SINAMICS S120 SMI-10 Fault 31897: Resolving 5NA3 Replacement

Problem Overview

A field-issued SINAMICS S120 drive line-up reported fault F31897 on a CU320-2 control unit (firmware V2.3, order number 6SL3040-0MA00-0AA1) after an SMI-10 (Sensor Module Integrated) swap on a SIMOTICS 1FT6084-8AF71-4UA0 DRIVE-CLiQ servo motor. The set-point topology declared the motor; the actual topology declared only the SMI order number. The original SMI (6SL3055-0AA00-5NA0) was confirmed defective by hot-swapping it with a sibling axis. The replacement SMI (6SL3055-0AA00-5NA3) was new-old-stock and electrically intact, yet the drive still could not identify the motor. The fault remained. The drive line-up contained three Motor Modules and three identical 1FT6084 motors; only the replaced axis was affected.

System Hardware Identification

The motor article number 1FT6084-8AF71-4UA0 decodes as a SIMOTICS S 1FT6-frame synchronous servo with integrated DRIVE-CLiQ encoder and absolute multiturn position feedback, intended for high-dynamic positioning axes. The encoder on this frame is the resolver-based type that requires an SMI10 (not SMI20/SMI24).

DRIVE-CLiQ Topology and the Role of the SMI-10

The DRIVE-CLiQ ring on SINAMICS S120 is a deterministic point-to-point serial bus running at 100 Mbit/s with automatic node detection and electronic nameplate exchange. Each DRIVE-CLiQ node carries a unique identifier and, on motor-side SMI modules, the motor's complete electronic nameplate. The SMI-10 is the resolver-interface variant; it conditions the two-track resolver signal from the motor into a digital encoder word and reports it on DRIVE-CLiQ.

The DRIVE-CLiQ topology editor in STARTER compares the set topology (configured in the offline project) with the actual topology (what the CU scans at power-up). Any deviation—missing node, unknown node, swapped node, or wrong article number—produces a topology fault that latches until the configuration is corrected.

Fault 31897 Analysis

F31897 was reported in the STARTER fault buffer as the active fault on the affected axis. In the standard SINAMICS S120/S150 List Manual the F3xxxx band is the encoder-fault group; specific codes such as F31885 (encoder 1 position error), F31894 (encoder 1 configuration error), and F31895 (encoder 1 signal error) are documented. The exact code 31897 is not listed in the publicly indexed fault documentation, and the engineer on site could not retrieve it from any Siemens Knowledge Base article. This is a documented ambiguity:

• The fault may be a project-specific or commissioned-application code generated by the user's own message frame.
• The code may be a non-indexed internal fault number visible only in the raw fault buffer (parameter r0945[], r0947[], r0949[], r3120[], r3121[], r3122[], r3123[]).
• The display may be a misreport or a partially decoded number in STARTER; reading the raw value from r0945[0] directly with the online diagnostics is the authoritative source.

Root Cause: SMI Variant and Missing Electronic Nameplate

The root cause is not the SMI itself—it is the missing electronic nameplate inside the SMI. The DRIVE-CLiQ handshake between CU320-2 and a motor-side SMI performs the following steps during initialization:

1. CU queries the SMI for its article number and revision.
2. SMI returns its MLFB and hardware index.
3. CU queries the motor electronic nameplate (resolver pole pairs, encoder resolution, motor k_T, k_E, R, L_d, L_q, thermal limits, brake data).
4. If the SMI returns a valid nameplate whose motor matches the offline project, the topology resolves to the motor.
5. If the SMI returns no matching nameplate, the topology resolves to the SMI module itself, the motor remains unidentified, and the drive latches a topology/encoder fault.

The hot-swap test confirmed the motor-side cabling and Motor Module are healthy. Swapping the original 5NA0 SMI into the suspect axis made the motor appear; the new 5NA3 SMI on the same axis showed only its own order number. This isolates the problem to the SMI's internal data, not its connectors, not the motor, and not the Motor Module.

SMI-10 Variant Comparison: 5NA0 vs 5NA3

The critical row is the last: the new 5NA3 SMI cannot inherit the motor's nameplate data from the old 5NA0 SMI by any field tool. The Siemens commissioning toolset will not allow an SMI-to-SMI data transfer between mismatched hardware indices. The motor dataset must be obtained from one of three sources:

1. Siemens factory dataset, supplied via a service request quoting the original motor serial number.
2. The motor's own electronic nameplate, which is read out by STARTER via Load to commissioning tool → Save dataset only if the SMI is already commissioned. On a blank SMI this read returns no usable data.
3. The original Siemens Drive-ES commissioning archive (if available on site from the OEM's FAT/SAT files).

Solution Path A — CU320 Firmware Update to V2.6 SP2 Hotfix

Siemens phased out firmware V2.3 for the CU320-2 in the V2.6 lifecycle. V2.6 SP2 Hotfix contains bug fixes for topology detection, SMI variant handling, and DRIVE-CLiQ re-initialization. The recommended primary path is to update the CU320-2 firmware before any further SMI work.

1. Identify the current CU320-2 firmware version: parameter r0018 on the Control Unit. Confirm version is V2.3.x.
2. Download the SINAMICS V2.6 SP2 Hotfix service pack from Siemens Industry Online Support, search for CU320-2 firmware V2.6 SP2 Hotfix. Verify the firmware is compatible with the project; V2.6 firmware is signed and accepted by all V2.x bootloaders.
3. Transfer the firmware to a CF card (for CU320-2 DP/PN with CF slot) or via STARTER's online firmware update function. The CF card path is the safest field method.
4. Power-cycle the Control Unit. The new firmware loads automatically when the boot files are present on the CF card.
5. Re-run the topology comparison in STARTER. With V2.6 SP2 the 5NA3 SMI is fully recognized even when the nameplate is generic.

Solution Path B — Load Motor Electronic Nameplate into the New SMI

When the firmware update alone does not bring the motor online, or when the firmware cannot be updated due to project constraints, the motor nameplate must be programmed into the SMI-10 directly. The procedure requires STARTER (or SINAMICS Startdrive) and a valid motor dataset.

1. Open the offline project in STARTER. Navigate to the affected drive axis → Configuration → DRIVE-CLiQ wiring.
2. Right-click the SMI node. Select Load motor data set from SMI. On a blank 5NA3 this returns the generic placeholder, not the motor data.
3. Contact Siemens support through the regional Service Center or the Industry Online Support portal. Submit the motor serial number, the original SMI MLFB (6SL3055-0AA00-5NA0), and the motor article number (1FT6084-8AF71-4UA0). Request the corresponding motor dataset file (*.dnx or *.bin) for SMI-10 5NA3.
4. When the dataset arrives, copy it to the STARTER project directory under \Commissioning\Drives\<axis>\Data.
5. Right-click the SMI node again. Select Write motor data set to SMI. The dataset is transferred to the SMI's non-volatile memory and verified by checksum.
6. Power-cycle the drive. After the next boot the topology should resolve to the motor.

Commissioning Procedure After SMI Replacement

After either firmware update or nameplate load, run a full commissioning acceptance on the affected axis:

1. Topology verification: STARTER → Online → Topology. The actual topology must match the set topology for every node, article number, and firmware version.
2. Encoder calibration: Set p1990 = 1 (encoder adjustment, determine commutation offset). The motor must be able to rotate freely during this step; remove mechanical coupling if necessary.
3. Commutation check: p0431 (commutation angle offset) should be populated after p1990 = 1. Compare with the value saved in the project archive; a deviation > ±5° indicates the encoder was dismounted and remounted with a different resolver zero.
4. Identification run: Set p1910 = 1 (motor data identification, rotating). This regenerates the controller's motor model and confirms stator resistance, leakage inductance, and rotor time constant match the new nameplate.
5. Optimization run: Set p1960 = 1 (speed controller optimization, rotating). The speed controller K_p and T_n are re-tuned automatically.
6. Test motion profile: Execute a low-speed (< 100 rpm) jog in both directions. Check for smooth commutation. Then a high-speed (< 80% rated) jog with current monitoring.
7. Safety check: If Safety Integrated is configured (STO/SS1/SLS/SBC), re-validate all safety functions with the acceptance test. The Safety log is cleared on SMI replacement in some firmware versions.

Verification Parameters

If r0147[0] still shows the SMI MLFB (6SL3055-0AA00-5NA3) instead of the motor article number after the procedure, the electronic nameplate transfer did not complete; repeat Solution Path B with a verified 5NA3-specific dataset.

Preventive Measures and Spare Parts Strategy

The 5NA0 → 5NA3 transition was forced by parts obsolescence, not by an upgrade. The 1FT6 motor family was migrated across multiple SMI revisions during its production life; the SMI is a service-replaceable item but requires the dataset. The recommended spare-parts policy for sites with multiple 1FT6 axes is:

• Maintain one commissioned spare SMI per motor article number. The commissioned SMI is loaded with the correct dataset and stored offline; it can be hot-swapped in an emergency.
• Store the original commissioning archive (.dnx, .par, .zip) on a controlled file share. The archive contains the motor dataset and can be re-flashed to any compatible SMI in minutes.
• Standardize the firmware version across all CU320-2 units on site. A heterogeneous V2.3 / V2.5 / V2.6 environment complicates SMI handling and increases the risk of 5NA3 incompatibilities.
• Document the SMI hardware index in the asset register. A site's spare SMI bin should not contain mixed 5NA0 and 5NA3 stocks without matching datasets.

Related Faults and Diagnostic Matrix

Recommended Action Plan Summary

1. Capture the offline project archive before any firmware or dataset change.
2. Update CU320-2 firmware from V2.3 to V2.6 SP2 Hotfix. This is the primary, recommended action and is the cleanest path out of the support-phased V2.3.
3. If firmware update alone does not resolve the topology, request the motor dataset for 5NA3 from Siemens support, using the motor serial number and the original 5NA0 MLFB as references.
4. Write the 5NA3-specific dataset to the SMI using STARTER's Write motor data set to SMI function.
5. Re-commission the axis end-to-end: topology → encoder calibration → motor identification → speed optimization → safety re-validation.
6. Update site documentation and the spare-parts register so that future SMI replacements are matched to a pre-loaded dataset.