A Fanuc servo amplifier showing ‘U’ on its 7-segment display means an FSSB communication error — the amplifier is not receiving the optical serial link (FSSB) that daisy-chains every amplifier to the CNC's servo card. On the CNC screen this pairs with FSSB alarms such as SV5136 ‘FSSB: number of amps is insufficient’ or SV5137/SV5197-series errors, because the CNC simply cannot see the amplifiers beyond the break. The beautiful part: the daisy-chain display pattern tells you almost exactly where the break is before you touch anything.
Read the row of displays first
FSSB runs from the CNC's servo card into each amplifier's COP10B (upstream/master-side) connector and out of its COP10A (downstream/slave-side) connector to the next unit. An amplifier with a live link shows ‘L’; one without shows ‘U’. So walk the amplifier row from the CNC side and note where L turns to U:
- L, L, U, U… → the break is between the last ‘L’ unit and the FIRST ‘U’ unit — the optical cable connecting them, or one of the two amplifiers it plugs into.
- U on the very first unit → the break is between the CNC and the first amplifier: that first optical cable or the CNC's servo card/axis card.
- All units suddenly U after cabinet work → someone disturbed an optical connector — check whichever cable was near the work first.
Step 1 — The optical cable at the break point
- Power OFF. Unplug the COP10B cable at the first ‘U’ unit and its other end at the last ‘L’ unit (or CNC).
- Inspect before replacing: look into the connector ends — oil film, coolant, or dust on the optical faces kills the link; clean gently with a lint-free wipe and IPA. Check the cable run for tight bends (FSSB optical cable has a minimum bend radius — sharp kinks from cable-tying are a classic self-inflicted failure), crush damage, and a snapped latch letting the plug sit half-seated.
- Reseat firmly at both ends — the connector should click home — and retest. A large share of U alarms end here.
- Still U → replace the optical cable (Fanuc's step (a)). Cables are cheap and fail far more often than the electronics on either end — always the first replacement.
Step 2 — If a known-good cable doesn't fix it
Fanuc's replacement order continues along the link, nearest-to-CNC first:
- The first ‘U’ amplifier (the receiving end): its COP10B optical receiver module has failed — the most likely electronic culprit. Board-level repair or exchange unit.
- The upstream ‘L’ amplifier (the transmitting end): its COP10A transmitter can fail while its own link (via COP10B) still shows L — it receives fine but passes nothing on. This asymmetry confuses people: an amplifier showing a healthy L can still be the one breaking the chain for everything after it.
- The CNC's servo (axis) card — last suspect, mainly when the very first cable and first amplifier are proven good and U starts at unit 1.
Swap-tests before buying anything
- Cable swap: exchange the suspect optical cable with a known-good one from elsewhere in the chain — if the U moves with the cable, done.
- Bypass test: connect the last ‘L’ unit's COP10A directly to the unit after the first ‘U’ (skipping the suspect amplifier). If the downstream units come up, the skipped amplifier is the fault. (Axis count will mismatch — expect SV5136 — but the link-level result is what you are testing. Do not run the machine like this; it is a five-minute diagnostic only.)
Prevention notes
- Route optical cables away from oil drip paths and never cable-tie them tight — loose loops, generous radius.
- Keep dust caps on any unused optical ports.
- Intermittent U under vibration (alarm during cutting, fine at idle) → a marginal connector or micro-cracked fibre — replace the cable rather than living with resets; every FSSB dropout is a full feed-hold event on the machine.
Related guides
- Fanuc SVM blinking dash (control power supply)
- Fanuc αi drive alarms — PSM, SVM & SPM codes explained
- Fanuc servo drive alarm codes