The SV0401 (V READY OFF) alarm is a frequent headache for CNC maintenance personnel. It signifies that the Servo Ready signal has been disconnected. When this happens, the system cannot provide power to the motors, bringing production to a halt.
In this article, we will break down the causes of SV0401 and show you how to use Diagnosis 358 to pinpoint the exact failure point.
Common Causes:
The SV0401 alarm may occur under the following conditions:
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The servo ready signal (VRDY) of the servo amplifier has not turned ON during startup, or it turns OFF during operation
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Chain Reaction: The alarm is triggered by another, primary servo alarm.
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External Hardware: Issues within the high-voltage/strong-current circuits surrounding the amplifier.
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Component Failure: A malfunction in the Servo Amplifier, Mainboard, or CNC Axis Card.
- VRDY
- Example connection around the amplifier
Step-by-Step Analysis Method
Step 1: Check for Concurrent Alarms
Before diving into SV0401, check if there are any other alarms on the screen. Always resolve other specific servo or power supply alarms first, as they often cause SV0401 as a secondary symptom.
Step 2: Utilize Diagnosis No. 358
This is the "secret weapon" for troubleshooting SV0401. This diagnostic parameter (available on systems like 0i-B/C/D/F, 16i/18i/21i, and 31i series) identifies exactly why the VRDY signal is missing.
How to read it:
Convert the value in DGN 358 into binary and check bits #5 through #14. When the system is healthy and ready, these bits turn to "1" sequentially from bit 5 upwards.
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Normal Value: 32737 (Binary: 111111111100000).
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The Rule: The first bit (reading from right to left) that is "0" is the cause of your SV0401 alarm.
Bit Breakdown for DGN 358:
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#5 HRDY: System monitor program started.
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#6 *ESP: Emergency Stop signal (Check external E-stop circuit).
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#7, #8, #9 MCON: Signal sent from NC to Amplifier to Power Supply.
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#10 CRDY: Common Power Supply (PSM) is ready.
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#11 RLY: Relay signal (Dynamic Brake relay drive).
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#12 INTL: Interlock signal (DB relay release state).
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#13 DRDY: Amplifier preparation finished (Hardware).
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#14 SRDY: Amplifier preparation finished (Software).
Interpreting DGN 358 Values
You can quickly identify the failure point based on the decimal value shown in Diagnosis 358:
| DGN 358 Value | Likely Root Cause |
| 417 | Emergency Stop (E-Stop) not released. |
| 993 | Power Supply Module (PSM) is not ready. |
| 4065 | DBM (Dynamic Brake Module) interlock not released. |
| 225 / 481 / 2017 / 8161 | Potential Servo Amplifier failure. |
| 97 | MCON signal not sent from NC to Amplifier. |
| 1441 | beta i series single-axis amplifier (no PSM) alarm. |
Hardware Troubleshooting & Solution Path
Common check items for SV0401 Alarm
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Is the Common Power Supply on?
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Has an emergency stop been canceled?
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Is MCC on? If there is an external MCC sequence in addition to the MCC contact of the Common Power Supply, check that sequence also. Check CX3 (MCC) interface pins 1 and 3. Ensure the power to drive the MCC coil is present. And check CX4 (ESP) interface: Pins 2 and 3.
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Is the power for driving MCC supplied?
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Is the breaker on?
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Has some alarm been issued in the Common Power Supply or spindle amplifier?
- Circuit Breakers: Check if any circuit breakers or fuses in the cabinet have tripped.
Final Solutions:
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Replace Servo Amplifier: If external circuits (MCC, ESP, 24V) are normal but DGN 358 still indicates an amplifier fault (#13/#14).
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Replace Mainboard/Servo Card: If the problem persists after replacing the amplifier, the issue may lie in the CNC's command generation hardware (Mainboard A or Servo Card B).
Conclusion
The SV0401 alarm doesn't have to be a mystery. By using Diagnosis 358, you can stop guessing and start fixing.
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