Fanuc alarm SV0493: SAFETY DO SIGNAL ERROR (CNC)

What it means

A safety position switch output or programmable safe-related output disagreed between the two transmission paths (via PMC and via FSSB I/O module) for longer than the period set in parameter No.1945. Dual-path signalling is how DCS achieves category-3 redundancy, so any persistent mismatch shuts power off immediately. Causes include faulty FSSB I/O module wiring or a configuration using extension-module outputs without the extension module fitted.

Likely causes

• Mismatch between PMC-path and FSSB-path safety outputs (wiring, relay, or I/O module fault)
• Extension FSSB I/O module absent while parameters 13880-13911 still enable its outputs
• Excessive delay in machine ladder updating the duplicated signals

Replacement parts

We supply the following units for this alarm — tested, warranty-backed:

• FSSB I/O module (basic/extension) for DCS

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