The "Switching on inhibited - STO terminal w/ 24 V (hardware)" fault in Siemens SINAMICS drives typically indicates missing 24V DC on the Safety Terminals (EP+ / STO) on the Power Module. Ensure \(24V\) is applied to EP terminals (e.g., X21:3 and X21:4 on booksize) and that the Control Unit (CU) is properly seated.

• Check EP Terminals (Power Module): Ensure 24V DC is present at the EP+ and EP- terminals (e.g., X21:3, 4 on PM240-2). Without this, the drive will show this fault even if Safety Integrated is not explicitly configured.
• Verify Wiring: Check for loose or broken wires in the emergency stop circuit connecting to the drive.
• Check CU Seating: Ensure the Control Unit (CU) is fully seated and properly connected to the Power Module (PM).
• Check Parameters: Verify that Safety Integrated functions are correctly parameterized for your application.
• Voltage Levels: Confirm the 24V supply is stable and not dropping below the threshold