SINAMICS G120 CU240E-2 Programming Without PM Module: System Limits and Field-Proven Workarounds

When a SINAMICS G120 Control Unit fails in a remote installation, the service engineer's first instinct is to ship a pre-programmed replacement CU to minimize downtime. With the modular G120 platform this workflow is straightforward in principle—parameters are stored on the CU—but the practical question of whether a CU can be programmed without a Power Module (PM) physically connected has no quick answer. The behavior observed by field engineers (CU halts during power-on, STARTER cannot complete commissioning, the operator panel reports a power-unit fault) is a documented system limit of the G120 architecture, not a commissioning error.

This reference covers the affected hardware, the boot sequence that creates the PM dependency, the parameter storage map, and the four field-proven workarounds that allow a remote CU replacement without dispatching a service engineer to the site.

Problem Statement and System Behavior

The reported service scenario:

• Drive: SINAMICS G120 with CU240E-2 DP (Siemens article 6SL3244-0BB12-1PA0) and PM240 Power Module (6SL3224-0BB31-8AA0), 18.5 kW, 400 V three-phase. The source transcript shows "22:00 kW" which is a transcription error; the 1-8 suffix in the Siemens article number decodes to 18.5 kW. The 22 kW variant in this voltage class is 6SL3224-0BB32-2AA0.
• Customer location: several thousand kilometers from the service office.
• Failure: a non-recoverable CU fault; the PM is functional and the motor is intact.
• Planned workflow: order a new CU, program it at the service bench using a parameter backup from the failed unit, ship the pre-programmed CU to the customer.
• Observed problem: the new CU cannot complete its power-on self-test or be commissioned when no PM is connected, even though the CU is supplied with external 24 V DC on the customer terminal strip.

This matches the documented G120 design. The Control Unit performs an automatic commissioning handshake with the Power Module at every power-up, and the handshake fails when no PM is detected. A backup CU cannot be programmed on its own; a PM is required as a stand-in for the customer's unit during bench commissioning.

Affected Hardware and PM-Dependent Boot

Verify part numbers, voltage class, and power rating before ordering a replacement. The seven-segment Siemens article number encodes the family, voltage class, and rating; a one-digit read error in the office can cost a week of shipping time.

Control Unit 6SL3244-0BB12-1PA0

Power Module 6SL3224-0BB31-8AA0

Boot sequence

The G120 boot is a coordinated handshake between the Control Unit firmware and the Power Module. Without a PM, the handshake fails at the first stage and the CU remains in a partial-boot state.

Two key facts follow from the boot sequence:

1. The 24 V external supply on the CU terminal strip is sufficient to start the firmware, light the operator panel, and even communicate over the fieldbus—but not to operate the drive or run commissioning.
2. The CU reads the PM's identification EEPROM (voltage class, current rating, I²t parameters, STO capability) over the ribbon between CU and PM. Without that identification the CU has no power section to control and refuses to enter the operating state.

The source dialogue confirms this behavior: the original poster observed that the CU halted during power-up, and a respondent stated that "the CU does a automatic commissioning during the startup" and that "if no PM is plugged then the CU stop working during power on." This is the expected behavior, not a fault in the replacement CU.

G120 Parameter Storage Architecture

Understanding where parameters are stored is the basis for every workaround below. The G120 uses three storage locations:

Three parameter numbers drive the save and load process:

• p0009 – device commissioning parameter filter (0 = ready, 1 = quick commissioning, 30 = transfer from MMC, 31 = factory reset).
• p0977 = 1 – copy all parameters from RAM to non-volatile flash (RAM-to-ROM).
• p0802 – transfer parameter image from MMC/SD card to flash; p0802 = 1 triggers the load.

Why a Standalone CU Cannot Be Commissioned

Combining the boot sequence and the parameter storage map explains why the new CU cannot be programmed at the bench without a PM:

1. STARTER (or TIA Portal Startdrive) online commissioning is a guided flow that requires the drive to be in p0010 = 1 (quick commissioning) and the CU to be responsive. With no PM, the CU does not advance past the handshake stage and STARTER reports "drive not in commissioning state" or a PROFIBUS timeout.
2. Direct parameter writes from a PC over the fieldbus are committed to RAM, but the RAM-to-ROM copy (p0977 = 1) is blocked when the firmware cannot detect a power section.
3. The MMC card "load to drive" procedure requires the CU to complete at least the basic commissioning with a PM detected; otherwise the CU raises a fault and aborts the load.

Net effect: the bench engineer cannot reach a state in which the new CU holds a complete, non-volatile parameter set without a PM connected.

Workaround 1: Bench-Program With a Test PM

The standard service procedure and the only method that works for every G120 variant. Use a PM at the bench to run a full commissioning, save the parameters, and ship the CU.

Prerequisites

• Test PM of the same voltage class as the customer's installation (400 V 3 AC for the 0BB family).
• The PM may be a lower power rating—the drive only needs the PM for identification and DC link power at zero load.
• External 24 V DC supply for the CU (24 V, 1 A minimum).
• Motor (optional but recommended for current loop tuning). A no-motor commissioning mode is also valid when the goal is parameter transfer only.
• PC with STARTER V5.4 SP3 or higher, or TIA Portal V15.1 or higher with the SINAMICS Startdrive option.

Procedure

1. Connect the new CU to the test PM at the bench. Apply 24 V to the CU. Apply 3 AC mains to the PM.
2. Connect the PC to the CU over the appropriate interface: USB-to-PROFIBUS (Siemens 6SL3255-0AA00-2CA0) for the DP variant, PROFINET cable for the PN variant.
3. Open the offline project that contains the customer's drive configuration, or load the *.dnx/*.par archive from the customer (see Workaround 2 for the read procedure).
4. Download to the target device. In STARTER: Target system → Download to target device. In TIA Portal: Online → Download to device. The CU accepts the parameter set, performs the quick commissioning, and stores the data in flash.
5. After the download completes, trigger RAM-to-ROM. Either set p0977 = 1 from STARTER, or from the BOP-2 navigate MENU → SAVE → RAM-ROM and confirm with OK. Wait for r3996 to return to 0.
6. Save the complete project to the PC. Power down, remove the CU, and ship it to the customer.

At the customer site

1. Power down the drive, disconnect 24 V from the failed CU, and unmate the PM connector.
2. Install the pre-programmed CU on the existing PM. The CU-to-PM ribbon is keyed; do not force it.
3. Apply 24 V and 3 AC mains. The CU boots, reads the PM, and runs quick commissioning against the parameters stored on the new CU. Most installations are back in r0002 = 14 (run) within 5 to 10 seconds.

Workaround 2: MMC/SD Card Cloning

Cloning is the most field-friendly procedure because it does not require a PC at the customer site once the initial read is complete. The MMC card is inserted in the CU's front-panel slot.

Reading parameters from the failed CU at the customer site

1. Power down the drive. Insert a blank SD card (FAT16 formatted, 32 MB to 2 GB) into the CU's card slot until it clicks.
2. Apply 24 V to the CU. Wait for the operator panel to display r0000 = 0.
3. From the BOP-2 (if fitted): MENU → UP (to the SAVE/LOAD level) → SAVE → 02: TO CARD. Confirm with OK. The CU writes the parameter image to the card as USERDATA in the root directory and as a *.clc clone file.
4. Alternative without BOP-2: STARTER or TIA Portal, when online with the failed CU, exports the parameter set using Target system → Upload from target device (load to PG/PC). The file is saved as a *.dnx or *.par archive.
5. Power down. Remove the SD card and ship it (or the exported file) to the service office.

Loading the parameter image onto the new CU at the bench

1. Connect the new CU to the bench PM (Workaround 1 prerequisite).
2. Insert the SD card with the customer image into the new CU. Power up.
3. From the BOP-2: MENU → UP → 03: FROM CARD. Confirm. The CU copies the parameter image from the card into its flash, then triggers an automatic restart.
4. From STARTER/TIA Portal with the project open and online: Target system → Download to target device (PC to target). The MMC card is bypassed and the project is loaded directly.
5. Verify that the new CU now reports the same motor data and fieldbus address as the failed unit. Specifically check p0010 = 0, r0002 = 3 (ready), and that the PROFIBUS node address in p0918 matches the customer PLC's configuration.
6. Save to flash with p0977 = 1. Remove the SD card. Ship the CU.

Workaround 3: STARTER and TIA Portal Offline Backup

The most robust procedure because the project file is portable, version-controlled, and human-readable. Use it in combination with Workaround 1 or 2.

STARTER (classic, pre-TIA Portal)

1. Connect the PC to the failed CU. Online connection over PROFIBUS, PROFINET, or the USB-to-PROFIBUS adapter (Siemens 6SL3255-0AA00-2CA0).
2. Open the project. Right-click the drive in the project tree and select Load to PG/PC. The CU's parameters are uploaded into the project.
3. Right-click the drive → Save project to PG. The complete project is committed to the local disk.
4. Compress the project to a *.zip or *.dnx archive. This is the master copy sent to the service office.

TIA Portal with SINAMICS Startdrive (V13 SP1 and higher)

1. Connect online to the failed CU. In the project tree, right-click the SINAMICS device → Load device (software) from device to project. The current parameter image is mirrored to the offline project.
2. Save the project. Optional: Project → Archive to produce a *.zap archive.
3. At the service office, open the archive in TIA Portal on a machine with a matching Startdrive version. Add the new CU as a replacement device with the same article number and firmware. The target configuration in the project must match the firmware version of the new CU; mismatches trigger a download error.
4. Download the project to the new CU at the bench using the Workaround 1 procedure.

Workaround 4: BOP-2, IOP-2, and Smart Access Transfer

For installations without a PC on site, the operator panels provide a direct path. The choice of panel depends on what was installed on the original drive.

Procedure with BOP-2:

1. Power down. Insert the SD card. Power up the failed CU.
2. BOP-2: press OK to enter menu. Navigate to SAVE/LOAD. Select 02: TO CARD. Press OK. Wait for "DONE" on the display.
3. Power down. Remove the card. Ship to the service office.
4. At the bench, insert the card in the new CU with the bench PM connected. Power up. BOP-2 → SAVE/LOAD → 03: FROM CARD. Press OK. The new CU is now parameter-matched to the failed unit.

Diagnostic Parameters and Fault Codes

Use these parameters and faults to confirm the PM-missing condition and to verify a successful parameter load.

Boot-time faults when no PM is present

Verification parameters

For the complete parameter reference, consult the SINAMICS G120 List Manual on the Siemens Industry Online Support portal.

Field-Proven Decision Tree

Use this flowchart to pick the right method given the resources available at the customer site and at the service bench.

Verification Procedure After a Spare-CU Installation

When the pre-programmed CU arrives at the customer site and is installed on the existing PM, the commissioning engineer should perform the following verification sequence before releasing the drive to production.

1. Visual check. Confirm the CU-to-PM ribbon is fully seated and latched. Confirm the 24 V terminal polarity at CU terminals 31 (0 V) and 32 (+24 V) is correct. A reversed polarity will not damage the CU but will prevent boot.
2. Power-up check. Apply 24 V to the CU only. Wait for the RDY LED to settle green. Read r0002 from the BOP-2 or STARTER—expect 1 (ready).
3. Mains application. Apply 3 AC 400 V to the PM. Wait for the PM's READY LED to settle. Read r0200 and r0207—expect the customer's PM article number and rated current.
4. Fieldbus check. From the customer's PLC, ping the PROFIBUS slave at the configured address. The CU should appear in the diagnostic buffer with no active F085xx faults.
5. No-load spin. Issue a small speed setpoint (e.g., 50 rpm) in jog mode. Confirm direction matches the motor wiring. Stop.
6. Parameter integrity. From STARTER/TIA Portal, compare a CSV export of the live CU with the offline project. Differences should be limited to runtime counters (r0947, r0046, r0021) and timestamps.
7. Document the swap. Update the asset register with the new CU serial number, the firmware version (read from r0018), and the date of the swap. Archive the failed CU for RMA.

Safety and Documentation Review

When a CU is replaced in the field, four safety-relevant items must be reviewed before the drive is returned to service. These are not part of the parameter image and are not restored by the cloning procedures above.

1. Safe Torque Off (STO) wiring. The PM240 article 6SL3224-0BB31-8AA0 does not include the STO function (the AA0 suffix is a non-STO variant). If the original installation relied on STO, the replacement PM must be a BA0 variant and the wiring to terminals X9 on the PM must be re-validated. Refer to the SINAMICS G120 product page for the STO wiring details.
2. Braking resistor. The PM240 in the FSC frame accepts a plug-in braking resistor via terminals DCP/R1 and R2. Confirm the resistor was carried over with the PM (not the CU); it does not survive a CU swap.
3. Output reactor and sine filter. If present, the reactor/filter is wired between the PM output (U2/V2/W2) and the motor. It is not part of the parameter image but its presence affects p0230, which should be reviewed.
4. EMC plate and shield termination. The shield plate is mounted on the PM, not the CU. The replacement CU installation reuses the existing shield plate; no action is required unless the cable screen is disturbed.

Frequently Asked Questions