Fanuc spindle alarm 77 occurs only with Dual Check Safety, and means the spindle amplifier's axis number check does not match the CNC's axis number check — the two redundant channels disagree about which axis/spindle this amplifier is supposed to be. Identity matters in a safety architecture: if the CNC believes it is monitoring spindle 1 while the amplifier believes it is spindle 2, every safety judgment built on that identity is unreliable — so the machine stops.
When it appears
AL-77 is overwhelmingly a configuration-change alarm: amplifiers swapped between positions, FSSB cabling reordered, a control board transplanted from another machine, axis assignments edited, or software reloaded. On an untouched machine it means one channel's stored identity data is failing.
Fanuc's remedy — and the check to do first
The manual: replace the CPU card in the CNC or the Spindle Amplifier control PCB. Before that:
- Re-verify the configuration on paper: the amplifier's position in the FSSB chain, the axis/spindle assignment settings, and — after any board swap — whether the board carries another machine's identity. If anything was recently rearranged, the arrangement is the suspect, not the silicon (the safety-flavoured cousin of AL-14 and AL-70).
- History check: companion self-test codes name the failing side — amplifier codes (AL-74/75/76) → amplifier board; CNC system alarms → CPU card.
- Then hardware: board-level repair or exact-spec exchange of the guilty side, accepted with a documented safety-function test.
Related guides
- Fanuc spindle alarm 70 (connection vs hardware setting)
- Fanuc spindle alarm 72 (speed check mismatch)
- Fanuc αi drive alarms — PSM, SVM & SPM codes