Resolving SINAMICS S210 Tech Alarm 108 After DRIVE-CLiQ Loss

1. Problem Overview

The SINAMICS S210 servo drive system — paired with a SIMOTICS S-1FK2 / S-1FT2 motor and controlled by a SIMATIC S7-1500 / ET 200SP through TIA Portal V17 or later — raises a non-acknowledgeable technology alarm when the DRIVE-CLiQ link between the drive and the encoder (or between two DRIVE-CLiQ nodes) is interrupted, even momentarily, while the controller is in RUN.

The user-visible symptom on a controller running firmware V5.2 on the S210 is:

• Alarm text: Tech Alarm 108 — Encoder initialization / DRIVE-CLiQ topology fault (exact wording depends on the technology object instance).
• The entry remains in the TIA Portal Alarm Display with state = active, not ack-able.
• Executing MC_Reset with Restart = TRUE returns Busy = FALSE, Done = FALSE, Error = TRUE (typically ErrorID = 16#8001 or a TO-specific state error) — the fault is not cleared.
• The axis cannot be re-enabled (MC_Power.Enable = TRUE sets Status = FALSE) until the encoder initialization sequence has been completed.

2. Affected Hardware, Firmware, and Software

3. Root Cause: Why the Alarm Cannot Be Acknowledged

DRIVE-CLiQ is a deterministic, ring-aware serial bus used inside SINAMICS drives to connect the Control Unit, the power module, the encoder, and (optionally) terminal modules and expansion modules. On the S210 platform, the DRIVE-CLiQ link between the converter and the SIMOTICS motor is implemented as a One Cable Connection (OCC) — power, encoder feedback, and electronic nameplate are all carried on the same hybrid cable.

When a DRIVE-CLiQ node is unplugged while the converter remains powered, the drive's internal component manager removes the lost device from the live topology. The technology object (TO) on the S7-1500 continues to receive process data frames (telegram 105), but the Sensor[n] block inside the S210 reports Communication OK = FALSE and State ≠ 7 (Encoder_Operation). This is the exact state the help file for Tech Alarm 108 points to.

Siemens' design philosophy on DRIVE-CLiQ is conservative for a reason:

1. Live topology changes can re-bind identifiers. A node that re-appears on a different port, or with a different component number, must not silently resume control of a running axis.
2. The motor's electronic nameplate is read once at first power-up. Encoder zero, commutation offset, and resolver calibration are established during initial DRIVE-CLiQ enumeration; they are not re-derived on the fly.
3. Acknowledgement is reserved for transient drive-side faults. Topology events (lost component, swapped component, invalid serial number) are intentionally non-clearable until the topology matches what was present at the last valid start-up.

That is why MC_Reset returns an error: the technology object refuses to leave the Encoder_Initialisation sub-state until the DRIVE-CLiQ component is re-detected and re-initialized. ACKnowledgement of a motor fault (F-class) is not the same as re-initialization of the encoder subsystem (A-class technology alarm).

4. Tech Alarm 108 vs. Drive Fault A01007

On a SINAMICS S120, the standard way to express the same condition is a drive-side fault code. On an S210 (which uses a unified alarm model inside TIA Portal), the visible event is Tech Alarm 108, but the underlying SINAMICS message ID is the same: F01007 / A01007 — DRIVE-CLiQ component POWER ON required.

Per the official Siemens knowledge base article ID 109771824 — A01007 SINAMICS S210 / SIMOTICS S-1F:

Cause: A DRIVE-CLiQ component must be switched on again (POWER ON).
Remedy: Switch off the power supply of the specified DRIVE-CLiQ component and switch it on.

This is not a suggestion — it is the documented Siemens response. Treat Tech Alarm 108 as the technology-object view of A01007.

5. Telegram 105 and the G1_STW Encoder Control Word

Siemens telegram 105 is the standard PROFIdrive / PROFINET telegram for a single-axis SINAMICS S210 under TIA Portal. Its PZD layout is:

The encoder control word G1_STW is what the S210 webserver shows in the second half of the diagnostic page. The bit that matters for the Tech 108 condition is:

Setting G1_STW.15 = 1 for at least one PROFIdrive cycle (typically ≥ 32 ms) tells the encoder channel in the S210 to clear its internal error latch and re-run the initialization state machine. This is the encoder-side equivalent of "MC_Reset".

6. Writing G1_STW from a SIMATIC PLC

G1_STW is not exposed as a separate TO_Speed.SetPoint… tag on the SINA_SPEED / SINA_POS technology object — it sits inside the raw PZD slot of the telegram, which is why the S210 webserver shows it under Control Words → G1_STW. To manipulate it from the PLC you have two options:

6.1 Option A — Direct tag-table write (recommended for one-time recovery)

1. In TIA Portal, open the watch table or the TO's I/O mapping.
2. Locate the output word mapped to PZD 4 of telegram 105. It will appear as a tag of the form <TO_name>.TO_Outputs[4] or a symbolically named DB tag such as "Axis_1".G1_STW if a data block was generated.
3. Force a value with Bit 15 = 1: e.g. W#16#8000 as the raw word, or a symbolic G1_STW.Ack := TRUE.
4. Hold the value for at least one PROFINET IRT cycle (≥ 1 ms in IRT, but Siemens recommends ≥ 32 ms to be safe).
5. Reset bit 15 to 0; re-run MC_Reset on the TO with Restart = TRUE.

6.2 Option B — MC_PostServo block (recommended for production code)

MC_PostServo from the MotionControl library accepts a free-form Output[0..5] array of WORD that is written to the telegram after the SINA_SPEED / SINA_POS output processing. The structure is:

// PLC code excerpt — SCL
"Axis_1".PostServo(    // MC_PostServo instance DB
    Execute := bTrigger,
    Output[0] := 0,             // STW1 still driven by MC_Power / MC_MoveVelocity
    Output[1] := 0,             // NSOLL_A
    Output[2] := 0,             // MOMRED
    Output[3] := W#16#8000,     // G1_STW low  — Bit 15 = Acknowledge
    Output[4] := 0,             // G1_STW high
    Output[5] := 0,             // A_DIGITAL
    Done => bDone,
    Busy => bBusy,
    Error => bErr,
    ErrorID => wErrId
);

Important: Output[0] and Output[3] are logical OR-merged with the values the technology object itself writes. Therefore you must keep the bits set by the TO (e.g. Output[3] from the S210 default map) and only add the 0x8000 bit. In practice, leaving Output[3] = W#16#8000 while the TO is in the encoder-error sub-state works because the TO writes zeros to G1_STW in that sub-state.

7. Step-by-Step Recovery Procedure

Use this decision flow whenever Tech Alarm 108 appears after a DRIVE-CLiQ event on an S210.

1. Confirm the alarm is the technology alarm, not a drive fault. In TIA Portal's Alarm Display, expand the entry. The cause text must contain "encoder" or "Sensor", and the alarm class must be TO-Alarm / Tech-Alarm, not Fault.
2. Inspect the S210 webserver. Navigate to https://<S210 IP>/ → Diagnostics → Control Words → G1_STW. If Bit 15 (Ack) is shown as the only missing handshake, proceed to step 3.
3. Inspect TO.AxisData.StatusSensor[n] from the TO-DB in the watch table. Confirm CommunicationOK = FALSE and State < 7.
4. Try software acknowledgment first. Use Option A or B from section 6 to pulse G1_STW.15. Verify that StatusSensor.CommunicationOK transitions to TRUE within 200 ms.
5. Run MC_Reset with Restart = TRUE. The TO should now accept the reset. Done = TRUE, StatusSensor.State = 7.
6. If step 4 fails (still CommunicationOK = FALSE or alarm persists), perform a controlled power cycle of the S210 (24 V logic + 400 V link).
7. After power-up, wait for the drive to reach Commissioning → Ready, then re-run the commissioning wizard if the topology is unchanged (it should be — only the cable was disturbed).
8. Re-enable the axis with MC_Power.Enable := TRUE and jog at low speed to confirm encoder zero and direction.

8. Power Cycle as the Last Resort

If the software ACK through G1_STW.15 does not clear the topology latch, the only guaranteed recovery is a full S210 power cycle. The procedure is:

1. Bring the axis to a safe state (stop any motion, clear MC_Power.Enable).
2. Open the main line contactor (or DC-link enable) to remove 400 V.
3. Remove the 24 V logic supply at connector X124.
4. Wait at least 30 seconds for the internal DC-link to discharge (verify with the status LED going dark).
5. Restore 24 V, then 400 V in the correct sequence per the S210 commissioning manual.
6. Wait for the drive to complete its DRIVE-CLiQ enumeration (≈ 5–10 s on a single-axis system).
7. Clear the residual MC_Reset alarm from the TO.

9. One Cable Connection (OCC) — A Common Field Trigger

The DRIVE-CLiQ link on the S210 motor side is implemented as a hybrid cable (Siemens OCC) carrying power, encoder, and electronic nameplate on a single M12 SPEEDCON connector. Field data shows that the most common trigger of Tech Alarm 108 in service is not a deliberate unplug, but a partial disconnect caused by:

• Vibration loosening the M12 SPEEDCON connector on the motor side.
• Pin damage in the hybrid cable (pins 1, 2, 4, 5 carry DRIVE-CLiQ; pins 3, A, B carry power).
• Water ingress at the cable gland on the motor junction box.

Before assuming a software-only path is sufficient, visually inspect and re-torque the OCC connector to the S210 manual's specification (hand-tight plus 1/4 turn on the SPEEDCON knurl). Re-terminate the cable if the pin block shows burn marks or oxidation.

10. Why MC_Reset Alone Fails

The MC_Reset function block on a SINAMICS S210 / S120 technology object maps to the drive's Acknowledgement mechanism. Internally, that is the rising edge of STW1.7 (Acknowledge fault). STW1.7 is recognized by the SINAMICS firmware only for faults whose fault class is configured to ACK_FM (acknowledgeable fault) in the expert list.

Topology errors such as A01007 / Tech 108 are configured as POWER_ON_REQUIRED in the drive firmware. They do not respond to STW1.7 and therefore do not respond to MC_Reset. The PROFIdrive profile (and the Siemens-specific extension) deliberately requires a hardware-side re-initialization for this class of error because the control-side state machine cannot prove that the encoder's commutation reference is still valid after the link was lost.

11. Webserver Diagnostic Sequence

The S210 webserver is a fast triage tool. Recommended order of inspection:

1. Home → confirm drive state is Commissioning or Ready, not Fault.
2. Diagnostics → Messages → read the active alarm; cross-reference the fault number with the S210 list manual.
3. Diagnostics → Control Words → inspect G1_STW and G1_ZSW. The Ack bit pattern tells you if the drive is waiting on a pulse from the PLC.
4. Diagnostics → Topology → check that the connected components (S210 itself + 1FK2/1FT2) match the commissioning baseline. A red X next to a component means a DRIVE-CLiQ node is missing.
5. Functions → Save / Restore → do not perform a factory reset. A factory reset will not clear A01007 and may erase the commissioning parameters, which is worse than the original problem.

12. Field-Validated Recovery Matrix

13. Escalation Path to Siemens Support

If software ACK and a power cycle both fail — usually a sign of a hardware defect in the S210's encoder interface, the OCC cable, or the motor's resolver track — escalate via a Siemens Support Request at support.industry.siemens.com. The minimum data set to attach is:

• S210 order number (MLFB) and serial number.
• Motor order number and serial number (must match the electronic nameplate).
• S210 firmware version (read from the webserver Identification page).
• TIA Portal project version and HSP version.
• A screenshot of the alarm (exact alarm text) and the topology page from the webserver.
• The TIA Portal watch-table capture of StatusSensor and G1_STW/G1_ZSW.
• The topology file saved from the S210 (Save / Restore → Save to memory card).

14. Prevention in New Installations

• Apply a cable tie strain relief on the OCC cable within 50 cm of the motor connector to absorb vibration.
• Set a 6-month preventive check on the SPEEDCON connector torque.
• For axes that can tolerate a brief stop, enable topology change detection in the expert list only with explicit Siemens consultation — it can mask legitimate fault conditions.
• Use the official Siemens HSCP (highly shielded hybrid cable) for cable runs > 10 m. Field failures of Tech 108 increase sharply on non-OCC-approved third-party hybrid cables longer than 15 m.

15. Quick Reference