SINAMICS S120 Motor Module RDY LED Red: Diagnostic and Recovery

SINAMICS S120 Booksize Single Motor Module: RDY LED Red Continuously, DC LED Off

Two SINAMICS S120 Booksize Single Motor Modules — catalog numbers 6SL3120-1TE21-8AA3 (18 A) and 6SL3120-1TE13-0AA3 (3 A) — enter a permanent fault state immediately on power-up. Both modules display the RDY LED steady red with the DC LED off, and neither powers its drive output stage. The original machine builder has password-locked the Starter / Startdrive project, which prevents online access to the fault buffer from the maintenance workstation. This reference documents the offline diagnostic procedure, the LED-state decoding, the firmware/configuration recovery path, and the hardware-verification steps that resolve the fault without requiring project online access.

1. Affected Components and System Topology

The Booksize Single Motor Modules are part of the SINAMICS S120 Function Manual drive family. Each module is a self-commutated inverter for one servomotor or induction motor and is connected to a higher-level Control Unit (typically CU320-2 or SIMOTION D4xx) over DRIVE-CLiQ.

Both modules share the same LED cluster on the front face:

• RDY – ready / fault indicator
• DC – DC-link status
• OPT – option (DRIVE-CLiQ link state)

A correctly commissioned module lights green on RDY with DC green once the line module has charged the DC bus to the working voltage.

2. RDY LED State Decode for Booksize Motor Modules

The RDY LED is the primary diagnostic indicator. Each colour and cadence maps to a distinct phase of the module's internal state machine.

The cited condition — RDY steady red, DC off — indicates that the module has cleared its boot phase, loaded firmware, and detected a fault that inhibits the DC-link pre-charge and inverter output. The DC LED remains off because the inverter has not been enabled and the gate-drive supply is suppressed by the fault handler.

2.1 SINAMICS Integrated reference

The status-display concept is shared with the SINAMICS Integrated variant inside the SIMATIC Drive Controller. The relevant rule — "RDY LED flashes red at 2 Hz: firmware could not be started (for example, update error, drive firmware not compatible with CPU or missing, CRC check failed)" — applies to every SINAMICS variant that hosts the same firmware stack and applies to the standalone Booksize modules in this article.

3. Root Cause Matrix

For an installation that has run ten years, the fault is almost always one of the following:

4. Diagnostic Procedure Without Online Project Access

Because the machine builder locked the Starter / Startdrive project, the user cannot retrieve the diagnostic buffer over PROFIBUS / PROFINET. The following procedure extracts the fault information using the panel, the operator controls, and direct hardware inspection.

4.1 Prerequisites

• One CU320-2 Control Unit (or SIMOTION D4xx) with a temporary service project that does not require the OEM password
• An empty SD card / CF card formatted FAT32
• SINAMICS Startdrive or Starter commissioning tool installed on a laptop
• DRIVE-CLiQ cable (RJ45, Siemens 6SL3060-4Axx-0AA0 or compatible)
• USB or Ethernet connection between laptop and CU320-2
• Class II CAT III 600 V meter for DC-link voltage checks

4.2 Step-by-step Offline Diagnosis

1. De-energise the cabinet. Open the line contactor, isolate the supply, wait 5 minutes, verify zero volts on the DC link at the test points of every module.
2. Swap modules to an identical spare slot (if available). If the second slot clears the fault, the issue is the upstream connector or DRIVE-CLiQ port, not the module.
3. Create an empty Startdrive project on the laptop. Insert the project into a CU320-2 with factory reset. Do not transfer the OEM project.
4. Connect the CU320-2 to the affected modules with a fresh DRIVE-CLiQ cable, observing the standard star topology rules.
5. Power up the CU320-2 only. Do not yet energise the line module. Read the LED state of the SINAMICS module.
6. Click "Accessible nodes" in Startdrive. The CU320-2 will detect the motor modules even with no project loaded.
7. Read the diagnostic buffer of each module: Drive unit > Diagnostics > Diagnostic buffer. Record every F-code (fault) and A-code (warning).
8. Read the firmware version from the device trace: Drive unit > Properties > Firmware version. Cross-check with the firmware matrix in the S120 Function Manual.
9. Save the trace files (file extension .trc) and export the fault buffer as XML for archival.
10. Compare topology. Open Topology > Graphic view. A red exclamation mark on a module indicates either a missing or wrong-addressed DRIVE-CLiQ node.

4.3 Reading the Fault Code Without Startdrive

If a CU320-2 is not available, the module exposes the fault code in two alternate ways:

1. 7-segment display on the front of the Control Unit (where one exists in the line-up) — the fault number scrolls through the digit positions. For example, "F08501" appears as F then 08501.
2. Flashing pattern on the RDY LED of the module itself when the CU is replaced by a STARTER diagnostic CU with an empty project.

5. Firmware and Memory Card Recovery

On a 10-year-old installation, the most common cause of the red RDY with CRC mismatch is corruption of the firmware stored on the module's internal memory or on the CompactFlash card of the Control Unit.

5.1 Required firmware

5.2 Firmware reload procedure

1. Insert the empty FAT32 SD card into the laptop.
2. Copy the matching firmware file (.bin) into the root of the card.
3. Power off the CU320-2.
4. Insert the card into the CU320-2 card slot.
5. Hold the DIAG button while powering on the CU320-2 to force a firmware download into the motor module. The RDY LED flashes green at 2 Hz during the write.
6. Wait until the RDY LED is steady green or red (not flashing). A steady red indicates the firmware loaded successfully but a fault still exists in the system — proceed to read the buffer again.

6. Hardware Verification

After the firmware state is verified, eliminate the hardware causes listed in the root-cause matrix.

6.1 24 V Control Supply Check

6.2 DC-Link Pre-Charge

1. Energise the line module only.
2. Measure DC link at the test points: target = 1.35 × V_line × √2 / √3 (for 400 V line ≈ 540 V DC).
3. If DC link voltage stays at zero, the line module pre-charge circuit is suspect; check the pre-charge resistor and contactor K1.

6.3 DRIVE-CLiQ Cable and Port Test

1. Unplug and re-plug every DRIVE-CLiQ connector on the affected module.
2. Inspect the RJ45 latch for cracks.
3. Replace any DRIVE-CLiQ cable longer than 70 m with the Siemens MC500 / MC800 media converter configuration.

6.4 Motor Insulation

1. Disconnect the motor leads at the module terminal.
2. Measure phase-to-phase and phase-to-ground insulation with a 500 V megohmmeter.
3. Reject if R < 1 MΩ. A low reading forces a motor-side overcurrent fault that the module reports as a hardware defect on its own diagnostics.

6.5 Power-Cycle and Watch the RDY Cadence

Reapply 24 V with the line contactor open. Observe the RDY LED for 30 seconds:

7. Replacement Considerations

If the module is electrically defective, a like-for-like replacement requires the same catalog number. With the OEM project locked, the replacement module must contain firmware matching the existing system, and the CU320-2 will write its topology automatically on next power-up if the same slot is used.

• 6SL3120-1TE13-0AA3 – 3 A booksize, 50 mm width, internal air cooling
• 6SL3120-1TE21-8AA3 – 18 A booksize, 100 mm width, internal air cooling

Use only Siemens-original spare parts. Counterfeit modules exist on the open market; verify the serial number against the Siemens product traceability database before installation.

8. Verification Procedure After Recovery

1. Power up. Confirm RDY = steady green, DC = green.
2. Open Startdrive and read the diagnostic buffer: zero faults, zero warnings.
3. Issue a speed setpoint of zero and enable the drive via the PLC. Confirm output current < rated idle current.
4. Run the motor at 10 % rated speed for 5 minutes. Monitor temperature; module baseplate temperature should stabilise < 60 °C in a properly ventilated cabinet.
5. Run the motor at rated speed and load. Capture the diagnostic trace during the run; expect zero faults after one full duty cycle.

9. Extended Troubleshooting Matrix

10. Preventive Recommendations for Legacy Systems

• Schedule a firmware / parameter backup every 12 months.
• Replace cabinet fans every 5 years; clogged fans raise module temperature and trigger faults.
• Inspect DRIVE-CLiQ connectors for oxidation at every major shutdown.
• Document the OEM password in a secure password vault before the OEM becomes unreachable.
• Keep a service SD card pre-loaded with the matching firmware family in the cabinet.