Troubleshooting Sinumerik 840D sl Fault F231835: Encoder Error

1. Overview of Fault 231835

Fault 231835 is a DRIVE-CLiQ communication fault on the SINUMERIK 840D sl and SINAMICS S120 platform that flags a cyclic data transfer error between the Control Unit (CU) and Encoder 1 of a connected drive object. The error is raised when the encoder and the CU stop exchanging cyclic telegrams in synchronism, which the drive firmware interprets as a loss of position feedback integrity. Because the closed-loop controller depends on deterministic, time-stamped encoder data, the fault response is immediate and may stop the axis via OFF2 on the infeed and an encoder-side reaction on the SERVO / VECTOR drive object.

The fault belongs to the family of DRIVE-CLiQ diagnostic messages and is documented in the SINAMICS S120/S150 List Manual and the SINUMERIK 840D sl Diagnostics Manual. It must not be confused with hardware-level encoder faults such as F31110, which indicate an SSI/EnDat wire breakage or an encoder-internal memory error rather than a transport-layer problem.

2. Affected Drive Objects and Reaction

The reaction field ENCODER (IASC/DCBRAKE, NONE) reflects the value of p0491 (Motor encoder fault response ENCODER). When p0491 is set to a value other than 0, the drive can apply IASC or DC-BRAKE procedures before pulse suppression. The reaction value is read at runtime from the configuration and is not fixed in firmware.

3. Decoding the Message Value (r0949 / r2124)

Every instance of F231835 is accompanied by a message value in r0949 (fault value) and r2124 (alarm value for the buffered HMI). The message value follows the encoding scheme:

0000yyxx hex
   |  |  |
   |  |  +-- xx = error cause (1 byte)
   |  +----- yy = component number (1 byte)
   +-------- always 0000

The component number yy refers to the DRIVE-CLiQ node as enumerated in the drive topology. Read the value via the HMI Diagnostics screen, STARTER / Startdrive, or directly from the CU by querying r0949 on the relevant drive object (DO).

4. Root Cause Analysis: Why the Cyclic Telegram Stops

DRIVE-CLiQ is a proprietary real-time Ethernet ring running at 100 Mbit/s with a deterministic cycle time tied to the current controller cycle. The cyclic telegram exchanged between the encoder module (SMC / SME / SMI) and the CU contains absolute position, commutation, temperature, and diagnostic data. Three failure modes correspond to the three causes in F231835:

1. 0x21 (cyclic telegram not received): The CU never sees a complete cyclic frame. Typical causes are a damaged cable, a disconnected Yamaichi connector, a broken wire inside the drag-chain section of the cable, EMI ingress from an unscreened routing path, or a defective encoder electronics stage.
2. 0x22 (timeout in the receive list): The CU receive list (slot assignment) times out because the node did not respond within the configured cycle. This points at a node that is alive electrically but has lost synchronism - often an encoder whose internal PLL is drifting, or a node experiencing a watchdog reset.
3. 0x40 (timeout in the send list): The CU itself fails to dispatch telegrams in time. This is a CU-side load problem (high utilization, parallel topology with many nodes, firmware bug) rather than a cable problem.

For practical troubleshooting, log the message value and classify the cause before swapping hardware. A cause of 0x40 is rarely solved by changing the encoder cable; a cause of 0x21 is rarely solved by a CU replacement.

5. Field-Proven Diagnostic Procedure

The following sequence has been validated in production Sinumerik 840D sl installations. Execute it before swapping any hardware.

1. Record topology. Use HMI Commissioning → Topology to dump the DRIVE-CLiQ wiring (DO number, node number, cable length, connector type). Compare against the as-built electrical drawing.
2. Read r0949 / r2124. Identify whether the cause is 0x21, 0x22, or 0x40. Take 8-10 samples over 24 hours to capture intermittent behavior.
3. Check r9976 / r9979. These provide system utilization and cycle time statistics. A cycle time slip >5% is a strong indicator of firmware/load-related DRIVE-CLiQ timing drift.
4. Inspect connectors. Yamaichi push-pull connectors on SMC/SME/SMI modules are a known failure point after thermal cycling. Disassemble, inspect pin contact resistance (<30 mΩ), and re-mate with a calibrated torque tool.
5. Validate shielding and grounding. DRIVE-CLiQ cables require 360° shield bonding at both ends with a defined ground reference on the cabinet backplane. Floating shield pigtails on the encoder side are a common source of intermittent 0x22 faults.
6. Check routing. DRIVE-CLiQ cables must maintain 200 mm clearance from VFD motor power cables and must not run parallel in the same duct for more than 1.5 m. A coupling factor of 0.05-0.15 is sufficient to corrupt cyclic data on marginal encoders.
7. Examine cable mechanics. Replace any cable showing jacket abrasion, kink marks, or pull-force beyond 50 N (fixed installation) / 20 N (flexible installation).
8. Capture a trace. Use STARTER / Startdrive Trace with r0565[0..7] (DRIVE-CLiQ diagnostic counters) sampled at 1 ms to record error events leading up to the fault.

6. Component Replacement Strategy

The official SINAMICS S120 List Manual recommends, in order, a POWER ON and replacement of the affected component. The field reality is that the encoder module (SMC/SME/SMI) is the dominant failure, followed by the motor-side Yamaichi connector and the cable assembly.

When a replacement is performed, document the original motor serial number, encoder type (EnDat 2.1 / 2.2, SSI, Resolver, TTL), and the DO number. After the swap, run an encoder calibration (p1990 = 1) and verify the commutation angle is within ±1.5° electrical of the stored value.

7. Encoder Signal Quality Degradation (Latent Fault)

Field experience with F231835 has shown that an encoder can be “alive” - reporting position and passing all commissioning tests - while exhibiting marginal A/B channel signal quality. Such encoders pass the standard commissioning self-test but fail under the tighter timing window of high-speed or thermally stressed operation. Symptoms include:

• Cause 0x22 with no clear pattern, but clustering around thermal cycles.
• Position actual value (r0483) showing ±1 count jitter at standstill.
• Velocity actual value (r0063) noise > 5% of rated speed at 50% mechanical load.
• Diagnostic LED on the encoder module (SMC/SME/SMI) flashing green with periodic red blink.

Such encoders cannot be reliably diagnosed without a lab-grade evaluation jig. Sending the motor to a certified repair vendor (for example, the motor OEM’s own repair center) with a written reference to F231835 0x22 is the field-proven resolution. Repair vendors will report degraded A/B signal amplitude and replace the encoder disc or photo-optical assembly.

8. Temporary Open-Loop Workaround

If the encoder cannot be repaired or replaced within a maintenance window, the drive can be operated in speed-controlled (open loop) mode by deactivating the encoder:

1. Open the relevant drive axis in STARTER / Startdrive.
2. Deactivate encoder 1 in the encoder configuration. The Sinumerik HMI commissioning view exposes this bit as P00145 (the HMI representation of the underlying SINAMICS configuration word that switches encoder 1 between active and inactive).
3. Switch the control mode of the axis from closed-loop to open-loop (frequency / voltage injection for induction motors, or sensorless field-oriented control for synchronous motors).
4. Save to non-volatile memory (p0971 = 1 on the DO) and POWER ON.

Once the repaired motor is reinstalled, restore the encoder activation and re-run the closed-loop commissioning (p1990). Always close the F231835 alarm buffer and document the workaround in the machine logbook.

9. Configuration of p0491 and p9916

p0491 - Motor encoder fault response ENCODER - selects what the drive does when an encoder-side fault (including F231835 on the encoder DO) is detected:

For axes where brake-stop is critical (vertical axes, spindle), set p0491 = 2. For horizontal axes, the default p0491 = 0 is acceptable.

p9916 - DRIVE-CLiQ data transfer error shutdown threshold slave - defines how many consecutive cyclic transfer errors may occur before the slave is declared failed. The default value is designed for typical installations. Lowering this value increases sensitivity (more aggressive faulting on marginal links); raising it extends tolerance but may mask intermittent issues. For long cable runs (> 50 m) in industrial environments, a small upward adjustment (10-20%) is acceptable if the fault is benign and reproducible.

10. Verification and Monitoring

After the corrective action, verify the fix with the following checklist:

1. Clear the fault buffer (p3981 = 1 on the affected DO) and POWER ON.
2. Run the axis through the full stroke at rated velocity and acceleration. Capture a STARTER trace of r0063 (velocity actual) and r0483 (position actual).
3. Monitor r0565[0..7] (DRIVE-CLiQ error counters) for at least 8 hours of production. A non-zero value after 8 hours indicates residual link issues.
4. Confirm r9976 (DRIVE-CLiQ utilization) is < 60% on the affected CU.
5. If the fault recurs with a new cause value (e.g., 0x40 after 0x21), escalate to a CU firmware bug check. Compare the firmware version against the published SINUMERIK Operate / SINAMICS S120 release notes; some firmware versions had F231835 0x40 root-caused in the changelog and fixed in a subsequent patch.

11. Related Faults and Diagnostic Patterns

F231835 is one of a cluster of DRIVE-CLiQ faults. Adjacent codes in the 2318xx range share the same diagnostic logic:

Fail-safe I/O modules on a SINAMICS-controlled system follow a related but separate diagnostic flow. The TIA Portal fail-safe modules documentation describes the diagnostics flow for ET 200pro and ET 200S fail-safe I/O: ET 200pro Fail-Safe I/O - Error Diagnostics.

Time-of-day clustering (night vs day), speed dependence, and ambient temperature dependence are the three dimensions that distinguish root causes:

• Time-of-day: thermal expansion, EMF pickup from a paralleling UPS, or HVAC cycling → marginal encoder, marginal cable.
• Speed dependence: lower speed → fewer CRC errors → cable or connector. Higher speed → firmware/determinism problem.
• Temperature dependence: oscillator drift → encoder module. The drive electronics are usually temperature-stabilized by the cabinet.

12. Field Commissioning Checklist

1. Verify the firmware version of the CU, NX, and all DRIVE-CLiQ slaves against the SINUMERIK Operate / SINAMICS release notes.
2. Check the rated cable lengths in the topology (p9910, p9911) and confirm p9912 matches the cable resistance value (for cables > 30 m).
3. Configure p0491 according to the axis safety class.
4. Configure p9916 to match the installation environment.
5. Run a 24-hour soak test on every drive object that triggered F231835 in the past 90 days.
6. Document the fault, the cause value, the replacement action, and the post-fix trace.