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How to Solve Fanuc 926 FSSB Alarm?

JAN. 20, 2026

In the Fanuc system, 926 FSSB Alarm is the kind of alarm that makes one's heart tight. Because it does not pick axis, do not pick working conditions, sometimes just power on the report, sometimes running suddenly stopped, the system directly pull the gate, all servo together lost connection. When many customers see 926 for the first time, their first reaction is often: “Is the system broken?” “Are all the servo amplifiers not communicating?” “Could this be a motherboard problem?”

To be honest, these guesses are not outrageous, but the 926 itself is not referring to a specific part is broken, it is really pointing to one thing: between the CNC and the servo amplifier, used to “talk” that the FSSB communication link has a problem.

First of all, let's make it clear: what is Fanuc's FSSB?

FSSB, full name Fanuc Serial Servo Bus, you can understand it as Fanuc servo system in the "high-speed private network”. All servo amplifiers and axis control boards are synchronized with the CNC in real time through this bus. Position, speed, enable, status, all run on this line. Once the FSSB communication instability, Fanuc's processing logic is very simple and very conservative: direct shutdown, do not give any "fluke run” opportunity.

So 926 alarm, the essence is not "an axis is bad”, but - the system has been unable to confirm that the servo system in a controllable, synchronized state.

Why does the 926 "look random” so often?

This is one of the most frustrating things about the 926.

Some machines are fine all day long, but when they are powered on the next day, they will be 926; some are fine when running under light load, but will jump when moving fast or linking multiple axes; others will restart a few times and be fine again, which makes people think that it is an "occasional occurrence”. But from the large number of cases we have actually dealt with, 926 is almost never unexplained, just that the problem is hidden deeper.

The most common is the FSSB fiber itself. Fiber has been pressed, bent, or the plug is not completely in place, seem to "still work”, but the signal quality has been degraded.Fanuc's FSSB signal integrity requirements are very high, a little bit of attenuation or loss of frames, is enough for the system to determine the communication abnormality.

The second high point, is a servo amplifier FSSB interface began to be unstable. This situation is particularly likely to "drag the whole system down”. Even if only one amplifier has a communication problem, the CNC will see the result: FSSB network abnormality → 926 → all stop.

There is also a problem with the Axis Card. Especially in a multi-axis system, if the status of a board is abnormal, the 926 is often the first "general alarm” that pops up.

As for interference and grounding, it can't be ignored. We have seen a lot of 926, the final root cause is poor grounding, or FSSB fiber and power lines too close to the long-term accumulation, communication stability is getting worse and worse.

What do we usually think when we judge 926 on site?

When we deal with 926 at Songwei, we will never "change the big parts” right away.

The logic is very simple: if 926 is a communication problem, then we will follow the communication link, one section at a time.

Let's start with the most easily overlooked, but most cost-effective place to look: Is the fiber optic plug loose? Has it been pulled? Has it been bent too far? Often times, the problem is clear enough when you rearrange the fiber optic alignment and replace it with a test fiber.

If the fiber to confirm that there is no problem, the beginning of the "isolation method”: the system of a servo amplifier temporarily disconnected to see if there is any change in the 926. If the alarm disappears after disconnecting one of the servo amplifiers, the direction is very clear.

The next step is to look at the amplifier itself, the axis card, or even the interface at the CNC side.

Repair, or replace? 926 It's realistic

If the problem is in the fiber optic or connector, it is almost ideal; if it is an anomaly in the FSSB interface of a single amplifier, repair or replacement will solve the problem completely; only in the case of damage to the axis card or the interface on the CNC side will there be a greater cost involved.

We have always emphasized that 926 is not a "take it easy” alarm, it is the system's way of telling you that it is not safe to communicate with the system.

Finally, say a very "field” conclusion

Fanuc 926 FSSB Alarm, the essence is not a servo control problem, but "servo can not communicate with each other properly. It will not be good by itself, and is rarely a "pure software problem”. As long as you start from the FSSB communication link, rather than focusing on a particular axis, a parameter to suspect, 926 can often be quickly and cleanly resolved.

If you're stuck on a 926 right now, and you're not sure if it's the fiber optics, the amplifier, or the axis card, the real time saver is to take the suspect part and verify it in a controlled environment, rather than repeat trial and error on the machine. That's what we've always done at Songwei.

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How to Solve Fanuc 926 FSSB Alarm?

Finally, say a very "field” conclusion

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