Resolving SINAMICS S120 F7800 Power Unit Missing Fault on CU320

Fault Overview

The F7800 – Power unit missing fault on a SINAMICS S120 drive line managed by a CU320-2 DP or CU320-2 PN Control Unit signals that the firmware has not detected an active DRIVE-CLiQ link to the expected Line Module (Smart Line Module, Active Line Module, Basic Line Module) or Motor Module within its configured topology. The fault is normally encountered when a previously commissioned system that has been in continuous service suddenly raises F7800 the first time 24 V electronics power is removed and re-applied. On some systems the fault can recur repeatedly on every cold start until the underlying defect is corrected.

F7800 is a STOP reaction II fault (pulse inhibit, OFF2 along the configured ramp), meaning the drive will not start in a controlled manner until the cause is removed and the fault buffer is acknowledged. The corresponding warning A7800 – Power unit missing appears in the same drive object just prior to the fault, giving a short window for diagnosis if the operator catches the warning state.

SINAMICS S120 Drive System Architecture

The SINAMICS S120 modular drive system is built from a Control Unit (CU320-2), one or more power units (Line Modules and Motor Modules), and any required expansion components such as Terminal Modules (TM), Sensor Modules (SM), and Hub Modules (DMC20 / DME20). All communication between the CU320-2 and the power units occurs over a proprietary DRIVE-CLiQ ring or star, depending on topology.

The CIM is the small plug-in board mounted on the front of every Booksize power unit. It carries the DRIVE-CLiQ transceiver, the 24 V electronics power supply interface for the power unit, and the CUD/CIM DAC ASIC that handles local handshaking. If the CIM is not fully seated, has thermal fatigue on the connector, or has a damaged DAC ASIC, the CU320 will lose communication with the power unit at the next power-up, even if the rest of the system is healthy.

F7800 Fault Definition and Trigger Conditions

F7800 is raised by the Control Unit when the expected power unit (configured by topology detection and saved in the project) does not respond within the DRIVE-CLiQ communication timeout window after the Control Unit boots. The List Manual categorizes the fault with the following attributes:

Typical trigger conditions observed in the field:

• 24 V supply to the CU320-2 is removed before the power unit has completed its shutdown sequence, leaving the CIM in an indeterminate state.
• DRIVE-CLiQ cable is marginal (intermittent contact, oxidation, or partially crushed) and fails on the first re-initialization.
• CIM DAC module on the Line Module has degraded and fails on cold power-up but recovers when warm.
• CU320-2 firmware boots faster than the downstream DRIVE-CLiQ nodes, causing a mismatch in the topology handshake.

Root Cause Matrix

DRIVE-CLiQ Topology and Cabling Verification

Before condemning any hardware, verify the DRIVE-CLiQ topology as detected by the CU320-2 matches the commissioned (project) topology. The DRIVE-CLiQ interface LEDs on every node provide a fast indication:

Verify each DRIVE-CLiQ cable is fully inserted and the connector's latch is locked. Cables that have been flexed during commissioning can develop partial opens at the crimp that pass continuity on a multimeter but fail at gigabit data rates. Siemens specifies a maximum DRIVE-CLiQ cable length of 70 m between two nodes, with a minimum bend radius of 5 × cable diameter. Always use genuine Siemens DRIVE-CLiQ cables (e.g., 6SL3060-4Axx-0AA0 family) – third-party RJ45 cables with RJ45 plugs and Cat5e cable will not meet the impedance and crosstalk budget.

CIM DAC Module Replacement Procedure

If the DRIVE-CLiQ cabling and topology are verified good, the next-highest-probability cause based on field reports is a degraded CIM (Control Interface Module), specifically the DAC ASIC which handshakes with the CU320 during power-up. The replacement procedure is as follows:

1. De-energize the cabinet. Wait 5 minutes after opening the DC bus shorting switches (DC link discharge LEDs must be off) for the DC link to discharge to safe voltage.
2. Disconnect the 24 V electronics supply to the Line/Motor Module.
3. Remove the upper DRIVE-CLiQ cables from the affected module to gain access to the CIM cover.
4. Release the four captive screws holding the CIM cover plate and slide the cover off, exposing the CIM printed circuit board.
5. Pull the CIM straight out using the integrated handle. The board is keyed and cannot be installed backwards.
6. Insert the replacement CIM (verify the spare part matches the module type and firmware generation).
7. Reinstall the cover, reconnect DRIVE-CLiQ cables, restore 24 V, then AC line power.
8. Power up and use STARTER/Startdrive to "Upload from drive (RAM to PG)" to capture the new topology, then "Load to ROM" to commit.

24 V Power Supply and Boot Sequence Considerations

The CU320-2 typically completes boot in 15 to 25 seconds and expects all configured DRIVE-CLiQ nodes to be powered and responsive within that window. The Bootloader uses a topology handshake over DRIVE-CLiQ to enumerate the connected components; if any node is missing, F7800 is raised with fault value 1 ("Power unit missing").

If the 24 V electronics supply is removed and re-applied in rapid succession (sometimes called a "brown-out" event), the CU320 can enter boot state while the Line Module's 24 V DC-DC converter is still ramping. In this race condition, the Line Module's CIM may not be ready when the CU polls it, and the handshake fails. Field experience shows three solutions in order of preference:

1. Sequence the 24 V supply with a small DC UPS or DC buffer module so the Line Module powers up first (recommend 3 to 5 second lead before the CU320-2).
2. Install a 24 V DC UPS sized for at least 30 seconds of hold-up, sufficient to ride through brief mains interruptions and allow the CU to come up cleanly each time.
3. Enable p0115[0] = 2 (Identification run on next POWER ON) – this is not a fix but forces the drive to re-identify topology at next power-up, which can clear spurious F7800 occurrences. Use only as a temporary mitigation while planning replacement of the CIM or supply.

Cold Weather / Thermal Marginal Conditions

The cold-weather F7800 symptom reported by field engineers is almost always related to:

• Thermal contraction of connector pin/socket interfaces inside the CIM socket, increasing contact resistance.
• Cold-brittle solder joints in the CIM DAC ASIC PCB, particularly in modules that have been through many thermal cycles.
• Capacitor ESR rise at low temperature in the Line Module's DC-DC supply, causing the CIM 5 V rail to come up too slowly.

Confirm by installing a thermostatic space heater set to 10 °C inside the cabinet, allowing the drive to fully warm (1 hour typical), then attempting POWER ON. If F7800 clears under warm conditions and reappears when the cabinet drops below 5 °C, the most cost-effective remediation is to replace the affected Line/Motor Module's CIM. Do not attempt to repair the CIM at board level – the DAC ASIC is not field-replaceable and reflow attempts will damage the multilayer board.

Diagnostic Procedure via STARTER / Startdrive

Use Siemens STARTER (commissioning software, SINAMICS commissioning) or Startdrive (in TIA Portal) to capture fault details. Steps:

1. Connect to the CU320-2 online via PROFIBUS, PROFINET, or Ethernet service interface (X127).
2. Open the drive object that raised F7800 (typically the infeed drive object, DO 1, but check r0002 to identify which DO is faulted).
3. Navigate to Diagnostics → Faults/Alarms and read r0947, r0948, r0949 to capture fault number, location, and value.
4. Check r0949[0] (fault value) – record the hex code, e.g., r0949[0] = 0001 hex indicates "no power unit connected".
5. Open Topology view and verify the displayed topology matches the physical cabinet. Any device showing as missing will be highlighted.
6. Open Trace function and record r3996 (DRIVE-CLiQ diagnostics) and r7836 (DRIVE-CLiQ telegram errors) for 60 seconds during a power cycle to capture the failure point.

For persistent F7800, additionally check the following parameters on the affected drive object:

• p9910 – Acceptance test mode (used for factory acceptance, do not modify).
• p7820 – DRIVE-CLiQ component number (verify the configured component matches the installed hardware).
• r7820 – Currently detected DRIVE-CLiQ component.
• p0101 – Drive object configuration; mismatches here will surface as F7800 on boot.

Commissioning Verification Checklist

After completing any repair or supply change, perform the following verification:

Preventive Measures

• Install a 24 V DC UPS sized for at least 30 seconds of hold-up time on the CU320-2 electronics supply.
• Sequence the 24 V supply so the Line Module is powered before the Control Unit, with 3 to 5 seconds lead.
• Maintain cabinet temperature between 5 °C and 40 °C using a cabinet heater and thermostat for cold-environment installations.
• Replace DRIVE-CLiQ cables at the first sign of intermittent behavior; do not re-seat marginal cables.
• Update CU320-2 firmware and Line/Motor Module firmware to the latest released version compatible with your STARTER version. Siemens periodically releases CIM firmware updates that improve cold-boot tolerance.
• Schedule periodic inspection of the 24 V terminal block and connector pins on each Line/Motor Module to detect oxidation early.

Related Fault Codes and Warnings

For a comprehensive list, refer to the SINAMICS S120/S150 List Manual, available on the Siemens Industry Online Support portal. The fault and warning descriptions include specific remediation guidance and links to the relevant function diagrams in the manual.

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