SINAMICS G120 with CU240E: STO Active but No Drive Fault or Alarm

This reference explains why a SINAMICS G120 inverter with a CU240E Control Unit halts the motor when Safe Torque Off (STO) is asserted through the on-board digital inputs (DI4/DI5) but never raises a fault, warning, or alarm visible in the operator panel, the TIA Portal diagnostics buffer, or the cyclic PROFINET status word. It also documents the parameter wiring required in Startdrive to surface the STO event as either a configurable fault (F) or alarm (A), and the telegram changes required to communicate that state to the PLC over PROFINET.

1. Problem Statement and Observable Behaviour

Configuration that exhibits the symptom:

• Control Unit: SINAMICS G120 with CU240E (CU240E-2 family, firmware V4.5–V4.7 SP range — confirm with parameter r0018).
• Power Module: PM240-2 (typical) or PM240, output rating project-specific.
• Safety function: STO via on-board terminals (DI4 and DI5 wired to the safety circuit, 24 V removed = STO active).
• Engineering: TIA Portal V14 with Startdrive V14 SP1 (or later SP of the same major version).
• Cyclic frame: Standard Telegram 1 (PZD1 + PZD2 = control/status word + speed setpoint/actual).

Observed behaviour:

1. Pressing the E-stop removes 24 V from DI4 and DI5.
2. The drive removes gating from the IGBTs within the STO debounce time (typically < 20 ms; exact value in p9651/p9851).
3. No fault code, no alarm code, no message appears on the BOP-2/IOP-2, in r0947/r2122, or in the TIA Portal online diagnostics.
4. Releasing the E-stop and pulsing ON/OFF1 resumes normal operation.

This is the intended default behaviour of the Safety Integrated (SI) basic functions on the CU240E-2 family. STO is a SIL 2 / PL d capable safe state; the absence of a fault is by design because the safety function being active is a normal state for the drive, not an error condition.

2. Why STO Generates a 'No Message' by Default

The Safety Integrated basic functions on the G120 distinguish two classes of events:

On the CU240E-2 platform, the default reaction to a STO request is the N class. From the SINAMICS G120 CU240E-2 List Manual (edition referenced in the field report: 09/2017, A5E33839529), the N class is used to indicate a normal operating state of a safety function. The drive does not treat the assertion of STO as a malfunction; it treats it as the correct response to a safety demand. Consequently, no entry is written to the fault buffer (r0947/r0949) and no bit is set in the alarm word (r2122).

3. Safety Status Word and Bit r9773

STO state is reported through the safety status word r9773 (Control Unit side) and its parallel mirror r9873 (motor-module/HYDRA side, only present on higher-feature CUs). On the CU240E-2 the relevant bits for the basic functions are:

The Siemens support recommendation in the field report is to use 9773.1 (the "STO active" cause bit). This is the bit that becomes '1' only after the drive has actually inhibited the output stage in response to the STO request — it is therefore the correct choice for an HMI/PLC-visible alarm.

4. The Default Wiring Path: r9773.1 → P2106 / P2112

The diagnostic message must be generated on top of the safety function, not as part of it. The mechanism is the SINAMICS Binector-Connector (BICO) interconnect, called BECO on the Startdrive HMI for the G120/CU240E-2 family.

4.1 Target parameters

For a CU240E-2 the G120 List Manual defines a pool of free binectors (p20030 through p20080 and similar ranges, depending on firmware) that exist purely as routing targets. Using p20080 as an intermediate makes the wiring portable across project revisions: any change of the source bit (e.g. from r9773.1 to r9773.0 if you also want to alarm on "STO selected, not yet active") is a single edit.

4.2 Why a free binector is used as a buffer

On older G120 firmware (≤ V4.5) some BICO sources could not be connected directly to p2106/p2112 because of a type/driver mismatch in the Startdrive selection dialog. Routing through a free binector (parameter of type BI with a matching r-number source) provides a guaranteed-valid edge. On V4.6+ the direct route is normally allowed, but the buffered route is still recommended because it isolates the safety-side source from the message-side sink and makes the configuration auditable.

5. Step-by-Step Configuration in TIA Portal V14 with Startdrive

5.1 Prerequisites

• TIA Portal V14 SP1 or later with Startdrive V14 SP1 (project matches the source environment).
• Online connection to the G120 (PROFINET or PROFIBUS) with the drive in commissioning or ready state.
• Know the drive password level: Safety Integrated commissioning is unlocked at expert level (parameter p0009 = expert or higher), but the wiring described here is not safety-parameter related and only requires access level Standard.
• Know the safety status word bit you are wiring. Verify by setting r9773 visible in the parameter monitor, asserting STO, and capturing the bit that goes high.

5.2 Procedure

1. Open the drive device view in the TIA project. Double-click the G120 → "Parameters" → online/offline parameter editor. Switch to Expert parameter list view (checkbox in the toolbar) so that p20080, p2106, and p2112 are visible.
2. Configure the source on the free binector.
   • Navigate to p20080.
   • Open the BICO interconnect dialog (Startdrive shows a "Connect to ..." button or a drop-down list depending on patch level).
   • Select source r9773.1 (STO active). If only the decimal form is offered, type 9773.1 in the source field.
3. Route the free binector to a fault or alarm sink. For an alarm (non-latching, drive remains ready):
   • Open p2112[0] (BI: External alarm 1, first index).
   • Set the source to p20080.
   Result: while STO is active, bit 0 of r2122 is set, alarm A07850 is raised, and the BOP-2 displays a flashing alarm.

For a fault (latching, drive transitions to fault state and requires acknowledge):

• Open p2106[0] (BI: External fault 1, first index).
• Set the source to p20080.

Result: F07860 is raised, OFF1 is inhibited until acknowledgment via p2103 or via the PLC control word bit 7.

4. (Optional) Customise the message text. Default messages F07860 and A07850 have generic text ("External fault 1" / "External alarm 1"). To make the text describe STO, use the message-class mapping in p2118 (fault) / p2119 (alarm) to point to a free message number; then enter the desired text via p2115[0..2]/p2121[0..2] in the message configuration. For most diagnostics, the default text combined with a comment in the project is sufficient.
5. Save and download to the drive. Use "RAM to ROM" (Startdrive: button "Copy RAM to ROM") if the parameter set must survive a power cycle without re-download from the PLC.
6. Online verify. Force the safety circuit off (E-stop or unplug DI4). Within 100 ms the new alarm/fault appears in the BOP-2 display and in r0947/r2122. Re-apply safety circuit and acknowledge as appropriate.

6. Telegram Selection for PROFINET Fault Communication

Standard Telegram 1 contains only two PZD words: control word 1 (STW1) and speed setpoint (NSOLL), with the corresponding status word 1 (ZSW1) and actual speed (NIST) on the feedback path. None of these words carry a per-bit fault/alarm indication that you can interpret as "STO active". The drive-side fault buffer is accessible only via acyclic record data (PROFINET index 0x002A / 0x002B) or via DP-V1 services — and even then the N-class message is, by definition, not in the buffer.

To make a visible alarm available to the PLC over cyclic PROFINET, use one of the following telegrams:

6.1 Free-telegram 999 mapping for STO diagnostics

1. In the G120 device view, open "Telegram configuration".
2. Set the receive and send PZD count to 4/4 (or whatever your application tolerates).
3. For the send direction, add a connector input: open the slot, select r2122 (active alarm word) as the source. Bit 0 corresponds to the "External alarm 1" line driven by p2112[0] — which is the one we wired in §5.
4. Compile, download the HWCN. The PLC will now receive bit 0 of the configured PZD set high whenever the STO-driven alarm is active.

7. Verification Matrix

8. Selecting Alarm vs Fault: Application Guidance

Choosing between p2112 (alarm) and p2106 (fault) is a process-design decision, not a safety decision.

• Use an alarm (A-class) when the application should be able to resume immediately after the safety circuit is re-armed. Typical for machinery that is expected to cycle through E-stops during normal operation (e.g., a guarded cell with frequent operator entry). The drive never enters a latched fault state; the HMI receives a record that the stop happened.
• Use a fault (F-class) when the operator or maintenance personnel must acknowledge the safety event before the machine can restart. Typical for installations where the STO event is treated as an incident that requires a documented reason for restart (food & beverage, regulated process lines). The fault latches and must be acknowledged either locally on the BOP-2 or remotely via the PLC (set STW1.7 rising edge after ZSW1.3 returns).

Do not wire the same r9773.1 bit to both p2106 and p2112 simultaneously. The fault reaction is dominant; the alarm will be suppressed while the fault is active, producing an inconsistent HMI display. Use a single sink per signal.

9. Alternative: Read the Safety Status Word Directly

If the goal is only to inform the PLC (not the operator), the cleanest path is to use Telegram 352 and read the safety status word 1 (S_ZSW1) bit 0 ("STO active"). This is the safety-validated status and is documented in the SINAMICS G120 Safety Integrated Function Manual. No BECO wiring is required, and the bit is guaranteed to be consistent with the actual STO state on both processor cores.

Recommended when the PLC is a fail-safe controller (F-CPU in TIA Portal) that already has a safety program and can act on S_ZSW1.0 directly inside a safety FB. In that case, leave p2106/p2112 unwired and use the safety-validated path.

10. Common Pitfalls and Edge Cases

1. Bit-mask mismatch after firmware update. A firmware upgrade (for example 4.6 → 4.7) can re-layout r9773. If the alarm stops appearing after an update, re-verify the bit by reading r9773 online and re-wire p20080 if necessary.
2. Drive reports the alarm but the PLC does not see it. Confirm the telegram length. If the project was upgraded but the device was not re-downloaded, the G120 may still be on Telegram 1 while the PLC expects 4 PZD. Re-download HWCN.
3. Alarm appears, drive refuses to start even after STO clears. This means p2106 (fault) was wired instead of p2112 (alarm). F07860 is latched. Send acknowledge via p2103[0] = STW1.7 from the PLC, or clear the fault locally.
4. Two errors with the same number appear. p2106 and p2112 are index-based. Make sure you wired index 0 of each. Index 1 and higher are for additional external fault/alarm sources if you need them.
5. Safety commissioning password. Wiring p20080 and p2106/p2112 does not require the safety commissioning password (p0010 for Safety Integrated is only p0010 = 95). The free binector and the external fault/alarm sources are standard parameters. The safety status source r9773 is a read-only safety diagnostic output that is visible at all access levels.
6. Stale message in the BOP-2 after RAM-to-ROM. A07850/F07860 is generated by the live interconnect state, not stored. A power cycle will clear the message from the display but the wiring remains in non-volatile memory.

11. Related Parameters Quick Reference

12. Diagnostic Flow (SVG)

13. Commissioning Checklist

• Drive is in "Ready to switch on" before E-stop is pressed.
• STO function test passed per SINAMICS Safety Integrated Function Manual test interval.
• p20080 = r9773.1 downloaded and saved to ROM.
• p2112[0] (alarm path) or p2106[0] (fault path) = p20080.
• Telegram upgraded to 352/354/999 if PLC needs the message bit cyclically.
• BOP-2 displays the expected message number within one update cycle after STO assertion.
• PLC receives the expected bit pattern on the configured PZD.
• Restart behaviour matches the design (alarm clears automatically; fault requires acknowledge).

14. References in the Field Manuals

For further detail, refer to the SINAMICS G120 CU240E-2 Operating Instructions, the List Manual A5E33839529 (edition 09/2017) cited in the field report, and the Safety Integrated Function Manual. Telegram structure for Siemens drives is documented in the PROFINET/PROFIBUS telegram reference applicable to the SINAMICS family.