Fixing SINAMICS S120 Motor Module 840D sl Topology Error After Swap

Replacing a single motor module on a SINUMERIK 840D sl with an order number that "looks the same" on the rating plate is one of the most common sources of persistent topology and Safety Integrated faults on a SINAMICS S120 line-up. The original module 6SL3121-1TE21-8AA0 and the physically similar replacement 6SL3120-1TE21-8AA3 are not functional equivalents: the two MLFBs differ in cooling method, firmware baseline, and — most importantly — in their built-in safety functionality. When the two are mixed on the same DRIVE-CLiQ topology, the NCK raises a component mismatch alarm and the drive will not be commissioned, even though both modules have the same 18 A current rating.

This reference walks through the root cause, the exact part-number differences, the supported repair paths, and the STARTER/HMI commissioning procedure that lets you either recover an old machine that no longer has its original project, or migrate cleanly to a successor module. The procedure is aligned with the SINAMICS S120/S150 Diagnostics Manual (DAsl_0108_en) and the standard SINUMERIK 840D sl commissioning flow.

1. Problem Statement

After a failure on an SINUMERIK 840D sl, the following sequence of events produces a non-recoverable commissioning state:

1. A 6SL3121-1TE21-8AA0 single motor module (Booksize, internal air cooling, no integrated safety) fails.
2. Because the original MLFB is end-of-life or out of stock, the maintenance team installs a 6SL3120-1TE21-8AA3 from spares.
3. On the next NCK ramp-up, the controller detects the topology mismatch, raises an alarm, and refuses to bring the axis into operation.
4. The original STEP 7 / SIMOTION project (with the original drive topology) is not on site, so the only NCK reference data the controller has is the topology it has learned by serial number — and that serial number no longer exists on the new module.

The reported symptom is typically a combination of the following alarms on the HMI:

2. Root Cause: Why 6SL3121-1TE21-8AA0 and 6SL3120-1TE21-8AA3 Are Not Drop-In Replacements

Siemens splits the S120 single motor module (Booksize) family across two MLFB stems: 6SL3121-1TExx-xAAx and 6SL3120-1TExx-xAAx. The digits are not random — they encode the cooling concept, the firmware scope, and the safety scope. The 1-AA0/1-AA3 suffixes are the safety flag.

Three technical gaps prevent a direct swap:

1. Cooling airflow: An internal-cooled module installed in a cabinet that was designed for cold-plate dissipation will derate or trip on overtemperature. The fan mounting pattern is also different, so the cabinet cut-out is not compatible.
2. Safety scope: The new module exposes additional telegram bits (PROFIsafe slot, F-DI/F-DO handling). The old target topology in the NCK has no safety configuration for this drive object. The drive raises F16400 / F16401 and stays in commissioning inhibit.
3. MLFB comparison in topology: The actual topology is built from each module's electronic rating plate. When the NCK compares actual vs. target, MLFBs that differ even in a single digit are treated as a different component, and the serial number learned for the old unit is no longer present.

This is the diagnostic that resolves the alarm. The fix is either (a) install a like-for-like module with no integrated safety, or (b) accept the safety-enabled module and re-engineer the slot, the safety message frame, and the F-runtime configuration.

3. Identification: Reading the Module's Electronic Rating Plate

Before you do anything on the SINUMERIK side, confirm exactly what is in the cabinet. With the controller powered, the actual topology is available on the HMI or in STARTER.

On the HMI (840D sl):

1. Commissioning → Drive system → Drives → <drive name> → Topology.
2. Open the slot. The status line shows the actual MLFB, the firmware version, the serial number, and the safety scope of the connected component.

In STARTER / SIMOTION Scout (online):

1. Connect to the target device (PROFIBUS or PROFINET).
2. Open the project view → Drive unit → right-click → Topology.
3. Double-click the slot. The dialog reports MLFB, hardware revision, safety ID, and serial number.

Cross-check the order number physically printed on the module's nameplate. The fault will reference the discrepancy between this nameplate and the value stored in the project.

4. Engineering the Slot: Two Supported Repair Paths

4.1 Path A — Replace with the correct, no-safety equivalent

The successor of the 6SL3121-1TE21-8AA0 in the no-safety Booksize line is the 6SL3121-1TE21-8AA3 (note the leading 21- not 20-). This MLFB has the same current rating (18 A), the same Booksize footprint, and the same external-cooling expectation as the failed unit, and it carries the same firmware train that supports the original safety scope. This is the path of least risk.

The 6SL3121-1TE21-8AA0 is end-of-life, but the 6SL3121-1TE21-8AA3 has the same electrical data sheet and a newer firmware release. From the NCK's perspective, this is a component of the same type — no further project work is required.

4.2 Path B — Re-engineer the slot for the safety-enabled module

If the cabinet on hand is only 6SL3120-1TE21-8AA3 and a no-safety equivalent is not deliverable, the slot must be migrated. This is the path that the original question implicitly asks about — and it is the path that cannot be done by simply rewriting the order number. The change crosses a functional boundary (drive with safety vs. drive without safety), and the SINUMERIK side has to be informed.

Steps on the SINUMERIK side:

1. Set the safety operating mode in the drive. The new module's safety functions are configured via p9501 (SI Motion enable) and the F-runtime group; the old target had these set to disabled.
2. Add a PROFIsafe slot on the drive. Without a PROFIsafe host (F-CPU in the SINUMERIK's PLC or an external F-CPU) the safety functions cannot be exercised — the drive will still raise F16400 / F01600 at every ramp-up.
3. Update the SINUMERIK machine data that maps the drive to the safety slot. On 840D sl, the relevant MDs are MD36900 / MD36901 family (safety message frame assignment) and the F-parameterization in the PLC's safety program (SIMATIC F-FB library).
4. Re-archive the project. The machine now has a different drive configuration; the original archive is no longer valid for service use.

5. Recovering Without the Original Project

The second issue raised in the source — no source program on site — is common on machines that have outlived their documentation set. The recovery path uses the NCK's own commissioning data and STARTER as a read-out tool.

1. Bring the controller up. The NCK will raise the same topology and component-mismatch alarms as above; clear them with Reset on the HMI only after the topology has been reassigned.
2. From the HMI: Commissioning → Drive system → Topology → Compare actual vs. target. If the only difference is the slot in question, accept the actual topology and let the NCK re-learn the slot.
3. Open STARTER on the engineering PC, connect to the NCK over PROFIBUS (PROFIBUS address of the 840D sl is typically 4, default). STARTER will read the actual drive parameters directly from the controller.
4. If the missing project was a SIMOTION-based configuration (as suggested by the presence of SIMOTION Scout 5.3 in the source), use Scout's Upload device to PG function. Scout reads the SIMOTION-side program image and rebuilds the project from it.

The new project will carry a warning that the Topology comparison level is set to topology, type, and serial number. Lower the comparison level to topology and type only for this slot so a future spare with a different serial number is accepted without alarms:

• Drive parameter: p9906 = 1 (component replacement without serial number binding), or
• STARTER path: Drive unit → Properties → Topology → Component comparison level → set to Type only for the affected slot.

6. STARTER Commissioning Procedure for the New Motor Module

STARTER is the canonical tool for SINAMICS parameter changes; SIMOTION Scout integrates it. Use the following sequence when the controller is up and the new module is recognized in the topology but not yet operational:

1. Open STARTER → Connect to target device → select the PROFIBUS/PROFINET node of the SINUMERIK.
2. After the project has been uploaded from the controller, expand Drive unit_xxx → Drives → SERVO_xxx (the drive object name in the topology).
3. Open Configuration → DDS / MDS and verify the motor ID. The motor data is unchanged, but the new motor module has a different electronic nameplate and the commissioning state may be First commissioning.
4. Run Automatic controller setting (right-click the drive → Commissioning → Automatic setting). This re-runs the motor identification, controller optimization, and basic safety parameterization.
5. Check the safety configuration: open Safety Integrated → Configuration. If the drive was previously in Safety without encoder and the new module supports Safety with encoder, choose the mode that matches the cabinet wiring.
6. Save the project to the PC and download back to the NCK. The controller will re-initialize the drive object on the next ramp-up.

7. Verification

After the re-commissioning, validate the system end-to-end before releasing the machine:

8. Safety Integrated Considerations Specific to the 6SL3120-1TE21-8AA3

If you are migrating a slot from a non-safety MoMo to a safety MoMo, the cabinet must support the new safety functions. The minimum hardware changes are:

• An F-DI cable from the safety relay / F-CPU to the new module's X41 connector (24 V, dual-channel).
• A PROFIsafe slot on the SINAMENS control unit, with the safety message frame configured (e.g., telegram 903 for SINUMERIK 840D sl).
• The PLC's safety program must be extended with the new drive's F-blocks. Without this, the safety functions cannot be exercised; the drive will report a deselection mismatch on every ramp-up.

Once the safety configuration is complete, the safety checksum on the SINAMICS side (p9700 / p9701) must be regenerated and accepted. A safety parameter change is not active until the CRC has been confirmed — a common cause of "everything looks right but the alarm is still there."

9. Common Pitfalls and How to Avoid Them

10. Summary of Action Items

1. Do not assume that the 6SL3120-1TE21-8AA3 is a drop-in replacement for the 6SL3121-1TE21-8AA0. The two differ in cooling, safety scope, and firmware baseline.
2. Read the actual electronic rating plate from the new module via the HMI or STARTER to confirm what is in the cabinet.
3. If the original non-safety equivalent is still available (6SL3121-1TE21-8AA3, leading digits 21-), use it. This is the lowest-risk path.
4. If a safety-enabled module is the only option, treat the swap as a functional change: update the safety configuration, the PROFIsafe slot, the message frame, and the PLC's F-blocks. Accept the safety CRC.
5. Recover the lost project by uploading the actual drive configuration from the NCK with STARTER or SIMOTION Scout. Archive the result immediately.
6. Verify with the procedure in Section 7 before releasing the machine to production.