FSSB, full name FANUC Serial Servo Bus, is basically the serial communication bus between a Fanuc CNC and the digital servo system. Think of it as a phone line running between the CNC and the servo system. The controller uses this line to pass data back and forth with each servo amplifier and motor. Whether servo initialization succeeds comes down largely to whether that phone line is working and whether the signal's clean.
* Something wrong with the fiber cable itself
This is one possible cause Fanuc's material spells out in black and white. FSSB fiber isn't as prone to a broken core as ordinary wire, but it's still delicate — it doesn't like being kinked, crushed, or yanked hard. What you commonly see in the shop: the outer jacket worn through at a cable track or a corner of the cabinet, someone accidentally folding the fiber into a sharp kink, a connector not pushed all the way in, a loose connector, a connector that got knocked, an elbow brushing the fiber during repair work, or the fiber ending up plugged into a slightly different spot than before during reassembly.
Worth flagging specifically: if the machine was fine before, just had maintenance or repair work done, and throws 5139 the very first time it's powered back up, odds are good you need to go back and check the fiber cable. But don't assume the fiber's dead the moment you see 5139 either — Fanuc only lists it as one of the suspects. The right approach is to look first, confirm, and only swap in a different fiber to test if you really have to — not run out for a new one the instant the alarm fires.
* Wrong wiring between the amplifier and other modules
Another important cause Fanuc mentions: an incorrect connection between the servo amplifier and other modules. This comes up all the time in actual repair work. Say a machine's been running fine, someone pulls the servo amplifier and swaps it for another, reconnects the wiring, and the moment it's powered up, 5139 hits. In that case, don't jump straight to "the new amplifier's bad" — the first thing worth doing is checking over whatever wires and connectors were just touched.
Similar situations tend to show up after repairing the cabinet, replacing the FSSB fiber, reinstalling the amplifier, cleaning inside the cabinet, tidying up a wire bundle, or swapping some other servo-related module. When checking, focus on: is the connector fully seated? Plugged into the wrong port? Is the fiber in the right spot? Does the current wiring match the machine's original configuration? Sometimes the problem is so simple it's almost annoying — a connector that missed being fully seated by half a millimeter.
