Backing Up and Replacing Sinamics S120 Double Motor Modules

1. Overview

The Sinamics S120 drive system is a modular, book-size servo/VFD platform built around a central Control Unit (CU) and one or more Motor Modules connected over DRIVE-CLiQ. In a typical S120 cabinet, the CU handles the closed-loop control, communication to the higher-level PLC (PROFIBUS, PROFINET, EtherNet/IP via gateway), and the configuration of all connected Line Modules and Motor Modules. The Motor Modules themselves contain only the IGBT power stage, gate-driver board, and the DRIVE-CLiQ interface; they do not store project-level parameter data independently of the CU and the offline commissioning project.

This architecture is important when a Double Motor Module (DMM) such as the 6SL3120-2TE21-0AA4 fails in the field. The replacement procedure is fundamentally a parameter download exercise: the new module must be commissioned, told its topology position, and fed the motor, encoder, and closed-loop data that previously lived on the failed module. Most of the data, however, does not come from the failed module — it comes from the offline project in STARTER (or Startdrive in TIA Portal) or from a backup of the CU320's non-volatile memory.

This article covers the full workflow: identifying the correct replacement part, performing an offline and online backup, physically swapping the module, downloading the configuration to the new hardware, and verifying the result with LED diagnostics and STARTER's online view.

2. Critical Part Number Identification

Before any backup or replacement work, confirm the exact part number on the device nameplate of the failed unit. The two article numbers in the original incident (one legitimate, one transcribed in error) are a textbook example of why a nameplate check is non-negotiable.

To read the MLFB physically:

1. Power down the cabinet, observe the five safety rules, and apply lockout/tagout on the Line Module DC bus.
2. Wait for the DC link capacitors to discharge (the Motor Module's DC-LINK LED will extinguish; Siemens specifies a 5-minute minimum after removal of the 24 V and line voltage).
3. Locate the adhesive label on the top or front face of the module. The order number (MLFB) is printed as a 16-character string beginning with 6SL3....
4. Cross-check against the spare-parts list in the Sinamics S120 Equipment Manual (GH6) - Control Units and Supplementary System Components.

All three part numbers above carry the 2TE21 power-stage family code, which decodes to a 600 V class, 9 A continuous double-axis booksize module. The trailing two-digit suffix (-0AA4, -0AD0) typically indicates the hardware revision or a later release with updated firmware; in the S120 spare-parts system, newer suffixes are functionally drop-in replacements as long as the firmware is at the matching level (see Section 8).

3. Prerequisites

Before powering off the cabinet or starting the swap, confirm the following are available. Skipping any of these typically forces an emergency call-back to the OEM or a 24-48 hour machine down-time.

3.1 Software and Licensing

• STARTER V5.x (or SCOUT for Sinumerik-integrated cabinets) with the current Service Pack installed. Available from the STARTER commissioning tool downloads portal.
• Startdrive V15 or later in TIA Portal if the cabinet is configured through TIA rather than classic STARTER.
• The latest drive firmware for the CU320-2 from the Sinamics S120 firmware update package.
• The original project archive (*.zip or *.sdf) of this machine, ideally with an MD5 checksum.

3.2 Hardware

• Replacement Double Motor Module with verified MLFB.
• DRIVE-CLiQ cable of the correct length for both ports X200 and X201 (Siemens part numbers 6SL3060-4Axx-0AA0 family; never re-use a stretched or kinked cable).
• USB or Ethernet cable to PG/PC.
• 24 V bench supply for module verification (optional, recommended).
• ESD wrist strap, torque screwdriver (1.2 Nm for the power terminals on a 9 A DMM), and the cabinet key.

3.3 Documentation

• Wiring diagram (circuit diagram) of the affected axis, including encoder cable pinout.
• Motor data sheet — type plate data is mandatory, the electronic nameplate (via DRIVE-CLiQ) is preferred when available.
• Functional safety configuration if the axis is part of an STO/SS1/SLS safety chain (typically configured in Safety Integrated Extended functions via PROFIsafe on CU320-2).

4. Backing Up the Sinamics S120 Drive Configuration

There are three independent backups that should be created and stored on the PG/PC. Each catches a different failure mode.

4.1 Offline Project Backup (highest fidelity)

1. Open the machine's STARTER project.
2. Connect to the target CU320 via TCP/IP or PROFIBUS, and click Online > Connect to target system.
3. Right-click the drive unit (e.g., SERVO_01) and choose Target device > Load to PG/PC. This reads every parameter from the CU320's non-volatile RAM into the offline project.
4. Right-click the drive unit again, choose Target device > Copy RAM to ROM to make sure the CU's internal flash is consistent.
5. Save the project: Project > Archive. Choose *.zip with the checkbox With decompilation information ticked. Store the archive on a network share AND on a USB stick.

4.2 CU320 Card Image Backup (closest to hardware state)

The CU320-2 contains a CompactFlash (CF) card on its faceplate. The image on that card is what the system boots from. To capture it:

1. De-energize the CU (remove the 24 V supply).
2. Press the eject button and remove the CF card.
3. Insert the CF card into a PC with a CF reader.
4. Create a sector-image backup with a tool such as dd (Linux) or Win32DiskImager (Windows). Store the *.img file as the second backup medium.

4.3 Component Trace (firmware/device identity)

In STARTER, navigate to Drive unit > Commissioning > Component trace and select Save trace to file. This XML file records the serial number, MLFB, hardware version, and firmware version of every DRIVE-CLiQ node. The replacement procedure in Section 6 needs this data to confirm the new module's identity and detect a firmware mismatch before commissioning.

5. Physical Replacement of the Double Motor Module

The Double Motor Module is a hot-swap candidate only when the cabinet's 24 V control supply and the DC link are both off, and only after the five safety rules. DRIVE-CLiQ is NOT a hot-plug interface in the S120 booksize lineup, and hot-swapping power electronics will void the warranty and may destroy the Control Unit.

5.1 Lockout / Tagout and Discharge

1. Open the line contactor via the cabinet's main disconnect.
2. Wait for the Line Module's DC-LINK LED to extinguish (typically 5 minutes for a 9-200 A booksize Line Module; the manual specifies the exact value for the size in use).
3. Remove the 24 V DC supply to the CU320 and the Motor Modules.
4. Verify zero voltage at the DC link with a properly rated meter (typically 1000 V CAT III minimum).

5.2 Removal of the Failed Module

1. Label every DRIVE-CLiQ cable at both ends (X200 / X201 / X202 if present) with a numbered label that matches the wiring diagram.
2. Disconnect the DRIVE-CLiQ cables by pulling on the connector body, never on the cable.
3. Disconnect the motor power leads at the front terminals U2, V2, W2, U1, V1, W1 (the 1 suffix is axis 1 of the DMM, the 2 suffix is axis 2). Label them.
4. If encoder cables are landed on the motor side of the DMM (rare; most S120 cabinets land encoders on the SMC/SME module in the motor junction box), label and disconnect them.
5. Loosen the module's two top retaining screws and tilt the bottom out of the booksize rail. Lift the module clear.

5.3 Installation of the New Module

1. Inspect the new module's nameplate and confirm the MLFB matches the PO and the failed unit's footprint (mechanical 50 mm or 100 mm booksize slot).
2. Visually inspect the connector pins on the rear of the module; any bent pin must be reported to the vendor before installation.
3. Hang the new module on the booksize rail and engage the lower latch. Torque the top retaining screws to 1.8 Nm.
4. Reconnect motor power leads first, then encoder leads, then DRIVE-CLiQ cables. Match each cable to the original numbered label.

6. Commissioning the New Module and Parameter Download

There are two valid ways to push the configuration onto the new module. Both end at the same state, but they differ in operator risk and audit trail.

6.1 Method A: Project Download (preferred)

1. Re-apply 24 V to the cabinet. The CU320 will boot, scan the DRIVE-CLiQ topology, and report the new module's presence.
2. In STARTER, go online. The new Motor Module will be shown as a non-Siemens or component not commissioned state because it has no project identity yet.
3. Right-click the affected axis in STARTER and choose Target device > Download to target device — not Load to PG/PC. The wizard will offer to load the drive unit (CU320 + all Motor Modules) or only the selected axis. Choose axis only for the failed axis.
4. STARTER will run the consistency check and offer to load the configuration to the module's RAM and ROM. Accept the ROM save.

6.2 Method B: Direct Upload from the Surviving Twin

Because the failure is on a Double Motor Module, the second axis of the DMM is typically still functional. In STARTER, you can copy parameters from the surviving axis to the new one using:

Right-click axis 1 (surviving) > Expert list > select all
Edit > Copy
Right-click axis 2 (new) > Expert list > Paste
Target device > Copy RAM to ROM

This is faster but it does not preserve any axis-specific fine-tuning (e.g., friction compensation, torque offset, motor encoder offset). Use Method A for the first time and treat Method B as a field emergency workaround.

6.3 Downloading Through the CU320 vs. Direct Connection to the Module

A frequent question, and the answer is unambiguous: commissioning a Motor Module on a Sinamics S120 is always done through the CU320, never directly to the module. Motor Modules do not have their own Ethernet/PROFIBUS service interface. They expose their parameters over DRIVE-CLiQ, and DRIVE-CLiQ terminates at the CU. The only supported path is:

PG/PC —Ethernet/PROFIBUS→ CU320 —DRIVE-CLiQ→ Motor Module

If a user attempts to attach a service tool directly to a Motor Module's service port (X21 on some 1-axis modules, not present on the 6SL3120-2TE21-0AA4 booksize DMM), they will see no response, because that port is intended only for a temporary service PG connection on single-axis 6SL3120-1xxxxx units without a CU. In a multi-axis CU320-driven cabinet, all parameter access is mediated by the CU.

7. Direct Connection vs. Control Unit Connection

The user-facing question from the source incident was whether the Motor Module can be commissioned by direct connection. The distinction matters because some S120 Motor Modules in the older 6SL3320-1xxxxx (chassis format) and 6SL3120-1xxxxx (single-axis booksize) product lines do expose a service port for one-off commissioning. The Double Motor Module 6SL3120-2TE21-0AA4 does not. Concretely:

The DRIVE-CLiQ topology is a star from the CU to the next downstream device; if a Motor Module has further DRIVE-CLiQ ports, it acts as a hub for sensors (SMC, SME, TM) or further Motor Modules. Replacing a hub module means the downstream topology is preserved as long as the cables are reconnected to the same numbered ports. Topology errors are the single most common commissioning fault after a swap and are covered in Section 10.

8. Firmware Compatibility and Component Trace

The S120 system rejects a module whose firmware version is incompatible with the rest of the drive unit. The rules are documented in the Sinamics S120 firmware description:

1. All components in a drive unit must share the same Sinamics firmware major version (e.g., V5.2 SP3). The CU320 enforces this at boot.
2. Newer module firmware can be downgraded to match the CU using STARTER's Online & Diagnostics > Firmware update function. Older module firmware cannot be upgraded past the CU's level without first upgrading the CU.
3. If the replacement module is a newer hardware revision (e.g., -0AD0 vs. -0AA4), expect the module to ship with the latest firmware for that hardware. Downgrade as required.

To read the current firmware on the new module before commissioning:

STARTER > go online > Drive unit > Component trace
Look for: SINAMICS S120 DOUBLE MOTOR MODULE
  - MLFB: 6SL3120-2TE21-0AD0
  - Hardware version: e.g., A
  - Firmware version: e.g., 5.2.3
  - Serial number: ...

If the firmware version of the new module is not the same as the surviving module's, perform a controlled downgrade through STARTER before downloading the project. Skipping this step produces a boot-time F01340 / F08501 cascade that is hard to recover from in the field.

9. LED States and Diagnostic Verification

Every Sinamics S120 Motor Module has two LEDs on the front face: RDY (ready) and DC LINK. Interpretation during and after a replacement:

After the download, the RDY LED on the new module should transition from green-flashing to green-steady within 2-3 seconds of the project being accepted. A persistent red flashing at this stage usually indicates one of three things: (a) a wrong topology caused by crossed DRIVE-CLiQ cables, (b) a firmware-version mismatch with the CU, or (c) a missing safety-integrated configuration that has not been downloaded yet.

10. Troubleshooting Matrix

The matrix below maps the most common faults encountered after a Double Motor Module replacement to their root cause and field-proven fix. Fault codes follow Sinamics S120/S150 conventions; see the Sinamics S120/S150 List Manual for the authoritative list.

10.1 Field-Proven Diagnosis Sequence

When the new module does not come up cleanly, run this sequence in STARTER before calling Siemens support or the OEM. It resolves approximately 80% of post-replacement incidents:

1. Go online and read the Topology view. Compare the actual topology (right pane) with the target topology (left pane). Any port highlighted red is wrong.
2. Open Diagnostics > Buffer on the CU320. The newest entry in r0947 is the most recent fault. Read its value text, not just the number.
3. Open the Component trace and verify every node's MLFB, hardware, and firmware version match the as-built list.
4. If the topology is correct and the firmware is consistent, force a Copy RAM to ROM and a power cycle. Many intermittent faults clear on a clean boot.
5. Only if all of the above are clean, run the Commissioning > Automatic controller setting wizard to re-identify the motor and encoder.

10.2 Safety Acceptance After a Swap

If the axis is part of a Safety Integrated Extended (SINAMICS Safety Integrated) chain, the replacement is not complete until the F-CPU's acceptance test has been re-run and signed off. The acceptance test is project-specific and cannot be downloaded with the parameter set. Most safety authorities (TÜV, IFA) require the test report to be archived for the lifetime of the machine, and to be re-executed whenever a Safety-relevant component is changed. Plan a 30-60 minute acceptance window in the maintenance schedule; do not skip it.

11. Frequently Asked Questions