SV0300 Request for Reference Return fault in Fanuc drive

SV0300 Request for Reference Return fault in Fanuc drive

Description

SV0300 (sometimes seen as APC alarm 300) is the most common "non-fatal" alarm. It simply states that the machine does not know where it is. Fanuc servo motors use Absolute Pulse Coders (APC). They are powered by a battery when the machine is off. If that battery voltage drops to zero, the counter inside the encoder stops. When you turn the machine back on, the counter reads "0," but the CNC knows this data is invalid because the battery flag was tripped. Therefore, the machine enters a state where it refuses to run automatic cycles until a "Reference Return" (Homing) is performed.

Cause

The loss of the absolute position data is the root cause.

• Dead Batteries: The 6V battery pack (usually yellow) or individual D-cells have died while the machine was powered off.
• Cable Disconnect: If the encoder cable was unplugged for maintenance while the drive was off, the capacitor in the encoder drains in about 20 minutes, losing the position.
• Drive Replacement: Installing a new drive requires transferring the parameters, but the "Position" is stored in the encoder, and the link between them must be re-established.

Solution

The solution depends on whether the machine has physical limit switches (Dogs) or is "Dogless."

1. Replace Batteries: Always replace the batteries first. Do this with the machine POWERED ON to prevent this happening again next time.
2. Standard Home (With Dogs): If the machine has physical switches, simply select "REF RETURN" mode and jog the axis toward the home position until the light turns on. The alarm will clear.
3. Dogless Homing (Parameter 1815): If the machine has no switches, you must manually teach the home position.
   • Move the axis to the align marks.
   • Go to Parameter 1815. Find the bit labeled "APZ".
   • Toggle it from 1 to 0, then back to 1.
   • Power cycle the machine. This resets the absolute zero point to the current location.