FANUC SP9120, SP9121, and SP9122 mean that an error occurred in the serial communication data exchanged between the CNC and the spindle amplifier. The CNC displays SP9120, SP9121, or SP9122, while the spindle amplifier displays C0, C1, or C2 respectively.
FANUC groups all three codes under the same fault description and troubleshooting procedure. The official alarm-list order is to replace the communication cable between the CNC and spindle amplifier, then replace the spindle amplifier control printed-circuit board, and finally replace the CNC main board or sub CPU board associated with the serial spindle.
| CNC alarm | Spindle amplifier display | FANUC description | Official repair order |
|---|---|---|---|
| SP9120 | C0 | Communication data error | Cable → spindle amplifier control board → CNC board |
| SP9121 | C1 | Communication data error | Cable → spindle amplifier control board → CNC board |
| SP9122 | C2 | Communication data error | Cable → spindle amplifier control board → CNC board |
For nearby spindle and converter alarms, see the Complete FANUC Spindle Alarm Codes SP9110–SP9216.
What Do FANUC SP9120, SP9121, and SP9122 Mean?
The CNC and spindle amplifier continuously exchange command, status, and diagnostic data through the serial spindle communication link. If the received data fails the amplifier's communication checks, the spindle amplifier generates C0, C1, or C2 and the CNC reports the corresponding SP alarm.
FANUC's maintenance manual states one common meaning for the three codes:
An error occurred in serial communication data between the CNC and Spindle Amplifier.
These alarms identify the communication path between two devices:
- The CNC serial spindle interface
- The CNC-to-spindle communication cable and any machine-specific intermediate connection
- The spindle amplifier communication interface and control printed-circuit board
They are not motor-feedback, spindle-sensor, power-circuit, cooling, or mechanical-load alarms.
Are C0, C1, and C2 Different Faults?
FANUC lists C0, C1, and C2 as separate amplifier displays, but the cited maintenance manuals group them together and provide the same fault description and corrective actions. The manuals do not assign three different field-service causes to the individual codes.
| Display | CNC alarm | Published service meaning |
|---|---|---|
| C0 | SP9120 | CNC-to-spindle-amplifier serial communication data error |
| C1 | SP9121 | CNC-to-spindle-amplifier serial communication data error |
| C2 | SP9122 | CNC-to-spindle-amplifier serial communication data error |
Always record the exact code shown, but do not assume that C0 means “cable,” C1 means “amplifier,” and C2 means “CNC.” FANUC does not publish that one-to-one interpretation in the applicable troubleshooting procedure.
Communication Path Involved in These Alarms
The exact connector arrangement depends on the CNC series, spindle amplifier generation, number of serial spindles, and machine builder's wiring design.
On many FANUC serial-spindle systems:
- A CNC serial spindle connector or connector panel uses a designation such as JA7A.
- The spindle amplifier side uses a designation such as JA7B.
- Some configurations make a direct connection from another CNC connector, such as JA41.
- Systems with multiple serial spindles can use a connector panel or branch arrangement.
- Certain installations use an optical I/O link adapter and optical cable because of distance, grounding, or electrical-noise requirements.
These connector names are examples, not universal instructions. Use the connection manual and machine electrical drawing for the exact CNC and amplifier installed.
1. Confirm the CNC Alarm and Amplifier Display
Before replacing hardware, confirm both ends of the alarm:
- SP9120 on the CNC corresponds to C0 on the spindle amplifier.
- SP9121 on the CNC corresponds to C1 on the spindle amplifier.
- SP9122 on the CNC corresponds to C2 on the spindle amplifier.
- The alarm is assigned to the expected spindle number when the machine has multiple spindles.
- No additional power-supply, amplifier-group, control-power, or CNC hardware alarm is present.
Record:
- Complete CNC series and model
- CNC software series and edition when available
- Complete spindle amplifier model and every suffix
- Number of serial spindles installed
- Spindle number associated with the alarm
- CNC-side and amplifier-side connector positions
- All simultaneous CNC and amplifier alarms
- Whether the alarm occurs at startup, intermittently, or during operation
- Any recent cable, CNC board, spindle amplifier, or cabinet work
Photograph the alarm screens and physical connections before switching off the machine.
2. Determine When the Communication Error Started
The occurrence history helps prioritize the inspection.
| When the alarm appeared | Checks to prioritize |
|---|---|
| Immediately after CNC or amplifier replacement | Model compatibility, board seating, connector position, and cable reconnection |
| Immediately after cabinet maintenance | Loose, displaced, damaged, or incorrectly connected serial spindle cable |
| At every power-up on a previously operating machine | Failed cable, spindle amplifier control board, or CNC serial spindle interface |
| Intermittently during operation | Loose contact, vibration, cable damage, contamination, shielding, grounding, or electrical noise |
| When another high-power device switches | Cable routing, grounding, shield termination, and electromagnetic interference |
| Only one spindle alarms on a multi-spindle machine | The cable branch, amplifier, or CNC channel assigned to that spindle |
If the error appeared immediately after work was performed, inspect that work before assuming that an unrelated board failed at the same time.
3. Inspect the CNC-to-Spindle Communication Cable
The communication cable is the first item in FANUC's official alarm-list procedure. With machine power isolated and the specified DC-link discharge time observed, inspect the entire link from the CNC to the spindle amplifier.
Check for:
- A connector that is not fully seated or locked
- A cable connected to the wrong spindle channel
- Bent, recessed, contaminated, oxidized, or heat-damaged contacts
- Broken conductors near connector exits and strain reliefs
- A crushed, stretched, cut, sharply bent, or oil-soaked cable
- Loose connector shells or damaged locking hardware
- Nonstandard splices or earlier cable repairs
- Incorrect pin assignment in a fabricated replacement cable
- An intermediate connector panel or adapter that is loose or unpowered
- A fault that changes when the cable or connector is moved during an approved test
Label both ends before disconnecting the cable. On a multi-spindle machine, an apparently identical connector can belong to a different serial spindle channel.
4. Check Cable Routing, Shielding, and Grounding
A serial communication cable can be physically continuous but still carry corrupted data because of installation problems. Compare the actual installation with the FANUC connection manual and machine-builder drawing.
Confirm that:
- The communication cable is separated from spindle-motor and servo-motor power leads.
- It is not routed beside contactors, reactors, braking circuits, transformers, or other high-noise devices.
- The original shield termination and cabinet clamping arrangement are intact.
- CNC and amplifier cabinets have the specified protective bonding and grounding.
- Excess cable is not coiled beside power components.
- An optical adapter, if fitted, has power and its optical connectors are clean and fully inserted.
- Cable length and cable construction comply with the applicable FANUC specification.
Do not alter shields or connect them at arbitrary points. Restore the documented machine arrangement.
5. Replace the Communication Cable With a Confirmed Compatible Cable
If visual inspection does not identify the fault, FANUC's alarm-list action is to replace the communication cable between the CNC and spindle amplifier.
The replacement must match:
- CNC-side connector type
- Spindle-amplifier-side connector type
- Pin-to-pin assignment
- Cable construction and twisted-pair arrangement
- Shielding and shield termination
- Length limit
- Intermediate connector-panel or optical-adapter configuration
- Machine-specific routing requirements
Do not select the replacement solely because the connectors fit. A cable with a different pin assignment, shield arrangement, or conductor specification can preserve the alarm or damage the communication interface.
When a known-good cable is available, change only the cable and retest. Replacing one item at a time preserves diagnostic value.
6. Check the Spindle Amplifier Control Printed-Circuit Board
If the correct known-good cable and installation do not clear C0, C1, or C2, FANUC identifies the spindle amplifier as the next possible fault location.
Before replacement:
- Verify the complete spindle amplifier model and suffix.
- Check that the control printed-circuit board is fully seated where the design permits inspection.
- Inspect the serial spindle connector for mechanical damage or contamination.
- Check whether the alarm began after a control-board replacement.
- Confirm that the installed control board and spindle software are compatible with the amplifier.
- Record any other amplifier status or self-test alarm.
Depending on the exact amplifier family and supported service procedure, replace either:
- The spindle amplifier control printed-circuit board, or
- The complete spindle amplifier module
Do not substitute a visually similar control board without verifying its complete FANUC specification, software support, and machine compatibility.
7. Check the CNC Serial Spindle Interface
If the communication cable and spindle amplifier are confirmed good, FANUC identifies the CNC as the remaining principal fault location.
The applicable component can be:
- The CNC main board
- A sub CPU board
- A serial spindle interface board or module
- A machine-specific connector panel or intermediate interface
Before replacing a CNC board:
- Confirm that the fault remains with a known-good compatible spindle amplifier and cable.
- Check the CNC-side connector for bent pins, contamination, or physical damage.
- Verify that a recently replaced board has the correct model, suffix, and supported software.
- Record or back up the machine data required by the CNC and machine-builder procedures.
- Identify whether the faulty serial spindle interface is integrated into the main board or located on a separate module.
Replacing a CNC main board can involve SRAM data, parameters, option configuration, PMC data, spindle settings, and machine-builder commissioning. Preserve the required backups before hardware replacement.
8. Verify Hardware and Software Compatibility
SP9120–SP9122 can appear after an incorrect replacement even when the cable is physically intact. Verify the complete configuration whenever the fault follows hardware work.
Check:
- Full CNC order specification
- CNC board or module part number and suffix
- CNC software series and edition
- Spindle amplifier model and every suffix
- Spindle software series and edition
- Number and assignment of serial spindles
- Machine-builder approved replacement or supersession information
- Connector arrangement for the installed CNC generation
Do not choose a spindle amplifier or CNC board only by appearance, capacity, or the first portion of its part number.
If a replacement is required, browse FANUC servo, spindle, and power drives or send REACO CNC the complete equipment labels for a compatibility check.
9. Retest the Serial Spindle Communication
After correcting the connection or replacing the confirmed faulty component:
- Restore every connector, shield clamp, protective cover, and ground connection.
- Confirm that the cable is connected to the correct spindle channel.
- Verify that no tools or loose conductors remain in the cabinet.
- Apply power according to the machine-builder procedure.
- Confirm that C0, C1, or C2 no longer appears on the spindle amplifier.
- Confirm that SP9120, SP9121, or SP9122 no longer appears on the CNC.
- Check every spindle and amplifier display for a new alarm.
- Run the spindle first under a controlled no-load test.
- Confirm spindle speed command, actual speed, orientation, and machine-specific spindle functions.
- If another alarm appears, diagnose the newly displayed code separately.
Difference Between SP9110 and SP9120–SP9122
Both alarm groups concern communication, but they refer to different paths.
| CNC alarm | Amplifier display | Communication path | Main components to inspect |
|---|---|---|---|
| SP9110 | b0 / B0 | Communication among the spindle amplifier, servo amplifier, and Power Supply | Inter-amplifier cable, connectors, and amplifier-group modules |
| SP9120 | C0 | Serial communication between the CNC and spindle amplifier | CNC-to-SP cable, spindle amplifier board, CNC serial spindle interface |
| SP9121 | C1 | Serial communication between the CNC and spindle amplifier | CNC-to-SP cable, spindle amplifier board, CNC serial spindle interface |
| SP9122 | C2 | Serial communication between the CNC and spindle amplifier | CNC-to-SP cable, spindle amplifier board, CNC serial spindle interface |
Do not use the CXA2 amplifier-group procedure for C0/C1/C2 unless the applicable machine drawing shows that it is part of the relevant path. The b0 alarm and C0–C2 alarms identify different communication links.
Difference Between SP9066 and SP9120–SP9122
| CNC alarm | Amplifier display | Meaning |
|---|---|---|
| SP9066 | 66 | Communication error between spindle amplifiers through the applicable inter-spindle link, commonly associated with JX4 |
| SP9120–SP9122 | C0, C1, or C2 | Serial communication data error between the CNC and spindle amplifier |
If the machine has multiple spindle amplifiers, identify whether the failed link is CNC-to-amplifier or amplifier-to-amplifier before replacing a cable.
FANUC SP9120–SP9122 Diagnostic Table
| Finding | Likely interpretation | Recommended action |
|---|---|---|
| CNC shows SP9120 and amplifier shows C0 | CNC-to-spindle serial data is abnormal | Follow the common C0/C1/C2 cable-board diagnostic order |
| CNC shows SP9121 and amplifier shows C1 | Same published communication path is abnormal | Record C1, then test the cable and interfaces |
| CNC shows SP9122 and amplifier shows C2 | Same published communication path is abnormal | Record C2, then test the cable and interfaces |
| Alarm appeared after cabinet work | Cable or connector may be loose, damaged, or misplaced | Compare every connection with the machine drawing |
| Alarm changes when the cable is moved | Cable conductor or connector contact may be intermittent | Replace the correct communication cable |
| Known-good cable clears the alarm | Original cable is defective | Install the confirmed compatible replacement and restore routing |
| Known-good cable does not clear the alarm | Spindle amplifier or CNC interface may be defective | Test the spindle amplifier control board before the CNC board |
| Known-good compatible spindle amplifier still alarms | CNC serial spindle interface becomes the primary suspect | Diagnose the main board, sub CPU, or serial spindle module |
| Alarm appeared after amplifier replacement | Compatibility or reconnection may be wrong | Verify full model, suffix, software, spindle assignment, and cable position |
| CNC reports SP9110/b0 instead | Fault is on the amplifier-group communication path | Follow the SP9110 procedure rather than the C0/C1/C2 procedure |
| CNC reports SP9066/66 instead | Fault is between spindle amplifiers | Inspect the inter-spindle communication path, commonly JX4 |
Recommended Troubleshooting Order
- Confirm SP9120/C0, SP9121/C1, or SP9122/C2 on both the CNC and spindle amplifier.
- Record every simultaneous alarm, module display, spindle number, model, suffix, and recent repair.
- Identify the exact CNC-to-spindle communication path from the machine electrical drawing.
- Isolate machine power and observe the required DC-link discharge time.
- Check that each connector is in the correct socket and fully locked.
- Inspect connector contacts, cable strain relief, intermediate panels, and adapters.
- Verify cable routing, shield termination, grounding, construction, and pin assignment.
- Replace the CNC-to-spindle communication cable with an exact compatible known-good cable.
- Retest before changing another component.
- If the alarm remains, inspect and replace the supported spindle amplifier control printed-circuit board or complete spindle amplifier.
- Retest again with the same confirmed cable.
- If the fault remains, diagnose and replace the CNC main board, sub CPU board, or serial spindle interface module specified for the system.
- Restore all machine data and complete the machine-builder commissioning procedure when a CNC board is replaced.
- Test all spindle functions and diagnose any newly displayed alarm separately.
What Not to Do With C0, C1, or C2
- Do not assign unsupported individual meanings to C0, C1, and C2.
- Do not replace the CNC main board before checking the communication cable.
- Do not replace the spindle motor, motor sensor, or spindle feedback cable for a CNC-to-amplifier communication alarm.
- Do not confuse SP9120–SP9122 with SP9110 amplifier-group communication.
- Do not confuse SP9120–SP9122 with SP9066 inter-spindle-amplifier communication.
- Do not disconnect or reconnect amplifier cables while power is applied.
- Do not work near the DC link until the specified discharge time has passed and zero voltage has been verified.
- Do not install a cable merely because its connector housing fits.
- Do not reroute the serial cable beside motor power leads or contactors.
- Do not replace a CNC board without preserving the required machine data.
- Do not change spindle parameters randomly to clear a physical communication fault.
Technical References
- FANUC Series 0i/0i Mate-MODEL D Maintenance Manual, B-64305EN/03, Alarm List: SP9120, SP9121, and SP9122
- FANUC AC Servo Motor αis/αi Series, AC Spindle Motor αi Series, Servo Amplifier αi Series Maintenance Manual, B-65285EN/04, Alarm Codes C0, C1, and C2
- FANUC AC Servo Motor βi-B/βi Series, AC Spindle Motor βi Series, Servo Amplifier βi-B Series Maintenance Manual, B-65425EN series, SP9120–SP9122
- Applicable FANUC CNC connection manual and machine-builder electrical documentation for the serial spindle cable, connector arrangement, and grounding requirements
Always use the manual edition that applies to the exact CNC, spindle amplifier, spindle software, connector arrangement, and machine.
Need Help With FANUC SP9120, SP9121, or SP9122?
REACO CNC provides independent testing, repair, compatibility checking, and replacement support for FANUC spindle amplifiers, spindle control boards, CNC main boards, sub CPU boards, serial spindle interface modules, and communication cables. Send us the CNC alarm screen, spindle amplifier display, complete CNC and amplifier models and suffixes, connector photos, cabinet photos, and details of when the communication alarm occurs.
Browse FANUC servo, spindle, and power drives, visit the FANUC Technical Support Center, or contact REACO CNC for assistance.
Reference Source: FANUC technical documentation. The connector names, cable specification, supported control board, CNC interface module, and replacement procedure depend on the installed CNC and spindle-amplifier generation.
