Fanuc SVM Alarm L (FSSB Communication Error, COP10A) — Causes, Diagnosis & Fixes

Alarm code L on a Fanuc servo amplifier is an FSSB communication error on the COP10A side — the amplifier's downstream optical link (the cable leaving its COP10A connector toward the next unit) has failed. It is the mirror twin of alarm U: ‘U’ means a unit cannot receive from upstream (COP10B side); the ‘L’ alarm case means a unit received fine but its transmit path onward (COP10A side) is broken. Same daisy chain, same physical break — diagnosed from the other end.

Reading the display row — and which unit the alarm names

FSSB enters each amplifier at COP10B (from the CNC side) and leaves at COP10A (toward the next amplifier). In the classic pattern L, L, U, U, the break sits between the last ‘L’ and the first ‘U’. The two alarm codes simply point at it from opposite sides:

• Alarm U work order (from the manual's Fig. 3.2.12): suspect the cable, then the first-U unit's receiver, then the last-L unit's transmitter.
• Alarm L work order (Fig. 3.2.13): the L-alarm attaches to the unit farthest from the CNC that still shows L — its COP10A transmit link is the failure. Fanuc's sequence: (a) replace that unit's COP10A optical cable (the one between the last L and first U — the same cable as in the U diagnosis), (b) replace the last-L amplifier itself (its COP10A transmitter has died), (c) replace the downstream amplifier on the other end of that cable.

In practice you rarely need to care which code led you there: the break is at the L→U boundary, and the cable joining them is always the first suspect.

Step 1 — The cable at the L→U boundary

1. Power OFF. Unplug the optical cable at the last-L unit's COP10A and at the first-U unit's COP10B.
2. Inspect and clean before replacing: oil film or dust on the optical faces, tight bends below the minimum radius (over-tight cable ties are the classic self-inflicted kill), crush damage in the loom, and broken latch clips leaving a plug half-seated. Clean with a lint-free wipe and IPA, reseat until it clicks, retest.
3. Still faulted → replace the cable. Optical cables fail far more often than the electronics on either end — cheap, fast, first.

Step 2 — If a known-good cable doesn't clear it: which amplifier?

1. The last-L amplifier (transmitter side) — Fanuc's next suspect for the L alarm. This is the counterintuitive one worth internalizing: the amplifier whose own display shows a healthy L can be the exact unit breaking the chain, because its COP10B receiver (which makes the L) and its COP10A transmitter (which feeds everyone downstream) are independent circuits. A failed transmit LED/driver takes down every unit after it while looking innocent itself.
2. The downstream amplifier (receiver side) — if the transmitter tests good, the first-U unit's COP10B receiver is the remaining suspect.
3. Swap logic settles it without spares guessing: move the suspect cable to a different position in the chain (does the fault follow?); or temporarily connect the last-L unit's COP10A to a different downstream amplifier's COP10B — if that unit links up, the transmitter is fine and the original receiver is bad. Five-minute tests, no purchases. (Axis-count alarms like SV5136 will appear during bypass tests — expected; link state is what you are checking. Diagnostic only, do not run the machine.)

Prevention — same rules as alarm U

• Generous loops, no tight ties, routed clear of oil drip paths; dust caps on unused optical ports.
• Intermittent FSSB dropouts under vibration (faults mid-cut, fine at idle) → replace the marginal cable; every dropout is a feed-hold on a machine mid-part.
• After any cabinet work near the amplifier row, reseat every optical connector that was within arm's reach — half-seated plugs from a bumped loom are the most common ‘mystery’ FSSB alarm.

Related guides

• Fanuc SVM alarm U (FSSB error, COP10B side)
• Fanuc SVM blinking dash (control power supply)
• Fanuc αi drive alarms — PSM, SVM & SPM codes explained