SV0403 Hardware/Software Limit Switch (+/-) fault in Fanuc drive

SV0403 Hardware/Software Limit Switch (+/-) fault in Fanuc drive

Description

SV0403 is a fundamental operational alarm indicating that the axis has traveled beyond its permitted range. Fanuc systems use two layers of protection: Hardware Overtravel (OT), which are physical switches at the end of the rails, and Software Overtravel (Stored Stroke Limits), which are virtual boundaries defined in Parameters 1320 and 1321. When this alarm triggers, the axis decelerates to a stop immediately and prevents further movement in that direction to protect the ballscrew and way covers from crushing.

Cause

• Operator Error: The most common cause is manually jogging the machine too far.
• Grid Shift: If the Reference Return (Home) position was set incorrectly, the software limits will be shifted. The machine "thinks" it has room to move, but hits the limit early.
• Switch Failure: A physical limit switch stuck in the closed position (filled with coolant sludge) or a broken wire in the 24V limit string.
• Parameter Corruption: If parameters 1320/1321 are set to "0" or invalid large numbers, the protection zone is undefined.

Solution

1. P + CAN Cancel: To move the axis off a software limit, power on the machine while holding the "P" and "CAN" (Cancel) keys on the keyboard. This temporarily bypasses the software check, allowing you to hand-wheel the axis back into the safe zone.
2. Check Switches: Go to the [SYSTEM] > [PMC] > [STATUS] screen. Watch the X/Y signals associated with the limit switches. Manually press the switch; the bit should toggle. If not, the switch is bad.
3. Reset Home: If the alarm occurs in the middle of the table, your Home Position is lost. Perform a Reference Return reset (APZ 1815).