Resolving CU320 RDY Red Fault After SINAMICS S120 Smart Line

1. Problem Overview

A field failure of a SINAMICS S120 5 kW Smart Line Module (catalog number 6SL3130-6AE15-0AA0) often forces maintenance to swap the unit for its successor variant, 6SL3130-6AE15-0AB1, on a machine that is already wired and commissioned. After the mechanical and electrical replacement is performed, the operator frequently observes that the RDY LED on the SINAMICS CU320 Control Unit switches from flashing green to a steady red, the drive system refuses to leave the start-up inhibit state, and the connected Single Motor Modules no longer respond to enable commands. Because the new module is documented by Siemens as a form-fit-function successor, the maintenance crew is left asking whether the parts are truly compatible and what diagnostic steps are required to bring the drive line-up back into service.

This article reconstructs the most common root causes for the "successor SLM installed, CU320 in fault" scenario on a 5 kW SINAMICS S120 Booksize line-up, and walks through the diagnostic and remediation procedure that an automation engineer or commissioning specialist should execute on site. It applies directly to Booksize Cold-Plate / internal-air-cooled Smart Line Modules in the 5 kW class, but most of the steps generalise to other SINAMICS S120 Active Line Module and Smart Line Module swaps as well.

2. Affected Part Numbers and Hardware Identification

Both modules belong to the SINAMICS S120 Booksize Smart Line Module family. The 5 kW class is the smallest member of the SLM range and has a different feature set from the 10 kW, 16 kW, and 36 kW units: it has no DRIVE-CLiQ-based digital inputs/outputs on the front of the module, no DQ interface, and a different firmware signature.

Although the successor is electrically and mechanically compatible, the topology component version stored in the DRIVE-CLiQ node of the new module is different from the version that was used when the project was commissioned. That difference is the principal cause of the CU320 refusing to leave the start-up state.

3. DRIVE-CLiQ Topology in the 5 kW Smart Line Module

Every active component in a SINAMICS S120 drive line-up communicates with the Control Unit through the proprietary DRIVE-CLiQ serial interface. The CU320 (and its successor CU320-2) automatically detects the connected components on power-up, reads an electronic nameplate from each, and compares what it finds against the offline project stored in its non-volatile memory. The 5 kW Smart Line Module has two DRIVE-CLiQ sockets:

• Port 0 (X200) — typically the uplink to the CU320 or to the upstream component in the topology chain.
• Port 1 (X201) — typically the downlink to the next component in the topology chain (e.g., the first Single Motor Module).

Each DRIVE-CLiQ node carries a topology component version — a 32-bit value that increments whenever Siemens changes the firmware image, hardware revision, or functional scope of the device. The CU320 stores the topology component version that was downloaded at the last successful commissioning, and rejects any node whose version number does not match the expected value, even when the catalog number is the same.

4. Why the CU320 Enters the Fault State

When the CU320 detects that the component type at a given DRIVE-CLiQ port no longer matches the expected component, or that the topology component version stored in the offline project does not match the version reported by the new module, it generates one of the following behaviour patterns:

1. The RDY LED flashes slowly red/yellow while the CU320 completes its initialisation; once the check fails, RDY switches to a constant red state and the drive line-up is placed in start-up inhibit.
2. The system reports fault F01360 ("Topological configuration of the drive unit is not supported / inconsistent") or F01355 ("Topology: Component with different topology component version detected"), depending on the firmware release installed on the CU320.
3. Communication to the downstream Single Motor Modules is interrupted because the SLM never leaves the "topology mismatch" state to forward DRIVE-CLiQ messages.

From the operator's perspective, the entire line-up is dead even though the Smart Line Module itself is healthy. The motor modules show no fault of their own; they are simply waiting for the SLM to complete its handshake with the CU320.

5. Firmware and Topology Version Compatibility

Always confirm the firmware release running on the CU320 before assuming that the swap is at fault. The minimum firmware levels for clean operation with the 6SL3130-6AE15-0AB1 are listed in the official Siemens product support pages for the 6SL3130 series. The successor module typically requires SINAMICS firmware V4.4 SP3 or higher on the CU320; older firmware images that were adequate for the -0AA0 predecessor may not know the topology signature of the -0AB1.

When in doubt, the offline project stored in the CU320 must be updated using STARTER (V5.x or earlier) or Startdrive (V15 / V16 / V17 in the TIA Portal) with a current Support Package (SSP) installed. The current SINAMICS firmware and Support Packages are distributed through the Siemens Industry Online Support portal at support.industry.siemens.com.

6. Diagnostic Procedure: Reading the Active Fault on the CU320

Before changing any project data, capture the actual alarm numbers reported by the CU320. The visible LED pattern is useful but not specific enough to identify the root cause.

1. Connect a PG/PC to the CU320 service port (X127) using a standard Ethernet cross-over or patch cable, and assign a fixed IP address in the 169.254.1.x range with subnet mask 255.255.255.0 (or use the Siemens S7-DoNet / SINAMICS Starter discovery service).
2. Open STARTER or Startdrive, build a temporary empty project, and go online to the CU320 using Accessible nodes or Online → Target system → Connect to target system.
3. Navigate to Diagnostics → Fault buffer on the CU320, and read the active and historical faults. Record the F-code, the affected topology component, the topology component version reported, and the expected version.
4. Open Topology view in the online tree and confirm the reported component type and the topology component version at the port where the SLM is connected.
5. Compare the online topology component version against the offline project. If the values differ, the CU320 will not bring the line-up to run-ready without operator intervention.

The drive objects listed below are the ones that typically carry the fault entry in a 5 kW SLM replacement scenario:

7. Resolution: Topological Re-Assignment and Firmware Alignment

Once the fault is captured, execute the following sequence. The order matters: do not simply download the existing project to the CU320 while the topology mismatch exists, as this can leave the new SLM unconfigured and produce additional warnings on the next boot.

1. Update the offline project in STARTER / Startdrive. Insert the new SLM (6SL3130-6AE15-0AB1) at the same topology slot, or right-click the existing SLM and use Replace device with the successor catalog number. The tool will refresh the topology component version automatically.
2. Install the latest SINAMICS Support Package matching the CU320 firmware. In STARTER this is done via Options → Support Packages; in Startdrive via Options → Support Packages. The CU320 firmware and SSP are versioned together and must be consistent.
3. Compile the project and verify that no consistency errors remain. Pay special attention to messages about DRIVE-CLiQ topology and about lead-time / follow-up topology for the new SLM.
4. Download the project to the CU320 using Online → Target system → Download to target system. Select the option to download to both the CU320 and the active line-up. The download will overwrite the offline topology stored in the CU320's non-volatile memory.
5. Accept the new topology when prompted. The CU320 will perform a topology comparison and will ask the operator to confirm the new component before storing it. Confirm only after visually verifying that the replaced SLM is the only change.

If the CU320 firmware itself is too old to recognise the successor's topology component version, perform a firmware update before the project download:

1. In STARTER, navigate to Online → Target system → Firmware update and select the correct CU320 firmware image (e.g. V5.2 SP3 or the latest released). Do not interrupt the power supply during the update.
2. After the firmware update completes, perform a power cycle of the CU320 only (not the whole cabinet) and verify the RDY LED transitions through the standard boot sequence.

8. Verification and Commissioning Checks

After the project is downloaded and the topology is accepted, perform the following verification sequence before returning the line-up to production:

1. LED state check. RDY on the CU320 should be steady green; the SLM RDY LED should be steady green; the Single Motor Modules should show green RDY within a few seconds of CU320 ready.
2. Fault buffer clear. Open the fault buffer on the CU320 and clear all historical faults. Power cycle the line-up to confirm that no new faults are raised during the second boot.
3. Topology check. In the online topology view, confirm that every DRIVE-CLiQ port shows the correct component type and the correct topology component version. Compare against the as-built documentation.
4. Function check — line module. Close the line contactor, enable the SLM, and verify that the DC link voltage reaches the regulated value (typically 600 V DC for a 400 V class line). The parameter r0070 in the SLM drive object shows the actual DC link voltage.
5. Function check — motor modules. For each Single Motor Module, issue a small jog command (with the safety circuit engaged and the motor uncoupled if necessary) to confirm the enable chain. The status word r0899 should reflect the expected state.
6. Documentation update. Update the cabinet layout drawing, the bill of materials, and the project backup to reflect the new catalog number. Archive the project (".zap17" or ".zap15") to the maintenance system.

9. Common Fault Codes Related to DRIVE-CLiQ Topology

The following fault codes are the ones most frequently observed in the field when a Smart Line Module is replaced with a successor. The complete list of faults and alarms is in the SINAMICS S120 List Manual, which is available from the Siemens Industry Online Support portal at support.industry.siemens.com.

10. Preventive Notes for Field Replacement

• Always carry the project archive on a USB stick or laptop in the field. A module swap is the moment you discover whether the project backup is current. The current SINAMICS project can be exported from STARTER or Startdrive as a single zip file (".zap" or ".zapXX").
• Pre-stage the firmware. If the cabinet uses an older CU320 firmware release, download the latest Support Package onto the maintenance laptop before going to the machine. Site-to-corporate network connections are frequently slow or unavailable in production environments.
• Verify the DRIVE-CLiQ cable on removal and re-installation. A cable that was borderline before the swap will fail when the new SLM drives a different topology comparison load.
• Use only Siemens DRIVE-CLiQ cables with the correct length. Third-party Category 5e cables will sometimes work electrically but may exceed the timing budget at higher DRIVE-CLiQ port speeds.
• Document the topology component version in the cabinet log after every commissioning. This single number is the difference between a 30-minute swap and a 3-hour swap.
• Check the SLM firmware via parameter r0128[0] in the SLM drive object after a swap. The version reported here is what STARTER and Startdrive use to decide whether the device is known.

11. Frequently Asked Questions