Complete Fanuc SV0401 and SV0404 V-READY Alarms Troubleshooting

Introduction

An abrupt stop during high-speed machining or a frozen startup sequence on a heavy-duty CNC milling machine often traces back to a breakdown in high-voltage communication. When a Fanuc system commands the servo amplifier to engage the motor contactor (MCON) and establish position control (PRDY), any failure to receive a velocity control ready signal (VRDY) will trigger an immediate SV0401 alarm, trapping the machine sequence to prevent driving a dead axis. Conversely, an improper VRDY-ON feedback signal when the system is commanded OFF will lock the controller with an SV0404 alarm to halt dangerous, uncommanded axis runaway. Attempting to force axis movement when these critical handshake signals are out of sync locks out all manual and automatic operations, threatening production timelines and indicating deep-seated electrical or mechanical faults.

Technical Summary

Technical Metric	Specification / Detail
Command Code	— (Hardware Servo Alarm)
Modal Group	—
Applicable Brands	Fanuc
Critical Parameters	Parameter 1800 bit 1 (CVR), Parameter 1804 bit 4 (IVO), Parameter 1804 bit 6 (SAK), Parameter 0010 bit 2 (OFFVY)
Main Safety Constraint	High voltage circuitry shuts down automatically if the PRDY vs VRDY sequence fails to protect hardware; vertical axes will violently drop under their own weight when power lines are disconnected if they are not physically secured.

Quick Read

• Identify the alarm type: SV0401 represents a Velocity Control Ready OFF fault during operation, whereas SV0404 represents an improper Velocity Control Ready ON state when it should be deactivated.
• Inspect physical communication: Check for a disconnected or broken FSSB optical cable between the Fanuc CNC control card and the servo amplifier module.
• Check power diagnostics: Verify if there is a blown control power supply fuse or a missing phase in the input power line feeding the servo module.
• Leverage diagnostic register DGN 358: Convert DGN 358 to binary to inspect bits 5 through 14, isolating the exact micro-step of the hardware boot failure.
• Assess mechanical restrictions: Ensure no mechanical clamp is engaged on the axis when FVF (follow-up behavior) is disabled by Parameter 1800 bit 2, as mechanical binding stalls the motor's startup excitation sequence.
• Suppress startup sequence anomalies: If using third-party amplifiers, modify Parameter 1800 bit 1 (CVR) to suppress false alarms caused by timing discrepancies.

Basic Concepts

The SV0401 and SV0404 faults are hardware-level servo alarms rather than programmable G-code commands. They are generated by the CNC controller to indicate a critical timing and state mismatch between the CNC logic's Position Control Ready (PRDY) signal and the servo amplifier's Velocity Control Ready (VRDY) signal. This handshake mechanism acts as a primary safety gate: the CNC demands power readiness via the MCON (Servo Activation Request) command, and the servo module must acknowledge by asserting VRDY within a designated millisecond window. If this handshake fails, the high-voltage lines are instantly severed to isolate the axis and protect the system hardware from damage.

Because these alarms represent system-level state violations, operators cannot bypass them through standard NC code. Instead, these conditions are managed entirely through the CNC's system parameters and diagnostic interfaces. Understanding this underlying logic is essential for advanced troubleshooting; the sequence relies on a synchronized dance of low-voltage relay logic, high-voltage power contactors, and high-speed FSSB fiber-optic communications. Any interruption in this chain breaks the loop, triggering immediate alarm states that lock down machine operations.

Command Structure

Although SV0401 and SV0404 alarms are hardware-level safety interrupts rather than standard motion commands, the parameters that regulate their sensitivity and error-suppression timing can be programmatically queried or modified. In typical Fanuc setups, these settings are established during commissioning by the machine tool builder. However, developers and advanced maintenance technicians can alter these parameters dynamically using programmable data input commands if custom diagnostics or temporary testing phases are required.

To perform these adjustments directly from a G-code program, the G10 L52 block is utilized to open parameter entry mode for servo parameters. The system is then commanded to target the specific parameter address, adjusting individual bits to toggle alarm suppression or timing tolerances. This programmatic access provides a reliable method to configure controls without manual keyboard input at the operator panel, though it must be closed using G11 to return the controller to normal execution mode.

The standard programming block syntax is structured as follows:

G10 L52 ; (Enter parameter writing mode)
N[ParameterNumber] R[Value] ; (Write value to target parameter address)
G11 ; (Exit parameter writing mode)

Parameter Address	Name	Function & Value Settings
Parameter 1800 bit 1 (CVR)	V-READY Alarm Suppression	Specifies if the CNC issues a servo alarm if VRDY turns ON before PRDY. A value of 0 generates the alarm; 1 suppresses the alarm.
Parameter 1804 bit 4 (IVO)	Emergency Stop Release Mode	Determines how an emergency stop state is released when the VRDY OFF alarm ignore signal is active. A value of 0 delays release until the ignore signal is 0; 1 releases it immediately.
Parameter 1804 bit 6 (SAK)	Servo Ready State Control	Determines the state of the servo ready signal (SA) when the VRDY OFF alarm ignore signal (IGNVRY) is active. Range is 0 (SA is set to 0) or 1 (SA remains set to 1).
Parameter 0010 bit 2 (OFFVY)	Legacy V-READY Suppression	On older legacy Fanuc controls, dictates whether an alarm occurs when VRDY is set to 1 before PRDY is set to 1. A value of 0 allows the alarm; 1 suppresses the alarm.

Brand Applications

Fanuc

Within Fanuc systems, the V-READY handshake is a safety-critical timing sequence that ensures the high-voltage servo drive is fully energized and communicating before axis motion is permitted. The CNC coordinates this using MCON (Servo Activation Request) and monitors the VRDY feedback line. If a designer integrates a custom third-party amplifier or wants to configure custom emergency stop release sequences, they can use specialized parameters to modify the handshake behavior. Specifically, Parameter 1800 bit 1 (CVR) and Parameter 0010 bit 2 (OFFVY) are utilized to suppress alarms if the timing of the incoming VRDY signal does not align with standard sequences.

Important Safety Precaution: Before programmatically modifying parameter registers using G10 L52, operators should perform a full system backup to prevent accidental data loss. Refer to the Fanuc Automatic Data Backup guide for complete instructions on creating stable restore points before editing servo parameters.

Brand Comparison

Fanuc CNC Series	IGNVRY Ladder Ignore Signal Support	V-READY Alarm Suppression Parameter	Diagnostic Tracing Mechanism
Fanuc Series 15i / Legacy	Supported via PMC ladder ignore signal (IGNVRY <Yy+1#6>).	Parameter 0010 bit 2 (OFFVY) used on early architectures.	Legacy diagnostic registers with basic alarm bit mapping.
Fanuc Series 16i / 18i / 21i	Supported via PMC ladder ignore signal (IGNVRY <Yy+1#6>) to bypass false startup faults.	Parameter 1800 bit 1 (CVR) introduced for standard suppression.	DGN 358 basic bitwise tracking for velocity ready status.
Fanuc Series 0i (Modern 0i-D/F)	Obsolete and permanently deleted; builders must remove IGNVRY from ladders.	Parameter 1800 bit 1 (CVR) standard for third-party modules.	Complete DGN 358 bitwise sequence (bits 5 to 14) for micro-step isolation.
Fanuc Series 30i / 31i / 32i	Completely unsupported; high-speed fiber-optic FSSB performs self-diagnostics.	Parameter 1800 bit 1 (CVR) and advanced digital servo parameters.	Full high-resolution DGN 358 diagnostic register for millisecond sequence analysis.

Technical Analysis

Within the Fanuc ecosystem, the V-READY handshake is characterized by a granular, bit-level diagnostic tracking architecture. Unlike generic systems that merely report a general servo ready failure, Fanuc provides a dedicated diagnostic register, DGN 358 (V ready-off information), designed specifically to dissect the SV0401 alarm. By converting the value of DGN 358 to its binary representation, a technician can inspect a chronological, step-by-step timeline of the amplifier's internal boot sequence. As the servo drive initializes, diagnostic bits 5 through 14 sequentially transition to '1'. This includes bit 6 for converter emergency stop release, bit 10 for converter ready status, and bit 12 for the dynamic brake relay release. The maintenance engineer simply identifies the first bit in the register that remains '0' to pinpoint the precise micro-step where the electrical handshake stalled.

Additionally, Fanuc accommodates custom hardware configurations by allowing machine builders to mathematically manipulate the handshake timing. Through Parameter 1800 bit 1 (CVR), the controller can suppress false VRDY-ON sequence alarms, which is particularly beneficial when integrating third-party amplifiers that may power up at a different rate than the CNC's native position controller. This timing flexibility, paired with the complete removal of the obsolete V-READY ignore signal (IGNVRY) from modern Series 30i, 31i, 32i, and 0i control cards, demonstrates a transition away from ladder-based software bypasses toward rigid, high-speed, fiber-optic FSSB hardware self-diagnostics.

Program Examples

To programmatically adjust the V-READY alarm suppression setting from within a machining program, a programmer can use G10 L52 data input mode. Below is the G-code configuration sequence for Fanuc controls:

G10 L52 ; (Initiate programmable parameter input mode for servo parameters)
N1800 R2 ; (Sets Parameter 1800 bit 1 to 1, CVR=1, to suppress the VRDY sequence alarm)
G11 ; (Cancel programmable parameter input mode and return to normal execution)
G00 X100.0 Y50.0 ; (Rapid traverse block that will instantly halt if SV0401 occurs)

Dry Run and Safety Verification: Before executing this program on a live workspace, a dry run must be performed. Ensure that the axis is mechanically clear of any obstructions and that the mechanical clamp is fully released. Power up the controller and check the diagnostic DGN 358 screens. Execute the program in single block mode with dry run enabled to verify that Parameter 1800 is correctly modified and that no hardware-level alarms lock the axis. Monitor the motor contactor (MCON) status to verify the handshake timing remains within stable operational bounds without throwing a sudden SV0401 fault.

Error Analysis

Brand & Alarm Code	Trigger Condition	Operator Symptom	Root Cause & Technical Fix
Fanuc SV0401 (V READY OFF)	The servo amplifier's velocity control ready signal (VRDY) does not turn on, or unexpectedly turns off, despite the CNC's position control ready signal (PRDY) being active (ON).	Immediate machine emergency stop, all axes locked out, and high-voltage circuit contactor is de-energized.	Verify FSSB optical cables, check for a blown control power supply fuse or missing input power phase, inspect DGN 358 bits to isolate the micro-step, or replace the defective servo module.
Fanuc SV0404 (V READY ON)	The servo ready signal (VRDY) remains ON even though the CNC has commanded the machine contactor signal (MCON) to turn off.	Immediate alarm lockout upon boot or immediately following an emergency stop trigger.	Inspect the servo amplifier module's internal power switching circuitry for hardware defects, or replace a faulty CNC axis control card.
Fanuc SV0414 (Digital Servo System Alarm)	The digital servo software detects an internal drive fault, such as current overload, encoder communication breakdown, or thermal runaway.	Axis motion halts instantly, and the system displays detailed digital servo diagnostic registers for deep analysis.	Refer to the Fanuc SV0414 Digital Servo System Alarm guide to troubleshoot FSSB optical lines, check the 2000-series servo parameters, and inspect the motor feedback cable shield.
Fanuc SV0411 (Servo Deviation Alarm)	The position deviation error in the servo loop exceeds the maximum allowable limit specified in the CNC parameters.	The axis fails to track the commanded path accurately and halts during rapid traverse or heavy cutting.	Refer to the Fanuc SV0411 Servo Deviation Alarm guide to check for mechanical binding on the axis, verify the mechanical clamp release, or adjust loop gain parameters.

Application Note

Mechanical binding from an engaged axis clamp during power-up will stall the motor's excitation sequence, preventing the servo from establishing its V-READY handshake and triggering an immediate startup lockout. This scenario is particularly critical when Parameter 1800 bit 2 (FVF) is set to disable follow-up behavior, as the CNC will not recalculate position deviation while the axis is in servo-off mode. If the physical clamp remains locked, the motor's initial attempt to verify position will draw excessive current or encounter high mechanical resistance. To avoid component damage, the CNC's logic board aborts the sequence before motion occurs. Maintenance teams must ensure that mechanical clamps are electrically interlocked with the machine ready logic, preventing any boot sequence from starting until all physical axes are completely free to travel.

Related Command Network

• G10 L52 (Programmable Parameter Entry): Used to open the servo parameter register database for automated editing of critical servo timing and alarm settings directly from the NC program.
• G11 (Parameter Entry Cancel): Exits the G10 parameter entry mode to restore the CNC to standard execution and prevent accidental modification of servo parameters.
• MCON (Servo Activation Request Signal): The critical CNC logic interface signal that commands the physical servo amplifier's motor contactor to close and prepare the high-voltage lines.
• DGN 358 (V Ready-Off Diagnostics): A dedicated diagnostic register that tracks the step-by-step boot sequence of the servo amplifier to identify the exact micro-step of a V-READY failure.

Conclusion

Resolving V-READY alarms requires a systematic approach that balances physical hardware inspections with digital diagnostic queries. Rather than blindly swapping expensive servo amplifiers or control cards, maintenance teams should convert DGN 358 to binary to pinpoint the exact sequence break, while double-checking FSSB fiber-optic continuity and incoming power phases. By maintaining clear synchronicity between CNC position signals and drive velocity feedback, machine operators can maintain optimal uptime and secure the electrical integrity of their Fanuc control loops.

Frequently Asked Questions

What is the difference between an SV0401 and an SV0404 alarm?

An SV0401 alarm indicates that the CNC's position control ready signal (PRDY) is active, but the servo amplifier has failed to return its velocity control ready signal (VRDY) or it has dropped during operation. An SV0404 alarm represents the opposite condition, where the VRDY signal remains ON even though the CNC has commanded the MCON signal to turn off, which typically points to a short-circuited relay or internal drive failure. To resolve SV0401, check the FSSB cables and input power phases; for SV0404, inspect the amplifier module's internal power circuits for physical damage.

How does Parameter 1800 bit 1 (CVR) help prevent false V-READY alarms?

Parameter 1800 bit 1 (CVR) determines if the CNC triggers a servo ready alarm when the velocity control ready signal (VRDY) goes active before the position control ready signal (PRDY) has been fully established. By setting CVR to 1, you suppress this timing mismatch alarm, which is highly useful when using custom or third-party servo amplifiers that boot up faster than the CNC controller. To apply this, use G10 L52 programmatically or change the bit via the MDI panel to accommodate custom drive timing.

How do you use diagnostic register DGN 358 to isolate a startup failure?

Diagnostic register DGN 358 provides a step-by-step binary timeline of the servo amplifier's boot sequence, with bits 5 through 14 transitioning from '0' to '1' as each phase succeeds. If the startup stalls and triggers SV0401, convert the value of DGN 358 to binary and find the lowest-indexed bit that remains '0'. This bit corresponds to a specific hardware check—such as bit 6 for the emergency stop release or bit 12 for the dynamic brake relay—allowing you to replace the failing sub-component rather than the entire drive unit.