Depending on the FANUC control series the feedback signal will be DRDY or VRDY as per the maintenance documentation. The primary areas to check are the servo amplifier, the signal wiring and the CNC axis control card.
Before replacing any components, check the signal status and the associated connections.
1. What Does Fanuc Alarm 404 Mean?
FANUC Alarm 404 is the servo amplifier READY signal not following the correct control sequence.
Two signals are involved:
- MCON: A servo enable command from the CNC to the servo amplifier.
- DRDY / VRDY: The READY feedback from the servo amplifier to the CNC. The terminology is different from one FANUC control generation to the next and in the various manuals.
Under normal circumstances the servo amp READY feedback should follow the CNC control sequence. Alarm 404 will occur if the feedback remains ON after MCON has turned OFF or if the feedback comes ON prior to MCON being turned on.
This could be an alarm from the servo amplifier or it could be an alarm from the connection and control hardware between the CNC and the amplifier.
2. Common Causes of FANUC Alarm 404
The main possible causes include:
- Faulty servo amplifier: the amplifier itself may be at fault and not be able to change its READY signal correctly.
- Poor connections: loose connectors, damaged cables, poor contact between the CNC and the servo amplifier.
- Faulty CNC axis control card: the axis control card may be failing to process or exchange the servo ready signals.
- Related CNC board failure: on certain FANUC controls the Main CPU board or the Option 2 board may also have to be checked.
The exact hardware involved will of course depend on the CNC series and system configuration.
3. How to Troubleshoot Fanuc Alarm 404 Step by Step
Step 1: Confirm the Alarm and Identify the Affected Axis
Check the CNC alarm screen, record the full alarm message and the axis number which is affected.
Also identify the FANUC CNC model and the servo amplifier model. Does the alarm occur on startup, following an emergency stop or during normal machine operation?
If any other servo or power related alarms occur at the same time then record those too as they may well point to the root of the problem.
Step 2: Inspect the Connections Between the CNC and Servo Amplifier
Check the signal connections between the CNC control hardware and the servo amplifier.
Inspect the connectors, cables and interfaces for loose connections, damaged pins and poor contact. For systems using FSSB optical communication, check the relevant optical cables and connections in accordance with the applicable FANUC manual.
A connection issue should be ruled out prior to replacing any of the electronic components.
Step 3: Check the Servo Amplifier
Check the servo amplifier status lights and see if it's showing any further alarms.
Verify its power and operating condition in accordance with the procedures set out in the relevant maintenance manual. If the amplifier is suspected of being defective it should be tested by a qualified technician.
Step 4: Check the CNC Axis Control Card
If the servo amplifier and its associated wiring looks OK, then have a look at the CNC axis control card.
The location and configuration of the card will vary from one FANUC system to another. You'll need to refer to the relevant hardware manual to establish exactly which board it is and what the checks are.
Step 5: Evaluate Other CNC Control Boards
If the earlier checks don't sort the alarm out then further investigation into the relevant CNC hardware may be required.
Depending on the system this could be the main CPU board, the Option 2 board or some other board involved in servo control.
Before you replace a CNC board, backup the machine parameters and any other pertinent data in accordance with the manufacturer's instructions.
4. Common Mistakes When Fixing Fanuc Alarm 404
Avoid these common troubleshooting mistakes:
- Replacing the servo amplifier without first checking its connections.
- Ignoring the READY feedback and just looking at the motor.
- Replace the CNC axis control card without testing the amplifier.
- Assuming all FANUC systems use the same board configuration.
- Changing the control parameters without actually identifying the root cause.
A systematic diagnosis will of course help to avoid unnecessary component replacement.
5. Safety Precautions During Troubleshooting
Servo amplifiers will have hazardous electrical voltages present even after the main power has been switched off. Always follow the machine manufacturer's shutdown and lockout procedures prior to disconnecting any cables or replacing any components.
Allow the DC bus to discharge for the specified period of time. Live signal measurements and electrical tests should only be carried out by competent personnel using the appropriate equipment.
6. Should You Repair or Replace the FANUC Servo Amplifier?
Should you repair or replace the FANUC servo amplifier? The right answer is going to be down to the actual fault.
If Alarm 404 is due to a loose connection or a damaged cable then repairing the connection may well cure the problem without the need to replace the amplifier at all.
If the servo amplifier is definitely at fault then repair or replacement will be required. If the amplifier itself is OK then the focus should move to the CNC axis control card and the associated hardware.
Before you go and buy a replacement, make sure you have the full part number and that it's compatible with the FANUC system you've got in. If it's a used amp then you'll also need to check the test status and the warranty.
7. How SongWei CNC Can Help
SongWei CNC supply new and used FANUC CNC spares (servo amplifiers, servo motors, CNC control boards etc) as well as FANUC spares testing and repair.
If you need help with FANUC Alarm 404 then please provide your CNC model, the axis in question, the servo amplifier model and a photo of the alarm. That will help to pinpoint the fault and identify the correct replacement part.
Conclusion
FANUC Alarm 404 is 'Servo amplifier READY feedback signal does not follow expected MCON control sequence'.
Start by identifying the affected axis and check the connections between the CNC and the servo amplifier. Then check the amplifier and the CNC axis control card. Check other relevant control boards as appropriate according to the FANUC system installed.
Confirm the actual faulty component before deciding whether repair or replacement is required.