What causes Siemens SINUMERIK alarm 21550?

Part program contains an illegal or unsupported command for the current machine configuration. Channel or axis configuration does not support the programmed function. Required machine option not enabled for the commanded function. Axis assignment or transformation configuration error. Program execution sequence error.

How do I fix Siemens SINUMERIK alarm 21550?

1. Identify the program block that triggered the alarm — note the channel and block number. 2. Review the commanded function against the SINUMERIK programming manual. 3. Check if the required machine option is enabled for this function. 4. Verify axis assignments and transformation settings are correct for the application. 5. Consult the Siemens SINUMERIK alarm manual for alarm 21550 specific remedy.

axismd.ai

Siemens SINUMERIK Controller

Spindle · Intermediate · 30-120 minutes

Alarm 21550

Siemens 840D 21550 - Channel axis Travel from hardware limit switch not possible.

Alarm 21550 occurs when a retraction command is issued to a slave axis in an axis coupling or transformation output axis, but the current machine state prevents safe retraction. This typically involves hardware limit switch conditions, unsynchronized coupling states, or invalid kinematic configurations that block the commanded axis movement direction.

Siemens SINUMERIKSpindleIntermediate30-120 minutes

21550

Siemens 840D 21550 - Channel axis Travel from hardware limit switch not possible.

Channel axis Travel from hardware limit switch not possible.

Root Cause Summary

Alarm 21550 occurs when a retraction command is issued to a slave axis in an axis coupling or transformation output axis, but the current machine state prevents safe retraction. This typically involves hardware limit switch conditions, unsynchronized coupling states, or invalid kinematic configurations that block the commanded axis movement direction.

Safety

Refer to Siemens documentation
Ensure qualified technician performs repairs

Causes and Fixes

1
Verify axis coupling configuration: Navigate to Diagnosis > Axis > Couplings menu and confirm MD20050 (CouplingType) value matches intended master-slave relationship.
2
Execute synchronization move: Command the master axis to move through full stroke to establish coupling synchronization, monitoring SD43010 (ActualSynchronizationError) to confirm convergence below MD20051 (SyncErrorTolerance).
Do not force retraction commands while SD43010 exceeds tolerance thresholds.
3
Retry retraction command after synchronization complete. Confirm coupling status bit in NCK Shows synchronized state before issuing G-code retract movement.

Service Recommended confidence

Seeing Multiple Alarms?

Seeing another alarm with this one? Enter the code.

Get machine-aware diagnosis

AxisMD logs alarms against your actual machines, tracks patterns, and gives AI context your manual doesn't have.

Start free

Was this guide helpful?

Always verify with your machine manual or a certified technician before performing service procedures.