FANUC PS0202 Alarm: Position LSI Overflow During Rigid Tapping

FANUC PS0202 means that the spindle distribution amount calculated during rigid tapping exceeded the internal limit of the CNC. FANUC alarm lists describe this condition as “spindle distribution value is too large” and identify it as a system error.

On applicable FANUC 0i-F machining-center systems, spindle distribution can be viewed in diagnosis data No. 451. The documented maximum for a serial spindle is 32,767 pulses per 8 ms. The distribution amount changes with the rigid-tapping S command and the position-coder or rigid-tapping gear-ratio settings. If the calculated amount exceeds the applicable limit, the CNC generates PS0202.

The first practical action is to reduce the commanded spindle speed and confirm the program. If the alarm remains at a reasonable speed, check the applicable gear-ratio and detector settings. On an analog-spindle configuration using differential encoder signals, also verify the A, /A, B, and /B feedback wiring. These conductors may be marked A, *A, B, *B or with overbars on the machine diagram.

For the preceding rigid-tapping alarms, see the FANUC PS0200 Illegal S Code Command guide and FANUC PS0201 Feedrate Not Found in Rigid Tap guide. For nearby CNC alarm numbers, see the FANUC CNC Alarm Codes 0201–0400.

Safety Warning Before Rigid-Tapping Tests

Rigid tapping synchronizes spindle rotation with the tapping-axis feed. Incorrect spindle speed, detector data, gear ratios, or feedback wiring can cause loss of synchronization, a broken tap, workpiece damage, or unexpected spindle and axis movement.

Do not change encoder conductors or CNC parameters with power applied. Isolate the machine according to the FANUC and machine-builder procedures, observe amplifier discharge time, and have electrical work performed by qualified personnel. Record and back up the original parameters before any change.

Test corrections under a controlled condition approved by the machine builder. Do not use a production workpiece as the first test, and do not assume that dry run, machine lock, feed hold, or override behaves normally during rigid tapping.

What Does “Position LSI Overflow” Mean?

In this alarm name, LSI refers to the large-scale integrated circuitry used to process position-related command data. “Overflow” means that the spindle distribution quantity required within one CNC processing interval exceeded the permitted internal numeric range.

This wording does not by itself prove that an LSI chip, CNC main board, spindle amplifier, motor, or encoder has failed. FANUC's PS0202 description identifies an excessive calculated distribution value. The cause can be a spindle-speed command that is too high for the active conversion, an incorrect detector or gear-ratio setting, or a configuration-specific feedback problem.

FANUC Series 0i-F machining-center documentation gives the following relationship:

• The distribution amount is displayed in diagnosis 451.

• For the cited serial-spindle configuration, the maximum is 32,767 pulses per 8 ms.

• The value changes according to the gear-ratio setting for the position coder or rigid-tapping command.

• A setting or command that exceeds the upper limit generates PS0202.

FANUC Series 500i-A alarm documentation states the corrective direction more simply: when PS0202 occurs because the spindle distribution value is too large, lower the spindle speed.

The precise processing interval and supported spindle configuration can differ by CNC series and software. Some FANUC manuals describe a 4 ms or 8 ms interval. Use the operator, parameter, and maintenance manuals for the exact control rather than applying one series' numerical limit to every machine.

Why Spindle Speed Affects PS0202

The CNC must convert spindle rotation into position-distribution pulses so that the tapping axis follows the spindle at the programmed thread lead. As spindle speed increases, the number of spindle pulses that must be distributed during each control interval also increases.

The general relationship is:

Distribution pulses per control interval = spindle speed × effective detector pulses per spindle revolution × control interval ÷ 60,000

This expression is useful for understanding the alarm, but the actual calculation can also include the position-coder gear ratio, spindle-to-detector relationship, and rigid-tapping conversion specified for the machine.

FANUC provides examples in which the maximum rigid-tapping speed depends on detector resolution and the processing interval. These examples are theoretical explanations of the internal limit, not recommended machine speeds. The allowable tapping speed must still comply with the spindle, tap, holder, workpiece, tapping axis, and machine-builder limits.

Difference Between PS0200 and PS0202

Both alarms may appear when the rigid-tapping S command is too high, but they represent different limits:

A spindle speed can therefore be within the programmed maximum-speed parameter yet still create an excessive internal distribution amount if the detector resolution or conversion ratio is incorrect. Conversely, an S command that simply exceeds the selected gear's permitted speed will normally be diagnosed as PS0200 on the cited systems.

1. Record the Alarm, Program, and Spindle Configuration

Before changing anything, record:

• The complete CNC series and model

• Whether the control is an M series or T series

• The exact alarm text and the block where PS0202 occurs

• The rigid-tapping command, such as G84 or G74

• The commanded S and F values

• The active spindle and gear range

• Whether the machine uses a FANUC serial spindle, built-in spindle, analog spindle, or another machine-builder configuration

• The position-coder or spindle-detector type and pulses per revolution

• Whether the alarm began after encoder, spindle, belt, gear, parameter, CNC, or drive work

This information is essential because the same PS0202 message can appear on several FANUC generations, while the detector interface, processing interval, diagnosis availability, and parameter assignments differ.

2. Reduce the Rigid-Tapping Spindle Speed

Check the S command used for rigid tapping. Confirm that it is the intended value, is expressed in the correct unit, and is not being replaced by a macro variable or post-processor output.

Reduce S to a conservative value approved for the tap and machine, then recalculate the feedrate so that the required thread lead remains unchanged:

• In a common M-series G94 feed-per-minute mode: F = S × thread lead

• In a common M-series G95 feed-per-revolution mode: F = thread lead

Do not reduce S while leaving a G94 F value calculated for the old speed. That would change the programmed lead and can damage the tap.

If lowering S clears PS0202, the result supports a spindle-distribution limit rather than proving a feedback hardware fault. Do not raise the speed again until the active detector resolution, gear ratios, and machine limits have been confirmed.

3. Check Diagnosis 451 on Applicable FANUC Systems

On the FANUC systems covered by the cited 0i-D, 0i-F, 16i/18i, and 30i documentation, diagnosis 451 represents spindle distribution during rigid tapping in detection units.

Use diagnosis 451 to compare the behavior at different approved spindle-speed commands. Record the value instead of relying only on whether the alarm appears. A value that rises with S and approaches the documented limit supports the distribution-overflow diagnosis.

Diagnosis numbering and display availability can vary. Do not assume that No. 451 has the same definition on every control without checking the applicable maintenance manual. On some systems, additional rigid-tapping diagnosis data includes spindle position error, accumulated spindle distribution, and synchronization-related values, but these should be interpreted using the exact manual rather than copied from another series.

4. Verify Position-Coder and Rigid-Tapping Conversion Settings

FANUC states that the spindle distribution amount changes according to the gear-ratio setting for the position coder or rigid-tapping command. Check the applicable parameters against:

• The machine builder's original parameter list

• A known-good CNC backup from the same machine

• The exact position-coder or detector specification

• The actual spindle-to-detector mechanical ratio

• The active spindle gear range and its selection signals

• The FANUC parameter manual for the installed CNC series

Pay particular attention if PS0202 appeared after parameter initialization, memory restoration, CNC replacement, spindle replacement, encoder replacement, belt or gear work, or a retrofit. A value copied from a similar machine may be wrong even if the CNC series is the same.

Do not change a gear-ratio value merely until the alarm disappears. An incorrect ratio can make the tapping axis follow the wrong spindle distance and produce an incorrect thread lead or loss of synchronization.

5. Check A, /A, B, and /B Wiring Only on Applicable Analog-Spindle Systems

For a simulated or analog-spindle configuration that receives incremental encoder feedback as differential A and B phases, technical guidance for PS0202 recommends checking the spindle encoder wiring and correcting the A, *A, B, and *B connections where necessary.

These signals are commonly understood as two complementary differential pairs:

Verify the wiring against the exact CNC interface, spindle drive, encoder, and machine electrical diagram. Check for:

• A and /A reversed within the pair

• B and /B reversed within the pair

• A-phase and B-phase pairs interchanged

• Open, shorted, loose, or high-resistance conductors

• Bent or pushed-back connector pins

• Incorrect connector pin assignments after encoder or cable replacement

• Damaged shielding, grounding, or cable routing that affects the differential signals

Do not randomly swap conductors while repeatedly testing the machine. Reversing a complementary pair or interchanging A and B changes phase polarity or phase relationship and can change the detected rotation direction. The correct arrangement must come from the applicable connection diagram and should be verified with suitable test equipment by qualified personnel.

This A/B-phase procedure is not a universal step for a FANUC serial spindle. If the installed detector communicates through a serial interface, identify and diagnose that interface according to its own maintenance manual rather than looking for four analog encoder conductors that may not exist.

6. Inspect the Encoder Feedback Circuit When Evidence Points to It

If the parameters and speed command are correct but an applicable analog feedback signal is abnormal, inspect the complete feedback path:

• Encoder connector at the spindle

• Intermediate connectors and terminal points

• Feedback cable continuity and insulation

• Differential-pair integrity

• Shield termination and grounding

• Separation from motor power and contactor wiring

• Encoder power supply at the specified voltage

• Encoder output waveform, amplitude, complement, and phase relationship using the approved test method

Replace the cable or encoder only when the test results identify it as abnormal. PS0202 alone does not prove that either component has failed.

7. Confirm the Mechanical Detector Ratio

Where the position coder is driven by a belt or gear rather than directly coupled to the spindle, confirm that the actual mechanical ratio matches the CNC settings. Check whether a pulley, gear, spindle cartridge, motor, or encoder was changed before the alarm began.

A loose or slipping encoder belt is more likely to create unstable or incorrect feedback than a repeatable mathematical overflow, but it should still be inspected when the machine uses this arrangement. Verify belt condition, tension, pulley security, coupling condition, and detector rotation without changing the original timing or orientation.

8. Perform a Controlled Verification Test

After correcting the identified cause:

1. Restore power according to the approved procedure.

2. Confirm that the original parameters and wiring match the documented configuration.

3. Verify S, F, feed mode, thread lead, tap, holder, and active spindle gear.

4. Observe diagnosis 451 where applicable.

5. Begin at a conservative approved spindle speed.

6. Perform a controlled rigid-tapping test without risking a production workpiece.

7. Confirm that PS0202 no longer appears and that the actual thread lead and direction are correct.

8. Increase speed only within the verified machine, detector, tool, and distribution limits.

If the alarm remains even at low speed with confirmed settings and correct feedback, stop changing unrelated parameters. The CNC, machine-builder configuration, spindle interface, and applicable diagnostics require deeper inspection.

Do Not Replace the CNC or Spindle Amplifier Based on PS0202 Alone

The phrase “system error” in the FANUC alarm list does not establish that the CNC main board or spindle amplifier is defective. The official description concerns an excessive spindle distribution value, and current FANUC alarm guidance first instructs the user to lower spindle speed.

Hardware replacement should be considered only after the command, detector resolution, gear-ratio settings, applicable feedback signals, and machine configuration have been verified. If another spindle or amplifier alarm is also present, diagnose that exact number separately.

FANUC PS0202 Diagnostic Table

Recommended Troubleshooting Order

1. Confirm PS0202 and record the exact rigid-tapping block.

2. Identify the CNC series, spindle type, detector type, pulses per revolution, and active gear.

3. Check that the programmed S value is reasonable and has not been altered by a macro or post processor.

4. Lower S and recalculate F so the programmed thread lead remains unchanged.

5. View and record diagnosis 451 on applicable FANUC systems.

6. Compare the position-coder and rigid-tapping gear-ratio settings with the machine builder's data or a known-good backup.

7. Confirm that the actual mechanical spindle-to-detector ratio matches the settings.

8. For an applicable analog-spindle interface, verify A, /A, B, and /B wiring, connectors, waveform, shielding, and grounding.

9. For a serial spindle, follow the serial feedback diagnostics instead of applying analog wiring instructions.

10. Perform a controlled test at a conservative approved speed.

11. If PS0202 remains, collect the program, parameters, diagnosis values, wiring information, and related alarms for deeper technical analysis.

Technical References

• FANUC Series 0i-MODEL F Operator's Manual (Machining Center System), B-64604EN-2/01, rigid-tapping spindle distribution limit and diagnosis data No. 451

• FANUC Series 0i-MODEL F Operator's Manual (Lathe System), B-64604EN-1/01, PS0202 pulse-limit explanation and calculation examples

• FANUC Series 0i-MODEL D / 0i Mate-MODEL D User's Manual (Machining Center System), B-64304EN-2, rigid-tapping distribution amount and diagnosis No. 451

• FANUC Series 30i/31i/32i Maintenance Manual, B-63945EN series, diagnosis data related to rigid tapping

• FANUC Series 500i-A Maintenance Manual, B-64805EN/03, PS0202 alarm description and instruction to lower spindle speed

• Beijing FANUC technical guidance supplied for PS0202 position LSI overflow, including the analog-spindle A, *A, B, and *B encoder-wiring check

Always use the manual edition that applies to the exact CNC series, software, spindle interface, position detector, machine builder, and rigid-tapping configuration.

Need Help With FANUC PS0202 Alarm?

REACO CNC provides independent FANUC technical support, spindle-feedback diagnosis, drive testing, repair, and parts compatibility checking. Send us the alarm screen, complete CNC model, machine model, tapping program around the alarm block, S and F values, diagnosis 451 value, spindle type, detector model, parameter backup, and details of any recent wiring or mechanical work.

Visit the FANUC Technical Support Center for more FANUC alarm guides, or contact REACO CNC for assistance.

Reference Source: Beijing FANUC. This article is based on FANUC technical documentation and the supplied troubleshooting guidance. The correct diagnosis and repair procedure depends on the exact CNC, software, spindle, detector, wiring, parameters, and machine-builder specification.