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FSSB is the fibre optic bus the CNC uses to talk to every axis amplifier. When it fails, it is not one axis complaining: half the machine is left without servo. This particular number usually means the control expected to find more amplifiers than it can actually see on the bus.
For reference only: alarms change with the manufacturer, the control and the machine PLC. Check the full code in the official manual before doing anything. Legal notice.
Always if it affects FSSB communication or several axes.
Reset after reestablishing communication and correct ignition sequence.
Choose the manual matching your control series, software version and machine builder. A support portal is a starting point; it does not establish applicability to every model.
The amplifiers do not each hang off their own control cable: they are daisy-chained on a fibre optic link that leaves the CNC, enters the first amplifier, leaves it towards the second and so on until the chain closes. Every cycle, each axis's command and the feedback coming back travel over that fibre. At power-up the control runs a bus recognition: it counts how many devices answer, in what order they sit and what each of them is, and compares that against what its axis configuration parameters say should be there. If it finds fewer than expected, or a device does not answer, or the order does not match what is declared, it cannot assign each axis to its amplifier and aborts with a servo bus alarm. Here is the key to diagnosis: because the link is a chain, an amplifier that does not start up also cuts communication for everything behind it. That is why a supply failure in a single amplifier looks as though several are missing, and why the fault point is usually right before the first missing axis rather than spread out. In practice there are three causes: damaged fibre, bent past its minimum radius or badly inserted; an amplifier with no supply or failed; or an axis configuration that no longer matches the installed hardware, typically after an intervention or a parameter restore.
In order, from the fastest and cheapest check to the most expensive.
01Note the exact number and which axes are missing
The alarm number from the FSSB group and the list of axes that never come ready are your starting data. If consecutive axes in the chain order are missing, the problem is at the first missing one or in the link before it. If one isolated axis in the middle is missing, it is more likely to be that device.
02Ask what was done beforehand
Electrical work, an amplifier change, a parameter restore or a job inside the cabinet the day before explain most cases. An FSSB alarm appearing out of nowhere on a machine nobody has touched is far less common than one appearing the day after the cabinet was opened.
03Do a full power cycle, respecting the sequence
Power down by the procedure, wait for the time the builder specifies and power up. Bus recognition happens at start-up, so if one amplifier was slow to power up last time, a clean restart may sort it. If it comes back the same, it is a hard fault.
04Do not touch the fibre optic cables
FSSB fibre is not unplugged or repositioned with the equipment energised, and it is not handled without the cleanliness and care it needs. Bending it below its minimum radius ruins it permanently. If you suspect the fibre, it is a maintenance job with the machine locked out.
05Let maintenance read the amplifier LEDs
Every amplifier has status indicators showing whether it has supply, whether it sees the bus and whether it has detected its own fault. Reading them means opening the cabinet, so it is electrical maintenance work. It is by far the information that pinpoints the break in the chain fastest.
06If hardware was changed, review the axis configuration
Assigning axes to the bus is a machine-specific configuration. If an amplifier was replaced with one of a different size or axis count, or a backup from another machine was restored, the configuration may no longer match. It is corrected using the builder's documentation and with a backup taken first, by maintenance.
The break is at that amplifier or in the fibre reaching it: no supply, failed, or a broken link. Confirm it by looking at that amplifier's LEDs. The axes behind it have nothing wrong with them, they are simply cut off from the chain.
Fibre badly inserted, bent while trunking was repositioned, or a connector not fully home. It is confirmed by the timing and by inspecting the fibre routing. Whoever did the work corrects it, with the machine locked out.
Degraded fibre or a marginal connector, or an amplifier that is slow to power up. It is confirmed by the pattern: only at start-up, never while running. Do not let it slide: this kind of fault develops into a drop-out in the middle of a part.
Absolute respect for the fibre routing and bend radius during any work inside the cabinet, and note it on the job sheet. A parameter backup including the axis configuration, saved and dated. Cabinet cleaning and thermal control, because heat degrades both the modules and the optical connectors. Spares identified by exact amplifier model. And a photograph of the inside of the cabinet showing the original routing: when somebody has to reposition something in two years' time, that photo is worth more than the manual.
Fanuc detects FSSB optical communication problem between CNC and amplifiers. It is an alarm from the Servo / ejes area, and in the catalog it is listed as “FSSB servo bus error”. On the machine you notice it like this: Several axes do not become ready, Missing bus or amplifier alarm, It may appear after electrical intervention.
On the shop floor it almost always comes from one of these causes: Damaged FSSB optical fiber, Unpowered amplifier, Incorrect connection order, Defective servo module. Rule out the most likely one on your machine first, before touching parameters or swapping anything.
Reset after reestablishing communication and correct ignition sequence. Resetting without fixing the cause first only hides the problem: FSSB / SV5136 trips again and, in the meantime, you keep working with the machine in bad shape.
With the machine stopped and in a safe state, start with these checks: Note affected topology/axes, Check the amplifier LEDs, Do not disconnect the fibre with the equipment energised. That alone will tell you whether it is the program or whether you need to raise a maintenance job.
With FSSB / SV5136 active, avoid this: Do not bypass safety devices, Do not manipulate energized electrical cabinet, Do not continue producing if the alarm reappears. Skipping any of these points is what turns a minor stop into an expensive breakdown.
Always if it affects FSSB communication or several axes. If you are not sure how far it goes, stop the machine and report it: a repeat alarm always works out cheaper than a crash.
It is critical: there is a real risk of damage to the machine, the tool or a person. Leave the machine stopped, do not keep resetting it to see if it goes away, and call maintenance before producing again. Guidance reference. Always confirm the full text in the exact machine/control manual.
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