SV0447 Hard Disconnect Alarm (External) fault in Fanuc drive

SV0447 Hard Disconnect Alarm (External) fault in Fanuc drive

Description

SV0447 is a "Hard Disconnect" specifically regarding an external separate detector (like a linear glass scale or rotary magnetic scale). The "Hard" designation means it is a hardware-level trip. The amplifier monitors the +5V power and the differential line pairs (A/A*, B/B*) from the scale. If the 5V supply line is cut, or if both A and A* signals go to the same voltage (loss of differential signal), the alarm triggers immediately. It means the CNC has zero connection to the scale.

Cause

This is a physical circuit break.

• Unplugged Cable: The most common cause is the connector simply falling off the linear scale reader head.
• Cut Cable: A chip conveyor or sliding door has physically severed the cable.
• Scale Failure: The reader head of the scale has failed internally and is no longer outputting voltage signals.
• Preamplifier Box: Many scales use a "Pre-Amp" or "ADB" box to convert sine waves to Fanuc serial data. If this box loses power or fails, SV0447 occurs.

Solution

1. Verify Connection: Trace the cable from the drive to the scale. Check all junction boxes and bulkheads.
2. Check Voltage: Unplug the connector at the scale end. Measure for +5VDC on the power pins. If you have 0V, the break is in the cable or the drive is not outputting power.
3. Bypass Scale: To keep the machine running, you can disable the scale. Go to Parameter 1815, find bit 1 (OPT). Change it from 1 to 0. You will also need to change the Flexible Gear Ratio (DMR) to match the motor encoder instead of the scale. This allows the machine to run on the motor encoder while you order parts.