SV0491 DCS: Safety I/O Error fault in Fanuc drive

SV0491 DCS: Safety I/O Error fault in Fanuc drive

Description

SV0491 is a Dual Check Safety (DCS) alarm. In a DCS environment, safe signals (like Emergency Stop, Door Interlock, or Safety Speed Monitoring) are transmitted via a redundant I/O link. SV0491 indicates that the two separate checking channels (Path A and Path B) for a specific I/O signal disagree. For example, Path A says the door is "Closed" (Logic 1), but Path B says the door is "Open" (Logic 0). The machine defaults to the safest state (Stop) and throws this alarm.

Cause

• Bad Safety Switch: A dual-channel tongue switch on a door has one broken contact. One side closes, the other stays open.
• Loose Wire: One of the two wires in the safety pair is loose at the terminal block (X11/X12 signals).
• I/O Module Failure: The Safety I/O board (or connector panel) has a failed opto-coupler on one channel.
• Timing: The two contacts on a mechanical switch closed with a time gap greater than the DCS parameter allowance (e.g., >500ms).

Solution

1. Check DCS Screen: Go to [SYSTEM] > [DCS] > [I/O]. Look for the signal that is "0/1" or "1/0". It should be "1/1" or "0/0".
2. Inspect Switch: Use a multimeter to ohm out the safety switch contacts. They must toggle simultaneously.
3. Wiring: Tighten all terminals on the yellow Safety I/O modules.