally performing encoders. Temporarily increase MD36300 by 20% as diagnostic test only—if alarms cease, tolerance may be set tighter than encoder quality supports. 7. Step 7: Swap encoder cable with known-good spare or temporarily route test cable outside machine to eliminate crush points. If alarm clears, original cable is faulty. If alarm persists with new cable and verified voltage/mechanical integrity, replace encoder assembly and verify MD31050/MD31060/MD31020 encoder type parameters match replacement unit specifications before re-enabling axis.

Siemens SINUMERIK codes

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Siemens SINUMERIK Controller

Servo/Axis · Intermediate · 30-90 minutes

Alarm 25040

Siemens Alarm 25040 - Encoder Monitoring Error

SIEMENS alarm 25040 fires when the SINUMERIK control detects encoder position feedback that exceeds the monitoring tolerance defined in MD36300. This occurs when encoder signal integrity degrades due to missing pulses, excessive jitter, or positional inconsistencies that violate the configured monitoring window.

Siemens SINUMERIKServo/AxisIntermediate30-90 minutes

25040

Siemens Alarm 25040 - Encoder Monitoring Error

Encoder Monitoring

Alarm 25040 indicates the SINUMERIK NCK or SINAMICS drive has detected encoder position feedback that violates the configured monitoring window defined in MD36300 (encoder monitoring tolerance). The control continuously compares successive encoder increments and triggers this alarm when signal integrity degrades due to missing pulses, excessive jitter, amplitude deviation, or positional inconsistency beyond threshold. This is a safety-critical fault requiring acknowledgment and correction before axis motion can resume.

Root Cause Summary

Safety

Before manually rotating motor shaft, engage machine master key to safe position and verify drive Safe Torque Off (STO) is active via HMI status display to prevent unexpected servo enable
When measuring encoder signals with oscilloscope or multimeter on energized system, use isolated test equipment and do not bridge encoder signal pins to ground as this will damage SINAMICS encoder interface module (SMC/SME)

Causes and Fixes

1
Check Encoder Cable Connections
Navigate to HMI > Diagnostics > Drive > Encoder and verify all encoder cable connections at motor, drive, and NCK terminals are secure and properly seated.
Power down axis drive before checking connections to prevent electrical hazards.
2
Inspect Cable Physical Condition
Examine encoder cable routing for damage, excessive bending, or proximity to power cables. Check for cuts, abrasion, or pinch points along cable path.
Maintain minimum 6-inch separation between encoder and power cables to prevent EMI.
3
Measure Encoder Signal Quality
Use HMI > Diagnostics > Drive > Encoder signals to monitor A/B channel amplitude and verify signals are within 0.5-3.3V differential range with proper 90-degree phase relationship.
Signal amplitude below 0.5V or above 3.3V indicates cable or encoder issues.

DIY Feasiblehigh confidence

Call a technician if:

·Encoder replacement required
·Servo parameter optimization needed
·Mechanical alignment issues identified

Prevention

Regular encoder cable inspection
Maintain proper cable routing separation
Monitor encoder temperature trends
Periodic mechanical coupling inspection

Common Mistakes

Setting MD36300 too high and masking real problems
Ignoring EMI sources near encoder cables
Replacing encoder without checking cable integrity first
Not documenting original parameter values before changes

Related Alarms

25041 25050 300500 300501

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