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Power off the machine. Inspect the serial pulse coder cable (gray/white cable) from the motor to the amplifier JF2 connector. Check for cuts, pinches, oil contamination, or damaged connectors. Verify cable shield continuity and proper grounding.

Always power off before disconnecting encoder cables to prevent damage to amplifier encoder interface circuits.

Swap the suspect axis cable with a known good axis cable if available. Power on and jog both axes. If alarm follows the cable, replace the cable. If alarm stays with original axis, proceed to step 3.

Check servo amplifier diagnostics: Navigate to DIAGNOSIS > SERVO > select affected axis. Monitor diagnostic numbers 300-305 (encoder feedback values) and 400 (serial pulse coder alarm detail). Record any abnormal values or additional alarm codes displayed.

If cable is confirmed good, the motor pulse coder is likely defective. Replace the servo motor or have the encoder unit replaced. After replacement, execute parameter initialization and perform serial pulse coder setup (parameter 1815 bit settings) and run automatic offset adjustment if required by motor type.

Encoder replacement requires recalibration. Back up all parameters and position data before motor replacement.

Navigate to SYSTEM > PARAM and verify parameter 1815 (serial pulse coder setting) for the affected axis matches the actual motor encoder type. Confirm bit 0 and bit 1 settings match motor specifications (αi, βi, or serial absolute encoder type).

Check parameter 1820 (pulse coder model selection) and parameter 1825 (serial pulse coder communication speed) are correctly set for your specific motor model. Refer to motor nameplate and amplifier specification manual for correct values.

Verify parameter 1827 (encoder output pulses per revolution) matches the motor encoder specification exactly. Common values are 1048576, 2097152, or 4194304 depending on motor series (αi vs βi series).

If parameters were changed or restored, power cycle the CNC completely (not just reset). Execute parameter write and perform a controlled axis reference point return. Monitor alarm recurrence during low-speed jogging before normal operation.

Parameter changes require full power cycle to take effect on encoder interface circuits.

Check servo amplifier LED indicators. A red LED or specific blink pattern on the amplifier indicates internal fault. Consult amplifier manual for LED codes. Check amplifier alarm history via DIAGNOSIS > SERVO > ALARM HISTORY for concurrent amplifier alarms.

Verify proper cabinet grounding. Measure resistance between amplifier chassis ground and machine earth ground (should be <0.1Ω). Check for loose ground connections at amplifier mounting bolts and ground bus bars. Inspect for improper cable routing near power lines or inverters.

Use calibrated low-resistance ohmmeter for ground measurements.

Check for electrical noise sources: inspect for damaged power cables, improperly shielded motor cables running parallel to encoder cables, or recent additions of VFDs/welders nearby. Separate encoder cables from power cables by minimum 300mm or use separate cable trays.

If noise sources are eliminated and alarm persists, swap the amplifier with another identical axis if available. If alarm follows the amplifier, replace the servo amplifier. After replacement, re-verify all servo parameters and perform servo tuning if necessary.

Amplifier replacement requires parameter backup and restoration. Some amplifiers have axis-specific calibration data.