Replacing Siemens SMC30 on SINAMICS S120: F31885 and A1482

Overview: SMC30 Role in a SINAMICS S120 Topology

The SMC30 (Sensor Module Cabinet-Mounted 30) is a DRIVE-CLiQ-attached encoder evaluation module used inside the SINAMICS S120 drive family. It converts external encoder signals (TTL, HTL, SSI, or EnDat 2.1, depending on variant) into DRIVE-CLiQ frames and forwards them to a Control Unit such as the CU320-2 DP (6SL3040-1MA01-0AA0) or to a SIMOTION controller like the D445 via DRIVE-CLiQ.

Common catalog numbers in service:

• 6SL3055-0AA00-5CA0 – SMC30 base variant (TTL/HTL)
• 6SL3055-0AA00-5CA1 – SMC30 with SSI/EnDat support
• 6SL3055-0AA00-5CA2 – SMC30 with extended functionality
• 6SL3055-0AA00-5CA3 – SMC30 current variant (firmware ≥4.x compatible)

When an SMC30 is replaced, the Control Unit must re-detect the new module and re-bind it into the DRIVE-CLiQ topology. A failed replacement typically surfaces as F31885 (cyclic DRIVE-CLiQ error on Encoder 1), A1482 (topology comparison – sensor module missing), or a non-illuminated/orange RDY LED on the new SMC30, with downstream SIMOTION STOP events.

Problem Symptoms Reported During SMC30 Replacement

The failure pattern most often reported when swapping an SMC30 on a CU320-2 DP project is:

1. F31885 – Encoder 1 DRIVE-CLiQ: cyclic data transfer error (fault value/component 17/33).
2. A1482 – Topology: comparison, sensor module missing in actual topology.
3. RDY LED of the new SMC30 stays orange (not green) – module is not yet identified by the Control Unit.
4. RDY LED of CU320-2 flashes (boot error) – Control Unit has not completed its startup handshake.
5. SIMOTION D445 transitions to STOP mode within seconds of run command, raising a follow-on array index out of bounds in MCC chart regelung task ueberwachung.

These five symptoms are coupled: the topology and DRIVE-CLiQ faults prevent the encoder from being commissioned, which in turn forces SIMOTION to stop and traps the program in an array access violation in the supervisory task.

Root Cause Analysis

According to the Siemens Service Manual Software and Hardware (ID 109807746), an SMC30 with a hardware defect must be replaced by an identical module, meaning the same part number and, ideally, the same or newer compatible firmware release. The replacement procedure assumes the following preconditions:

• The defective SMC30 has been identified via the diagnostic buffer or LED state.
• The spare SMC30 is electrically and functionally identical.
• DRIVE-CLiQ wiring and 24 V supply to the module are intact.

When these preconditions are violated, the most common root causes are:

Pre-Replacement Verification (Before Touching the Module)

Before removing the suspected SMC30, perform these checks to confirm it is the actual point of failure rather than the upstream DRIVE-CLiQ port, cable, or motor-side encoder:

1. Inspect the DRIVE-CLiQ port on the Motor Module. A defective port on the Line/Motor Module side will mimic a defective SMC30. Visually inspect the RJ45 socket for bent pins, contamination, or oxidation.
2. Move the new SMC30 to a free port on the CU320-2. If it enumerates there, the original port on the Motor Module or its cable is suspect – not the SMC30 itself.
3. Measure 24 V DC at the SMC30 terminals (X520 pin 1/2 = 24 V, pin 3/4 = 0 V). A brown-out below 20.4 V causes F31885 with fault value 33.
4. Compare part numbers and firmware versions on the nameplate of the old and new module. The new module must support the same encoder protocol (TTL/HTL vs SSI/EnDat) and must be at or above the minimum firmware level required by the S120 firmware installed on the CU320-2.

Step-by-Step Replacement Procedure

1. Power down the drive line-up completely, including the 24 V DC electronics supply. Wait for the DC-link capacitors to discharge (≥5 min on S120).
2. Note the DRIVE-CLiQ port where the original SMC30 was connected and the wiring of X520 (encoder interface). Capture this from the STARTER / Startdrive online topology before powering down.
3. Remove the defective SMC30 by unlatching it from the DIN rail or backplane. Disconnect X520 first to avoid tension on the DRIVE-CLiQ port.
4. Install the identical replacement SMC30. Mechanical keying prevents incorrect mounting. Tighten control terminals to 0.6 Nm.
5. Reconnect the encoder cable at X520. For HTL/TTL encoders, wire pairs A/A\* and B/B\* using shielded twisted pair with shield bonded at the cabinet entry only.
6. Reconnect the DRIVE-CLiQ cable to the original port. Use only Siemens DRIVE-CLiQ cables (6SL3060-4Axx-0AA0 series); non-Siemens Cat5e cables are not qualified and cause intermittent F31885 faults.
7. Restore 24 V and 400/480 V line in the correct sequence. Power up the CU320-2 first, then the Line Module, then the Motor Modules.
8. Observe the SMC30 RDY LED sequence: red → red flashing (boot) → red/green alternating (firmware check) → green steady (operational). An orange state during this sequence is normal during a firmware download initiated by the CU.

Firmware Compatibility – The Most Common Hidden Cause

A new SMC30 typically ships with a newer firmware than the original. The CU320-2 will detect the version mismatch and may attempt an automatic downgrade or upgrade. During this process:

• Do not remove power or disconnect the DRIVE-CLiQ cable.
• Wait until the LED changes from a fast alternating red/green to a steady green.
• If the firmware download fails, the CU will raise F01360 (Topology: actual topology indicates an inadmissible firmware version) or A1482.

To force a controlled upgrade, use STARTER (or Startdrive in TIA Portal) under Online → Accessible nodes, identify the new SMC30, and execute Load firmware to drive object. The supported target version is dictated by the CU320-2 firmware; consult the S120 firmware release notes before loading.

Resolving the A1482 Topology Warning

Warning A1482 is raised when the actual DRIVE-CLiQ topology discovered by the CU320-2 does not match the offline project. The two paths forward are:

1. Accept the new topology: in STARTER, go to Drive unit → Topology → Compare with offline, tick the new SMC30, and click Accept actual topology. Save and download to the CF card of the CU320-2.
2. Reassign the existing topology: in TIA Portal or STARTER, change the offline part number or firmware version of the sensor module to match the new SMC30. This is the correct approach when the replacement is functionally identical but differs in hardware revision.

After either change, perform a POWER ON to the CU. The RDY LED should transition to steady green and A1482 should clear within 5 seconds of the new topology being accepted.

RDY LED Reference on CU320-2 and SMC30

SIMOTION D445 STOP Mode – Root Cause and Recovery

When the SINAMICS drive fails to commission, the SIMOTION D445 raises a STOP event with the following stack signature in the diagnostic buffer:

Incoming event → STOP reason 5/6
Task: timerinterrupttask_1 (ueberwachung)
Program: regelung, MCC chart line 727
Fault: read access to array outside array bounds

This is a follow-on fault, not the cause. The supervisory task ueberwachung is reading from an encoder status array that was never populated because the drive never reached Operation following the F31885. After resolving the DRIVE-CLiQ and topology issues:

1. Open the diagnostic buffer in SIMOTION SCOUT.
2. Double-click the entry to navigate directly to the offending MCC chart line.
3. Add a guard condition on the array index: IF (driveState[encIndex] = OPERATION) THEN ...
4. Download the project to the D445 and run a controlled restart.

Per Siemens guidance, a STOP reason of 5 indicates a write-out-of-bounds condition, while 6 indicates a read-out-of-bounds. The MCC chart shows the code at line 727; double-clicking the diagnostic entry opens the chart directly to that line for editing.

DRIVE-CLiQ Cabling and Port Best Practices

DRIVE-CLiQ is a deterministic, point-to-point serial protocol running at 100 Mbit/s. Cable length limits are 70 m between two DRIVE-CLiQ nodes when using 6SL3060-4Axx-0AA0 cables. Common wiring mistakes that produce F31885 fault value 33:

• Using a standard Cat5e patch cord instead of a Siemens DRIVE-CLiQ cable.
• Routing the DRIVE-CLiQ cable parallel to a motor cable for more than 1 m without a metal divider.
• Plugging the cable into the wrong port of the SMC30 or the Motor Module.
• Exceeding 20 m between a Motor Module and SMC30 on a long-axis configuration.

Verification After Replacement

1. Online diagnostic buffer in STARTER must show F31885 cleared and A1482 removed.
2. STARTER topology view must show the new SMC30 in green at the correct port, with the correct serial number and firmware version.
3. Encoder identification in Parameters → Sensor modules → SMC30 must show the expected encoder protocol (e.g., p0400[0] = 9999 for user-defined, or a specific SSI/EnDat selection).
4. SIMOTION online must show D445 in RUN with no array out-of-bounds entries in the diagnostic buffer.
5. Mechanical test: command a low-speed jog and verify actual speed, position feedback, and encoder count direction in the trace.

Frequently Asked Questions