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Check amplifier LED status. A red LED or specific codes on αi/βi series amplifiers indicate encoder feedback errors. Verify power to the external encoder (typically 5VDC on pin connections).

Navigate to DIAGNOSIS > SERVO and check diagnostic numbers D0300-D0307 for the affected axis to verify external encoder pulse count integrity and compare with motor encoder feedback.

Inspect external encoder cable (JD1A/JD1B connector at amplifier) for physical damage, proper shield grounding, and separation from power cables. Check connector pins for contamination or bent contacts. Measure cable continuity and insulation resistance (>10MΩ to ground).

Disconnect power before checking cable continuity and resistance

Temporarily increase Parameter 1826 (count miss detection level for external encoder) from default 500 pulses to 1000-2000 to determine if alarm is marginal. If alarm stops, cable/encoder is degrading. Replace cable and/or encoder rather than permanently increasing threshold.

Increasing P1826 masks the problem and reduces position accuracy - use only for diagnostics

Check DIAGNOSIS > SERVO screen D0320-D0327 (external position deviation) during alarm occurrence to quantify the position error magnitude. Compare against Parameter 1826 setting (default typically 500 encoder pulses).

Manually inspect mechanical coupling between motor and external detector. For rotary encoders, check coupling set screws and shaft key engagement. For linear scales, verify scale head mounting bracket tightness and proper gap (typically 0.5-1.0mm from scale tape).

Enable servo OFF mode or ensure axis brakes are applied before manual inspection

Perform a slow axis jog test while monitoring D0320-D0327 deviation values. Sudden jumps or gradual drift indicate mechanical issues. Check ballscrew bearing preload and backlash per machine manufacturer specifications.

Verify Parameter 1815 bit#0 (external position deviation detection enable) is correctly set to 1, and Parameter 1825 (detection level selection) matches your encoder configuration. Confirm CMR (Count Miss Ratio) setting in Parameter 1827 if using dual-check system.

Swap the external encoder cable with a known good spare or from a working axis (if same encoder type) to isolate cable vs. encoder failure. If alarm follows the cable, replace cable. If alarm stays with axis, suspect encoder or amplifier.

Check encoder signal quality using DIAGNOSIS > SERVO waveform monitor if available (31i/32i series) or measure A/B phase signals at amplifier connector with oscilloscope. Verify 90-degree phase relationship, amplitude (differential RS422 signals ±2-3V), and clean square waves without noise or dropouts.

Use differential probes or proper grounding when measuring encoder signals to avoid ground loops

Review alarm history (ALARM > HISTORY) to determine if alarm is intermittent or continuous. Intermittent alarms suggest bearing noise in encoder or cable flexing issues. Continuous alarms indicate complete encoder failure or amplifier circuit damage.