Resolving SINAMICS S120 F07802 Fault on Basic Line Module Infeed

The SINAMICS S120 drive system reports fault F07802 when a Motor Module detects that the supplying Line Module (infeed) is not ready or has not been switched on. The fault is latching, surfaces on the bottom-left drive object of a multi-axis group after a hot process interruption, and reappears on every FN (Fault Acknowledge) reset until the infeed handshake chain is re-established. This reference walks through the parameter chain that gates the drive's ON command, explains why a 24 V control-power cut clears the latched state, and provides a verification sequence for the Basic Line Module (BLM) topology.

1. Problem Details: How F07802 Behaves in the Field

On a four-axis SINAMICS S120 cabinet (one BLM, four Motor Modules) the operators observed the following sequence:

• Bottom-left Motor Module refused to start while the three other axes were already running.
• The HMI / Starter fault buffer showed F07802 on the affected drive object.
• Pressing the FN button on the Control Unit (CU320-2) acknowledged the fault and cleared the buffer, but a new start command immediately regenerated F07802.
• Removing the 24 V control supply from the Control Unit, waiting for the capacitors to discharge, and re-applying control power cleared the condition without touching the line voltage or the motor cables.

This symptom pattern is diagnostic: the drive object is correctly receiving the ON command from the controller, but the infeed-ready gate is open. Cycling the control voltage forces the topology re-initialisation that resets the gate logic inside the CU320-2 firmware.

2. Root Cause: The Infeed-Ready Gate

F07802 belongs to the S120 Drive fault group and is documented in the SINAMICS S120/S150 List Manual under fault value 7802. The reaction is OFF2 (pulse cancellation via the Motor Module's gate driver) and the acknowledgement mode is IMMEDIATELY after the cause has been removed. Removing the cause is not the same as pressing FN; the cause lives in the infeed handshake chain.

For a Basic Line Module, the readiness path is:

1. The controller raises the infeed ON command at p0840 on the BLM drive object.
2. The BLM closes its pre-charge / line contactor and reports infeed running at

   r0863.0 (infeed in operation).

3. The Motor Module, in Safety / sequencing, monitors the infeed-ready bit at its own parameter r0864.0 (infeed ready), which is sourced from the inter-DO BICO interconnection BI: p0864 = r0863.0 of the BLM.
4. If the Motor Module does not see the infeed-ready feedback, the drive's own ON command path is blocked, and a start attempt raises F07802.

Resetting via FN clears the fault bit, but the gate driver will re-evaluate p0864 on the next pulse enable. If the BLM is still reporting not-ready at that point, the fault re-latches instantly. Only when the BLM has fully run up — including all DRIVE-CLiQ topology checks between CU and the BLM — does the feedback reach a stable '1'.

3. Drive Objects and Parameter Layout

The following table lists the parameters that participate in the S120 infeed-ready handshake. All parameter numbers refer to the SINAMICS S120 firmware family used with the CU320-2 Control Unit (V4.x / V5.x). Refer to the List Manual for the firmware revision installed in your cabinet.

4. Infeed Type Differences: Why BLM Behaves Differently from ALM/SLM

Three Line Module families are available in the S120 ecosystem. The infeed-ready handshake has subtle differences that matter when the fault is re-occurring.

On a BLM the infeed-ready path is essentially 'line contactor closed + DRIVE-CLiQ link to CU is healthy'. There is no active DC-bus regulator to misbehave, so a sticking fault almost always traces back to:

• DRIVE-CLiQ communication loss between the CU320-2 and the BLM (X200 / X201 port loose, fibre damage, EMI on copper variant).
• The 24 V supply at X124 of the BLM sagging under a hot thermal condition, causing the BLM's internal logic to drop r0863.0 while the contactor state remains valid in the CU image.
• Off / On / Off toggling of p0840 faster than the pre-charge time, which on BLM can take up to a few seconds at first start.

5. Step-by-Step: Restoring the Infeed-Ready Chain

Apply the following procedure from the SINAMICS S120 Commissioning Manual with STARTER and the SINAMICS S120 List Manual.

5.1 Prerequisites

• STARTER (≥ V5.4) or Startdrive (≥ V16) installed on the engineering PC.
• Online access to the S120 over PROFIBUS, PROFINET, or Ethernet — the project must match the installed firmware version.
• Read-only parameter access is sufficient for diagnosis; a download is not required to inspect the handshake bits.
• Operator lock-out / tag-out on the motor and line disconnect; the BLM main contactor will be exercised.

5.2 Inspect the BLM

1. Connect STARTER to the CU320-2 and go online.
2. Open Drive Navigator → Line Module and select the BLM drive object.
3. Read p0840[0] (infeed ON source). The expected BI source is normally a sequencer bit, the terminal input, or a higher-level PLC tag. Confirm it is '1' before continuing.
4. Read r0863.0 (infeed in operation).
   Case A: r0863.0 = 0 while p0840 = 1 — the BLM has not actually run up. Continue with step 5.3.
   Case B: r0863.0 = 1 but the Motor Module still raises F07802 — the BICO chain is broken. Continue with step 5.4.

5.3 Force-Run the BLM

1. Open the BLM expert list and locate p8540[0] (signal source for command ON / OFF1 extension) and p8480[0] (pre-charge control).
2. Temporarily set p970 = 1 to take the BLM into a manual commissioning mode if access is locked by the controller.
3. Read the BLM's own alarm buffer. Common alerts that prevent r0863.0 from going high:\li>
   • A06900 — line contactor does not pull in. Check 230 V / 24 V coil supply.
   • F07800 / F07801 — drive cannot start because DC link is not pre-charged. Check pre-charge resistors and fuses.
   • F30002 — DC link overvoltage on pre-charge. Check line quality.
4. Once the BLM starts cleanly, confirm r0863.0 = 1 and the DC-link voltage (r0070) is within the BLM's expected window (typically 540–570 V DC on a 400 V three-phase supply).

5.4 Verify the Motor Module BICO Source

1. Switch to the affected Motor Module DO.
2. Open p0864 in the expert list. The BI source must point to the BLM's r0863.0 (or ALM's r863.0 per the project).
3. If the BI is unset (value 0 or pointing to a now-deleted drive object), reconnect it:
   BI: p0864 = r0863.0 of the BLM DO.
4. Save to ROM (p0977 = 1) so the configuration survives a power cycle.

5.5 Power-Cycle the 24 V Control Section

If the parameter chain is correct but the fault is still sticky, exercise the procedure the operators used in the original incident. This is the field-proven recovery action when a BLM handshake is latched but all parameters are valid:

1. Open the cabinet and verify the DC-link has discharged below 50 V (BLM LED / DC-link test point).
2. Remove the 24 V supply at the CU320-2 (X124) and at any DMC20 / DME20 hubs in the DRIVE-CLiQ ring.
3. Wait a minimum of 30 seconds to allow DRIVE-CLiQ transceivers to fully reset.
4. Re-apply 24 V and observe the LED sequence on the CU: green flashing → green steady. Watch the BLM: orange flashing → green steady.
5. Re-issue the start command. The drive should now accept ON1.

The control-power cut is functionally equivalent to a warm reboot of the CU and the BLM. It clears the latched handshake state in the firmware's run-up state machine and forces a fresh DRIVE-CLiQ topology discovery. It is the correct recovery step when the BLM's pre-charge contactor logic has stalled, not when there is a true hardware fault.

6. Verification

After any of the above steps, perform the following confirmation sequence:

1. With the BLM running, read r0863.0 = 1 on the BLM.
2. On the Motor Module, read r0864.0 = 1 and confirm the BI source of p0864 matches the BLM's r0863.0 (right-click → 'Go to source' in STARTER).
3. Issue ON1 from the controller. The Motor Module should pull into pulses within < 50 ms; if it does not, check r0046.0 (control word effective) for a missing bit.
4. Run the motor under no-load for 30 s, then load it to the operating point. Monitor r0046 for any new warnings.
5. Cycle the start command five times in quick succession to confirm the BLM is not stalling during pre-charge. If F07802 reappears only on hot restart, increase p0347 (de-magnetising time) or the infeed's p0851 (pre-charge retry) according to the project.

7. Commissioning Notes and Common Pitfalls

• Topological addressing. DRIVE-CLiQ port order on the CU320-2 determines the drive object order. If a BLM is moved from X200 to X201 after a download, the DO numbering shifts and p0864 BI sources may point to a now-invalid drive object. This is the most common cause of 'phantom' F07802 after a configuration change.
• Hub behaviour. When DMC20 or DME20 DRIVE-CLiQ hubs are used, the BLM and Motor Modules must all be on the same hub segment for the r0863.0 signal to be visible to the Motor Module. Cross-segment signals do not propagate.
• Hot restart. A hot Motor Module may force a brief power dip on the DC link at the moment of the restart. The BLM's undervoltage monitoring (p0279 family) can momentarily drop r0863.0, and the Motor Module will raise F07802. The fix is to verify the BLM's undervoltage threshold and to allow a longer pre-charge time.
• Firmware variants. On older S120 firmware (V4.4 and below), the infeed-ready feedback may be sourced from r863.0 on the BLM rather than r0863.0. Always check the actual BI source in the project before assuming which parameter the engineer's notes reference.
• Safety Integrated. When Safety Integrated (STO) is wired, the STO gate takes priority over the infeed-ready gate. If STO is missing, the symptom is an OFF2 through a different alarm (F01611 for missing STO) rather than F07802. Always clear the safety chain first.

8. Related Faults and Diagnostic Map

9. Documentation and Learning Path

For a structured approach to the S120 platform the following Siemens support pages are the authoritative starting points:

• SINAMICS S120 Getting Started with STARTER — first commissioning workflow.
• SINAMICS S120 Commissioning Manual with STARTER — full parameter and function reference.
• SINAMICS S120 General Information — system overview and topology.
• SINAMICS S120 Equipment Manual — hardware description for BLM, SLM, ALM and Motor Modules.

Read-only access in STARTER is sufficient to inspect p0840, p0864, r0863.0 and the topology view, so parameter familiarity can be built up without write access.

10. FAQ