SV0384 Soft Phase Alarm (Separate Detector) fault in Fanuc drive

SV0384 Soft Phase Alarm (Separate Detector) fault in Fanuc drive

Description

SV0384 is a feedback integrity alarm for external linear or rotary scales (Separate Detectors). It indicates a mismatch in the "Soft Phase" data. External scales generate sine waves (A/B phase) or serial data. The CNC monitors the phase relationship to determine position. SV0384 means the CNC detected a transition that is logically impossible (e.g., both A and B signals changing state simultaneously), or the digital signature of the position packet was corrupted.

Cause

• Dirty Scale: Contamination (oil/coolant) on the glass scale is the #1 cause. It refracts the light, causing the A/B phase signals to distort or fade.
• Alignment: The read-head is rubbing against the scale body or the gap is too wide.
• Cable Impedance: Using a non-twisted pair cable or a cable that is too long, causing signal skew (where Signal A arrives slightly later than Signal B).

Solution

1. Clean Reader Head: Disassemble the scale guard and clean the glass and read-head with isopropyl alcohol.
2. Check Air Purge: Ensure positive air pressure is supplied to the scale to keep contaminants out.
3. Oscilloscope: The best way to diagnose is to put an oscilloscope on the A and B signals. They should be clean sine/square waves offset by 90 degrees. If they are fuzzy or not offset, the head is bad.