Resolving SINAMICS G120 CU250S-2 PN Drive Enable Failure

1. Problem Overview

The SINAMICS G120 drive with a CU250S-2 PN Control Unit (catalog number family 6SL3246-0BA22-1FA0) refuses to enable during commissioning. With Startdrive V14 SP1 Update 6 under TIA Portal V14 SP1 Update 6, an engineer clicks Activate on the Control Panel, the Set button briefly enables for approximately one second, and then disables again. No fault or alarm is shown in the active buffer. The motor identification routine never completes, leaving p1300/p1910/p1960 unable to write back their measured values.

This symptom is not a wiring defect and is not a motor-data problem. It is the classic behavior of a drive whose enable stack is incomplete at the moment the Control Panel requests master control + ON. The single most common cause is r46.8 = "Safety enable missing", which the Control Panel does not override by design.

2. Root Cause Analysis

The CU250S-2 PN evaluates an internal enable stack every millisecond. The status of this stack is exposed in parameter r46 (CO/BO: Missing enable signal / enable diagnostics). Each bit in r46 indicates which prerequisite is blocking the ON command:

When the Control Panel "Set" button is pressed and the drive enables for one second then disables, the trace of r46 immediately before disable reveals the blocking bit. In 80% of CU250S-2 PN commissioning problems, the trace shows r46.8 = 1.

2.1 Why the Drive Briefly Enables

Clicking Activate causes Startdrive to issue STW1 = 0x047E (master control acquired, no fault, ramp enable, setpoint enable, OFF1=1, OFF2=1, OFF3=1, enable inverter=1). The drive moves to the Operation enable state for the time it takes the safety chain to re-evaluate STO and PROFIsafe. If the safety answer is "no enable" the inverter drops the pulse enable within one scanner cycle (typically 4–16 ms), and the Control Panel reads disabled within the next polling interval.

3. CU250S-2 PN Control Unit Architecture

The CU250S-2 PN is the Safety Integrated capable variant of the G120 family. It supports the following integrated safety functions:

• STO – Safe Torque Off (SIL 3 / PL e per IEC 61508 / ISO 13849-1)
• SS1 – Safe Stop 1 (time-controlled and acceleration-monitored variants)
• SLS – Safely Limited Speed
• SDI – Safe Direction
• SSM – Safe Speed Monitor

These functions are enabled through two parallel signal paths:

1. Onboard fail-safe digital inputs – F-DI 0 / F-DI 1 wired to the EP terminals of the CU (terminals X131 / X132). Used when p9601.0 = 0 and p9601.1 = 1.
2. PROFIsafe over PROFINET – Safety control word S_STW1 / safety status word S_ZSW1 in telegram 30, 31, 901, or 902. Used when p9601.0 = 1 and p9601.2 = 1.

If the Safety Integrated configuration is left at the default extended configuration (Startdrive wizard > "Configure Safety Integrated" > "PROFIsafe" selected) but no F-host is online, the safety telemetry contains 0x0000 and the drive interprets this as STO active. The drive is correct; the configuration is wrong.

4. Reading r2 and r46 During Commissioning

Use Startdrive's Commissioning > Parameter View or the Starter/Startdrive trace to monitor the following parameters live:

Add the r46 parameter to a watchlist or trace. Capture it at 8 ms cycle time. Trigger on the rising edge of r46.0 to capture the failure snapshot.

5. Safety Integrated Configuration Walkthrough

5.1 Choose the Safety Source

In Startdrive's project tree, open SINAMICS G120 > Control Unit > Safety Integrated > Configuration. Three decisions are required:

5.2 STO via Terminal Wiring (CU250S-2 PN Only)

If the application genuinely uses onboard STO:

1. Wire F-DI 0 (terminal X131.1) and F-DI 1 (X131.2 or X132 depending on variant) to 24 V DC during normal operation.
2. Set p9601.1 = 1.
3. Set p9650 (STO debounce time, 1–30000 ms) and p9651 (discrepancy time, 1–30000 ms).
4. Run Acceptance test in Startdrive under Safety Integrated > Acceptance.
5. For bench commissioning without the safety PLC or wiring, temporarily set p9620 = 0 (STO permanently de-selected) until commissioning completes.

5.3 STO via PROFIsafe

The PROFIsafe telegram must be added to the PROFINET subslot configuration. In TIA Portal:

1. Open the G120 device view > Device configuration > PROFINET interface.
2. Add the SINAMICS F-module to slot 0 or as a separate subslot.
3. Set the F-source address and F-target address to match p9610 and the F-host configuration.
4. Generate the safety program in the F-CPU with SFDCSPL blocks providing S_STW1 to the G120.
5. Ensure S_STW1 bit 0 (STO) and bit 1 (SS1) are set to 1 in normal operation. Bit 0 = 0 always requests STO.

If the F-CPU is not loaded yet or is in stop mode, the drive receives S_STW1 = 0x0000 and r46.8 = 1.

6. Control Panel Master Control Acquisition

The Control Panel must own the master control bit (STW1.10) before it can issue ON commands. Parameter p8544[0] defines the signal source that releases master control back to the fieldbus. For Control Panel operation, set:

This forces the inverter to interpret the master control release bit (STW1.10) as active from PLC only when 1. For the Control Panel path, Startdrive automatically toggles p8544 internally; however, if p8544 has been overwritten by a previous commissioning, the Control Panel will fail to acquire master control. Verify:

// Ladder logic equivalent — not used by Startdrive, but illustrates the BICO signal
// p8544[0] = r2090.10 if you want fieldbus bit 10 to control master control
// Set to 0 to deactivate the override (always give master control back)

The state of master control is visible in r8511.10 ("Fieldbus master control active") and r8511.12 ("Control Unit master control active"). During Control Panel operation, r8511.10 should toggle.

7. Step-by-Step Troubleshooting Procedure

1. Connect online in Startdrive > Online > Connect to target system.
2. Open parameter view and add r46 to the watchlist. Cycle time 8 ms.
3. Read p9601. If any bit is 1, Safety Integrated is enabled and will block unless r46.8 receives a valid signal.
4. For first commissioning without safety wired:
   1. Set p0010 = 95 (Safety Integrated commissioning).
   2. Set p9601.0 = 0 and p9601.1 = 0.
   3. Set p9620 = 0 (terminal STO permanently de-selected).
   4. Set p9612 = 0 (no PROFIsafe response expected).
   5. Set p0010 = 0 to exit commissioning.
   6. Copy RAM to ROM (p0977 = 1) or click Save to memory card.
   7. Power-cycle the CU to commit safety parameter changes.
5. Verify r46.8 = 0 with no active command. All r46 bits should be 0.
6. Open Control Panel and click Take master control. Confirm Drive active indicator shows the panel as commander.
7. Click Activate. The Set button should now enable and remain enabled for the duration of the identification. r46 bits remain at 0.
8. Run motor data identification: p1900 = 2 (rotating measurement) or p1910 = 1 (static measurement) and start.
9. Save results: After identification completes, the Control Panel prompts to save. Click Save (RAM to ROM).

8. Control Word and Status Word Reference

The standard PROFIdrive telegram (e.g., telegram 1) carries the following bits:

The status word ZSW1 (r2) provides the actual state. Key bits:

A drive that briefly enables and then disables shows ZSW1 transitioning through 0x0E37 → 0x0E33 → 0x0E31 → 0x0E07. The intermediate 0x0E33 / 0x0E31 indicates the Safety Integrated demand.

9. Verification Tests

After the corrective action, run the following sequence to confirm the drive is healthy:

1. Static read: Read r46 = 0x0000 with no command active.
2. Control Panel test: Take master control, click ON, observe ZSW1 bit 2 = 1 for at least 30 s.
3. Setpoint response: With 50% setpoint via the panel slider, verify n_actual approaches n_set without overshoot > 5%.
4. Stop test: Click OFF1 from panel. Drive decelerates to 0 within the configured ramp (p1121). ZSW1 bit 2 = 0.
5. Coast test: Click OFF2 from panel. Pulses are removed immediately. Motor coasts.
6. Quick stop test: Click OFF3. Drive decelerates using p1135.
7. Fault reset: Trigger a fault by setting p2100[0] = 0 and p2101[0] = 0 (induces F0001 / overcurrent). Click Acknowledge on the panel. Fault clears.
8. Parameter save: Execute RAM to ROM (p0977 = 1). Power-cycle the CU. Confirm drive re-initializes with the saved parameters.

10. Common Pitfalls and Edge Cases

11. Recovery After Enabling Safety Integrated for Production

Once bench commissioning is complete, re-enable Safety Integrated for the production system:

1. Set p0010 = 95.
2. Set p9601 per the actual wiring (terminals or PROFIsafe).
3. Set p9610 = F-host PROFIsafe address.
4. Set p9611 = required telegram (30 default).
5. Set p9620 = correct BICO source for STO if using terminals.
6. Set p9650 (STO debounce, default 1 ms) and p9651 (discrepancy, default 100 ms) per machine risk assessment.
7. Exit commissioning: p0010 = 0.
8. Save to ROM and power-cycle.
9. Perform the acceptance test from Startdrive and archive the signed report.

12. Related Parameters Quick Reference

For the complete parameter list of CU250S-2 PN, refer to the SINAMICS G120 List Manual in the Siemens Industry Online Support documentation portal.

13. Diagnostic Flowchart (SVG)

14. Reference Documentation

• SINAMICS G120 Operating Instructions — CU250S-2 PN commissioning, terminal wiring, telegram selection
• SINAMICS G120 List Manual — full parameter reference including r46, r2, r977
• TIA Portal S7-1500 Manual Collection — Configuring SINAMICS G120 — Startdrive project integration