Fanuc CNC Alarm 401 (VRDY OFF): Servo Amplifier & Magnetic Contactor In-Depth Diagnostic Guide
1. Understanding the VRDY (Velocity Control Ready) Signal
On Fanuc CNC controllers (including Series 0i-MC/MD/MF, 16i/18i/21i, and 31i-B), the VRDY signal stands for Velocity Control Ready. It represents a critical hardware and software handshake sent by the Servo Amplifier Module (SVM) to the Main CPU confirming that the high-voltage DC Link bus (300VDC or 600VDC) is energized and stabilized. If this handshake fails during axis servo-on execution, the CNC immediately aborts operation with Alarm 401 (SERVO ALARM: X/Y/Z AXIS VRDY OFF).
2. Power-Up Sequence of the Fanuc Servo Architecture
Accurate troubleshooting requires mastering the exact timing sequence of the servo power stage: 1. CNC Power ON: CPU boots and verifies the integrity of the dual-channel Emergency Stop circuit. 2. MCON Output: Once E-Stop is released, the Fanuc I/O unit issues the MCON command to pilot relays. 3. MCC Pull-In: The Magnetic Contactor (MCC) closes, supplying 3-phase 200VAC to the Power Supply Module (PSM). 4. Pre-Charge Phase: AC power flows through inrush current limiting resistors, charging the high-capacitance DC bus to approximately 280-310VDC. 5. Internal Relay Bypass: Once nominal DC bus voltage is detected, internal power relays bypass the charging resistors. 6. VRDY Feedback via FSSB: The SVM transmits the VRDY ready confirmation over the FSSB (Fanuc Serial Servo Bus) optical link. If feedback is not received within 500ms, Alarm 401 trips.
3. Diagnostic Matrix for Fanuc PSM/SVM 7-Segment Status Codes
| 7-Segment Code | System Status | Probable Root Cause | Recommended Immediate Action |
|---|---|---|---|
| Single Dash (-) | Standby awaiting MCON | E-Stop circuit open or MCC coil unpowered | Inspect E-Stop loop, over-travel switches, and MCC 200VAC/24VDC coil supply |
| Number 00 | Power stage ready | Drive is primed, but FSSB optical loop is severed | Check optical cables COP10A/COP10B between CNC CPU and servo drives |
| Number 01 | Main circuit overload | Current surge or shorted motor phase winding | Megger motor windings at 500VDC and check mechanical axis binding |
| Number 02 | DC Link voltage sag | Incoming AC line dropped below 170VAC during surge | Verify 3-phase mains balance and inspect MCC contact points for pitting |
| Number 03 | DC Link overvoltage | Excessive regenerative braking energy accumulated | Measure dynamic regenerative discharge resistor resistance across terminals |
| Blank (No LED) | Missing 24VDC control supply | Internal fuses blown or auxiliary PSU defective | Measure 24VDC across CXA2A/CXA2B and inspect internal ceramic micro-fuses |
4. Systematic Step-by-Step Diagnostic Protocol
- Inspect CNC Diagnosis Screen: Navigate to [SYSTEM] > [DGNOS] and inspect parameter `DGN 0200` and `DGN 0201`. Check if the `VRDY` bit is 0 or 1. A 0 indicates the hardware handshake never reached the axis interpolator.
- Verify 24VDC Control Power: Measure voltage across terminals `CXA2A` and `CXA2B` on the Servo Module. Voltage must remain above 21.6VDC under all conditions. Excessive ripple triggers false voltage sag trips.
- Listen for MCC Acoustic Feedback: Upon hitting Reset or Machine Ready, listen for the solid physical snap of the main contactor. Absence of this click isolates the fault to the I/O unit or safety chain.
- Inspect MCC Auxiliary Contacts: The normally-open (NO) feedback contact on the MCC informs the CNC of mechanical closure. If contact resistance exceeds 1.0 Ohm due to carbon oxidation, Alarm 401 is flagged.
- Audit FSSB Optical Transmission: Unseat the `COP10A` optical plug and inspect the transceiver core. A crisp orange/red optical beam should be clearly visible with zero oil fogging or dust contamination.
5. Static IGBT Power Module Diode Verification
If the MCC pulls in but immediately chatters or trips the main breaker within 100ms, an IGBT transistor module is likely short-circuited. Execute this static diode test: 1. Disconnect main power breaker and wait until the red 'CHARGE' indicator LED has fully discharged. 2. Disconnect motor power cables and DC copper busbars from terminals U, V, and W. 3. Switch digital multimeter to Diode Test mode. 4. Place negative lead on DC (+) busbar, probe U, V, W with positive lead. Expect balanced forward voltage drops of 0.35V to 0.48V across all three phases. Reversing probe polarity must show OL (Open). 5. Place positive lead on DC (-) busbar, probe U, V, W with negative lead. Confirm matching forward bias readings. 6. Any phase reading 0.00V or generating an audible continuity beep confirms a blown IGBT transistor requiring professional module replacement.
High Voltage Electrical Safety Precaution
6. Preventive Maintenance Protocols to Eliminate Recurring Alarm 401
- Clean External Heat Sinks: Oil mist and conductive dust clogging rear cooling fins cause internal thermal accumulation, forcing over-temperature sensors to drop VRDY.
- Schedule Cooling Fan Replacements: Fanuc fans incorporate speed feedback tachometers; failing bearings drop fan RPM below safety thresholds, instantly blocking the Ready state.
- Annual Busbar Torquing: Mechanical machining vibrations loosen high-current busbar screws over time, generating localized contact heating and phase imbalances.
- Maintain FSSB Cable Bend Radius: Never pinch or sharply bend optical fiber cables. Maintain a minimum bend radius of 50mm to prevent optical attenuation.