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Power off the machine. Inspect the pulse coder cable from the motor encoder to the servo amplifier CN2 connector (or CN1 on older amplifiers). Look for cable damage, excessive bending near connectors, contamination, or loose backshells.

Ensure main power is OFF and capacitors are discharged before touching amplifier connections.

Disconnect and reconnect both ends of the pulse coder cable. Verify connector pins are not bent or corroded. Check that connector locking mechanisms are fully engaged and tightened to proper torque specification.

Measure cable continuity using a multimeter. Check all signal lines and shield ground. Replace the pulse coder cable if any intermittent connections or resistance above 1-2 ohms is detected on signal lines.

After cable replacement or reconnection, power up and check DIAGNOSIS > SERIAL on the control. Navigate to the serial pulse coder screen and verify the axis shows encoder position data updating. Also check parameter 1815#0 (absolute encoder setting) is set correctly for your encoder type.

Check for alarm 309 or 320 series alarms which specifically indicate battery voltage issues. Navigate to SYSTEM > DIAGNOSIS > APC and check battery voltage display for the affected axis. Replace encoder battery if voltage is below 3.0V DC.

Encoder battery replacement must be done with power ON to avoid losing absolute position reference.

If battery is good, perform an encoder reset. Navigate to SYSTEM > PMC > SETTING > INITIALIZE and execute the absolute pulse coder reset for the affected axis. Then perform a reference position return (G28) to re-establish the machine coordinate system.

Encoder initialization will lose current absolute position data. Record machine position before performing reset.

Check amplifier LED indicators. The encoder communication LED (typically labeled SPL or SER) should be steady green. Blinking red or no LED indicates encoder communication failure. Refer to amplifier manual for specific LED codes for your alpha/beta series amplifier.

If alarm persists after battery replacement and reset, the encoder internal electronics have likely failed. Replace the motor or encoder assembly and perform absolute position setup using parameter 1815 and reference position establishment procedure per Fanuc connection manual.

Verify encoder power supply from amplifier. Check DIAGNOSIS > SERVO and look for the encoder +5V supply voltage (should be 4.8-5.2V). If voltage is out of range, the amplifier encoder power circuit has failed and amplifier requires replacement.

Verify parameter 1815 (serial encoder interface setup) matches the encoder type installed on the motor. Check parameter 1820 (encoder serial number) is correctly set. Incorrect parameters will prevent communication initialization. Compare with backup parameters or machine documentation.

Swap the pulse coder cable with an identical working axis to isolate amplifier vs. encoder failure. If alarm moves to the other axis, encoder or motor is faulty. If alarm stays with original axis, amplifier is faulty.

Only swap cables between identical axis configurations with same encoder types to prevent damage.

If amplifier is determined faulty, replace the servo amplifier module. After replacement, set all axis servo parameters (2000-2200 series) and encoder parameters (1815, 1820) from backup. Perform reference position return and verify position accuracy with test movements.

Record all servo tuning parameters before amplifier replacement. Incorrect parameters can cause runaway or instability.