Siemens alarm 25021: Axis %1 zero mark monitoring of passive encoder

What it means

Zero-mark monitoring of the passive measuring system responded: the reference mark relationship of the second encoder became implausible between sampling instants. The control cross-checks both encoders; a mismatch indicates signal problems or mechanical slip of the second system.

Likely causes

• intermittent zero-mark pulses from the second encoder
• slipping or loose coupling of the direct measuring system
• electrical noise from damaged screening
• encoder electronics beginning to fail

Replacement parts

We supply the following units for this alarm — tested, warranty-backed:

• linear/direct measuring system (per machine spec)

  encoder · Common cause

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• 6FX5002 series encoder signal cable

  cable · Common cause

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