SINAMICS G120 CU240E-2 V4.4: Resolving Switch On Disable

Problem Statement

A SINAMICS G120 drive assembled from a CU240E-2 Control Unit and a PM240 Power Module, running firmware V4.4 (build A02-4.4), refuses to start when an ON command is applied. The wiring side is healthy: 24 VDC on terminal 9 reaches terminal 5, terminal 28 (drive enable), 69 (signal ground reference) and 34 (electronic ground) are correctly strapped, the digital input reports HIGH inside STARTER or SINAMICS Startdrive, parameter P840 = 722.0 ties the ON/OFF1 source to DI0, and the drive does not raise any fault or alarm. The Status Word 1 (ZSW1) bits for OFF1 and OFF2 read healthy, but the operating display still resolves to "Switching on inhibited" / "Switch on disable". The IOP-2 or BOP-2 Control Panel reports the same state and refuses to start, while the same panel still indicates "Ready to run" at the top line — a contradiction that, in practice, is the single most useful diagnostic fingerprint of a master-control-inhibit or missing secondary enable condition.

This article walks through the diagnostic logic that explains this contradiction, isolates the two real root causes that produce this fingerprint (P0806 master-control inhibit and one or more of the five non-OFF1 enable conditions still at zero), and provides the parameter-by-parameter recovery procedure. It also documents the firmware-update path for a CU240E-2 that is locked at V4.4 and that is no longer covered by warranty service.

Affected Hardware and Firmware Identification

Confirm the exact part numbers before any parameter work. The combination reported in this case is:

The firmware version is read from r0018 in STARTER / Startdrive or directly on the IOP-2 Diagnostics → Drive version screen. If the build is anything earlier than V4.5 you cannot use the integrated web server for an update, and SD-card-based updates require intermediate steps that are documented later in this article.

Symptoms Observed in the Field

• ON command via DI0 (24 V on terminal 5) does not start the motor; no F-code fault, no A-code alarm.
• ZSW1 bits 1 (OFF2 inactive) and 0 (Ready to switch on) read healthy in r898[0].
• IOP-2 / BOP-2: pressing the green RUN key returns the message "Switch on disable of drive is active".
• Header line on IOP-2 reads "Ready to run", but the RUN command is rejected.
• Control source appears to be local; no PLC is connected, or the PROFINET/PROFIBUS cable is unplugged.
• P0806 = 1 is active in the parameter list (BI: Inhibit master control).

Drive Operating State Machine

SINAMICS G120 follows the PROFIdrive state machine defined in the PROFIdrive profile. Every transition depends on the six enable conditions, not just on the ON/OFF1 bit. The state machine is what produces the contradiction: the drive can simultaneously be "Ready" in the control word sense (ZSW bit 0 = 1) and "Switching on inhibited" in the state-machine sense (ZSW bit 6 = 0).

The drive is parked in S2 "Switching on inhibited". The IOP-2 banner text "Ready to run" is a translation of the German "Einschaltbereit" and refers to the ZSW1 state, not to the run state S5. The two are not contradictory — they are two different fields.

Status Word 1 (ZSW1) Bit Definitions

ZSW1 is read from r898[0] for the currently active command data set. The bit numbers below follow the PROFIdrive profile mapping used in STARTER / Startdrive Control/Status view.

The fingerprint of a P0806 inhibit is therefore: ZSW bit 0 = 1, bit 1 = 1, bit 2 = 1, bit 3 = 0, bit 6 = 1. The drive reports Ready to switch on and OFF2 / OFF3 inactive, but it is still in state S2 and will not energize the inverter.

Six Enable Conditions Required to Run

Every SINAMICS G120 — regardless of control source — must satisfy all six enable conditions simultaneously before the inverter fires the IGBTs. Each enable has its own binector input parameter and its own default source.

The user in this case has correctly wired enable 1 (P840 = 722.0). STARTER confirms DI0 is HIGH. The mistake is that the remaining five enables are not guaranteed to be at static 1 by the factory default after a parameter reset in every command data set — and one of them, the master-control-inhibit P0806, can be parked at 1 by an older commissioning script or by a previous PLC-based project that left an inhibit latched.

Critical Parameter: P0806 Master Control Inhibit

P0806 is described in the SINAMICS G120 List Manual as "BI: Inhibit master control". Its function is conceptually simple: when the signal at P0806 evaluates to 1, the drive ignores control commands from any source that is not the active local control source. The active local control source is the one selected through P0700 (signal source selection: 0 = factory default, 1 = operator panel, 2 = terminal, 5 = PROFIBUS, 6 = PROFINET, etc.).

The trap is that P0806 = 1 is a static signal source. The drive is not going to evaluate DI0 on P0806 — it is going to evaluate the literal constant 1. Result: master control is permanently inhibited, and only a "low-priority" command channel (typically the IOP-2 menu "Manual / Jog" entry when the drive is configured for it) can override it. In the field case, the user pressed RUN on the IOP-2, the panel tried to issue the command, and the message "Switch on disable of drive is active" appeared, which is the exact string STARTER / Startdrive return when P0806 is asserting master-control inhibit.

The fix is to either:

1. Set P0806 = 0 (constant zero — inhibit permanently released), or
2. Connect P0806 to a binector that the application can drive low during normal operation, for example to a DI 722.x controlled by a "System ready" contact, or
3. Route P0806 to a word bit of the active control word that the PLC can drop low, for example 2080.0 (BICO output for a control bit). This is the standard approach for PROFIdrive-controlled plants.

Step-by-Step Diagnostic Procedure

1. Power the drive up and read the active operating state. Connect via STARTER (USB or Ethernet, depending on CU variant) or Startdrive. In the Commissioning → Control panel view the operating state is shown as text; in the expert list read r0002 (operating state as numeric) and r899 (operating state as text label). Confirm "Switching on inhibited" or numeric 6.
2. Read ZSW1 bit-by-bit. Open r898[0]. Confirm bit 0 = 1, bit 1 = 1, bit 2 = 1, bit 3 = 0, bit 6 = 1.
3. Read STW1 bit-by-bit. Open r896[0] and confirm that all bits you expect to be HIGH are actually HIGH. The most common bit map for a terminal-controlled V4.4 drive is 0x447E (binary 0100 0100 0111 1110) when DI0 is HIGH and the drive is sitting in S2.
4. Inspect the six enable source parameters. Read P840[0], P844[0], P848[0], P852[0], P1140[0], P1142[0] and confirm each is connected to a value that resolves to 1 at runtime (use Control Word diagnostics → Trace if uncertain).
5. Inspect P0806. If P0806 = 1, set it to 0 and re-test before changing anything else.
6. Power-cycle the drive. Many field cases of an apparently stuck "Switching on inhibited" are cleared by a clean power-down, 5-minute DC-link discharge, and power-up. If the drive sits at S2 again, the parameter work above is required.
7. Reissue the ON command with a 0→1 edge. A static 1 on DI0 will not start the drive if the drive has been parked in S2. The first attempt must be a rising edge. Many V4.4 STARTER builds will not detect this and will keep reporting "inhibited" until the DI drops and rises again.

Using STARTER and IOP-2 to Simulate Enables

The IOP-2 Control Panel screen has a "Simulate enable signals" checkbox (or, in older builds, four small checkboxes for OFF2/OFF3/RFG enable/Setpoint enable) that drives the five non-OFF1 enables with static 1 signals regardless of their parameter sources. This is the fastest way to test whether P0806 alone is the culprit:

1. Open STARTER / Startdrive → go online.
2. Right-click the drive → Commissioning → Control panel.
3. Tick "Drive acquires master control" and "Simulate enable signals".
4. Press the green RUN button. If the drive starts, P0806 is not the issue; one of the five enable sources is.
5. If the drive still says "inhibited", then the master control is being blocked — P0806, P0700 mismatch, or an active safety stop on terminal 28/69.

The same logic is exposed on the IOP-2 under Menu → Diagnostics → Control / Status words. The IOP-2 will display the bits of r898 and the simulated source, which makes it the most useful handheld tool for a no-laptop field diagnosis.

Parameter Reset and CDS Selection

If you want to recover the default wiring of the six enable parameters without loading a complete project, use the following short sequence. Run it online, not in offline mode, because the user-defined defaults (saved with P0971 = 1) take precedence over the factory defaults.

; --- Enable parameters — restore to factory defaults ---
P0003 = 3                  ; expert access level (read/write everything)
P0010 = 30                 ; restore factory settings on next P0970
P0970 = 1                  ; factory reset (writes defaults to RAM, copies to EEPROM on P0971)
P0971 = 1                  ; save parameters to non-volatile memory

; --- Re-wire ON/OFF1 to DI0 on the CU ---
P0700[0] = 2               ; signal source = terminal / digital inputs
P0701[0] = 1               ; DI0 = ON/OFF1 (function selector — only relevant on terminal source)
P840[0]  = 722.0           ; ON/OFF1 ← DI0 of CU

; --- Releases the master-control inhibit ---
P0806[0]  = 0              ; inhibit master control = constant 0 (released)

; --- Five non-OFF1 enables to static 1 ---
P844[0]  = 1               ; OFF2 ← static 1
P848[0]  = 1               ; OFF3 ← static 1
P852[0]  = 1               ; Enable operation ← static 1
P1140[0] = 1               ; RFG enable ← static 1
P1142[0] = 1               ; Setpoint enable ← static 1

P0971 = 1                  ; save
; Power-cycle the drive to flush RAM to EEPROM and re-init the state machine

If the drive is configured for multiple command data sets (CDS) — common on PROFIdrive plants that swap between auto and manual control — the [0] indices above are only the first CDS. Verify P0820[0..n] and repeat for each active CDS, or set P0009 = 1 temporarily to copy the CDS-0 wiring to all sets.

Firmware Update Path from V4.4

Siemens documents the firmware update for the SINAMICS G120 CU240E-2 through several channels, and not all of them are available at V4.4. The decision tree is:

The community guidance in the field case — that "FW updates can only be performed from FW 4.5 upwards" — is partially correct: it is true for the SD-card one-step procedure and for the integrated web server. It is not true for STARTER or Startdrive, which can push any firmware image to any G120 CU240E-2 supported by the tool. To update a V4.4 CU240E-2 to a current G120 firmware:

1. Download the current firmware package from Siemens Industry Online Support. Search for SINAMICS G120 CU240E-2 firmware and pick the highest *.upd file compatible with the hardware variant (the part number on the front of the CU is the binding key — do not cross HW variants).
2. Connect a USB cable (or Ethernet, depending on the CU variant) to the PC. Open STARTER or Startdrive. Go online to the drive.
3. Open Online → Drive → Load firmware to drive. Select the *.upd file. STARTER verifies the file, then flashes the CU. Do not interrupt the process; the LED pattern on the CU is a slow green/yellow blink during write, and a steady green at the end.
4. After the flash, STARTER prompts to perform a Factory Reset. Accept it. This returns the parameter set to the FW-default wiring (P0806 = 0, all six enables default, P840 = 0). Re-enter your motor and IO configuration, then re-issue the ON command.

If the only available tool is an SD card and the CU is on V4.4, the workaround is to put an intermediate firmware (e.g., V4.6 or V4.7) on the SD card, write it to the CU, then load the final target firmware via STARTER. Siemens Support can supply the legacy intermediate images on request — they are not always on the public download portal because of the version-age filter.

Verification Procedure

After applying the fix, run the following checklist to confirm a healthy run state.

1. Power the drive up. Read r0002 = 4 (Ready, run-capable) before the first ON command.
2. Apply DI0 = 24 V (rising edge). Read r0002 = 5 (Run) and r898.3 = 1 (COK, inverter enabled).
3. Confirm r0079 (torque setpoint) and r0021 (motor speed actual) are non-zero and tracking the setpoint.
4. Read r0002 = 5 for at least 5 seconds without an alarm being raised. If F0700, F0701, or F07900 appears, the contactor and motor wiring is suspect; the diagnostic that follows is the same as the original failure and the motor simply will not spin up.
5. Set P1210 = 0 (auto-restart disabled) for the rest of the commissioning to keep the state machine predictable.
6. Take a STARTER trace of r898, r0021, and r0079 for 30 seconds. Save the .trc file as part of the commissioning record. This is the standard evidence package for warranty and for any future re-commissioning.

Related Field Cases and Cross-References

The "Switching on inhibited" state is also the symptom of a number of issues that look identical at first glance but are caused by different parameters. The matrix below is a quick triage aid.

Spare-Part and Re-Commissioning Considerations

For a unit that is out of warranty and that you intend to keep in service:

• Save a project backup to the SD card slot of the CU (firmware 4.5+) or to a separate file in STARTER (any version). File name convention: PROJECT_NAME_CU240E2_YYYYMMDD.zip.
• Note the Hardware version (printed on the CU front panel) and the Firmware version (read from r0018). If you ever swap CUs, you must load a matching firmware onto the new CU before loading the parameter backup, otherwise the CDS-0 wiring will be silently re-mapped.
• Replace the CU fan filter at the panel (if installed) every 24 months; the CU is rated for 0 to +50 °C at the inlet, and overheating is the most common cause of intermittent enable loss in this drive family.
• If the drive is mission-critical, keep an older V4.4/V4.5 firmware on a second SD card as a hot standby. The CU can boot from a card in the SD slot if the internal flash is corrupted — a useful recovery path if a future flash write fails mid-cycle.