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Check amplifier LED status. FANUC servo amplifiers will typically show red LED alarm indication. Record LED pattern for reference.

Power off and inspect external encoder cable connections at both encoder head and amplifier CNA2 or CNA3 connector. Check for bent pins, contamination, or loose retention clips.

Always power down before disconnecting encoder cables to prevent damage to encoder electronics.

Measure cable continuity and insulation resistance. Check shield continuity to ground. Replace encoder cable if resistance values are abnormal or cable shows physical damage.

Navigate to SYSTEM > DIAGNOSIS > #300 series diagnostics. Check encoder data diagnostics for the affected axis to confirm encoder pulse detection after cable verification.

Check parameters 1815 (external encoder serial number) and 1816 (external encoder type) for the affected axis. Verify correct encoder model is specified matching installed hardware.

Temporarily swap external encoder cable with known good axis (if available) to isolate cable vs encoder fault. If alarm follows cable, replace cable. If alarm stays with axis, encoder is suspect.

Ensure axes being swapped use identical encoder types and amplifier models.

Inspect encoder head for physical damage, contamination, or excessive runout. Clean optical disk with isopropyl alcohol if accessible. Check encoder mounting and coupling for mechanical issues.

Replace external pulse coder if hardware fault confirmed. After replacement, verify parameter 1815 matches new encoder serial number and execute encoder setup procedure per axis.

Navigate to SYSTEM > PARAM and verify parameter 1815#0-#3 (external encoder protocol selection), parameter 1825 (encoder communication settings), and parameter 1850 (external scale settings) match encoder manufacturer specifications.

Check parameter 1023 (serial encoder type) bits for correct serial protocol selection (0=SERIAL, 1=BiSS-C). Incorrect setting prevents proper encoder communication.

Changing encoder protocol parameters requires CNC power cycle and may require encoder recalibration.

Inspect encoder cable routing. Ensure separation from AC power cables, servo motor power lines, and spindle cables. Verify cable shield is grounded at amplifier end only, not both ends.

Use oscilloscope to check encoder serial signal quality at amplifier connector if noise suspected. Look for clean differential signal levels per encoder specification (typically ±2-5V differential).

Oscilloscope measurements on live encoder signals require proper differential probe technique to avoid signal corruption.