What causes FANUC alarm 384?

Damaged or broken external pulse coder cable causing signal loss or noise. Poor cable shielding or improper grounding allowing electrical interference to corrupt phase signals. Faulty or failing external pulse coder encoder unit on the affected axis. Loose or corroded connector pins at the pulse coder or servo amplifier end. Cable routed too close to high-voltage power lines or servo drive cables causing EMI interference.

How do I fix FANUC alarm 384?

1. Identify the affected axis from the alarm message and visually inspect the entire external pulse coder cable from encoder to amplifier for cuts, pinches, or damage from machine movement. 2. Check all connectors at both the pulse coder and the servo amplifier for loose pins, corrosion, or contamination and reseat them firmly. 3. Verify the cable shield is properly grounded at one end only and that the cable is routed away from power cables and servo drive wiring by at least 10 cm. 4. Swap the pulse coder cable with a known good cable if available and clear the alarm to see if the fault follows the cable or stays on the axis. 5. If the alarm persists after replacing the cable, replace the external pulse coder unit itself as the encoder may have internal damage or bearing wear causing signal degradation. 6. After any repair, perform a reference return on the affected axis and monitor for alarm recurrence during normal machine operation.

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FANUC Controller

Servo/Axis · Intermediate · 45-120 minutes depending on cable access

Alarm 384

Fanuc Alarm 384 - n AXIS: SOFT PHASE ALARM (EXT)

Alarm 384 indicates a phase error detected in the external pulse coder (separate encoder) on the specified axis. This occurs when the A/B phase pulse relationship from the external encoder is invalid, typically due to cable damage, EMI interference, connector issues, or encoder hardware failure. The servo amplifier continuously monitors phase signal integrity and triggers this alarm when signal quality falls outside acceptable thresholds.

FANUCServo/AxisIntermediate45-120 minutes depending on cable access

384

Fanuc Alarm 384 - n AXIS: SOFT PHASE ALARM (EXT)

SERVO ALARM 384

Root Cause Summary

Alarm 384 indicates a phase error detected in the external pulse coder (separate encoder) on the specified axis. This occurs when the A/B phase pulse relationship from the external encoder is invalid, typically due to cable damage, EMI interference, connector issues, or encoder hardware failure. The servo amplifier continuously monitors phase signal integrity and triggers this alarm when signal quality falls outside acceptable thresholds.

Causes and Fixes

1
Power off and inspect the external encoder cable from the scale/rotary encoder to the amplifier. Check for pinched, cut, or damaged sections especially near cable carriers and moving components.
Always power down before disconnecting encoder cables to prevent amplifier damage.
2
Verify all encoder connector pins are straight and properly seated at both the encoder head and amplifier CN3 or CN4 connector (depends on amplifier model). Clean contacts with electrical contact cleaner if oxidation is present.
3
Check cable shield continuity and verify it is grounded at the amplifier end only (not both ends). Measure shield resistance to ground - should be less than 1 ohm at amplifier connection.
4
Temporarily reroute encoder cable away from power cables, spindle motor cables, and high-frequency noise sources. Maintain minimum 100mm separation. If alarm clears, permanent cable rerouting is required.

DIY Feasiblehigh confidence

Call a technician if:

·Encoder replacement required
·Amplifier module replacement needed
·Oscilloscope measurements show complex signal issues

Prevention

Regular encoder cable inspection during PM
Maintain proper cable separation from power circuits
Monitor encoder signal quality during routine diagnostics
Keep spare encoder cables in stock

Common Mistakes

Grounding encoder shield at both ends
Routing encoder cables with power cables
Ignoring intermittent alarm occurrences
Not checking parameter settings after amplifier replacement

Related Alarms

383 385 401 414

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