STO – switching delay

What does F20.11 mean?

A switching delay has been detected between the two Safe Torque Off (STO) channels. This indicates a potential malfunction within the safety circuit, which compromises the safety integrity of the drive and risks uncontrolled motor movement if not resolved.

• Asymmetrical response times of the two safety relays or output transistors within the STO circuit.
• External wiring impedance differences or capacitance imbalances between STO input channels (e.g., X12A and X12B).
• Degradation or fault in internal STO monitoring hardware (e.g., timing circuits or comparators).
• Incorrect STO wiring causing differential voltage drop or signal skew between the two channels.
• External safety contactors or relays connected to STO inputs exhibiting excessive bounce or slow operation.

Click steps to track your progress.

1. 1

   Disconnect power. Measure the resistance and capacitance of the external wiring connected to each STO input channel (e.g., terminals X12A and X12B) to ensure they are matched.

2. 2

   Use an oscilloscope to simultaneously monitor the switching times of STO input signals (e.g., at X12A and X12B) and compare the rising/falling edge delays.

3. 3

   Verify that the configured STO delay time parameters (e.g., Parameter P6-20, P6-21) are within the safety system's specifications.

4. 4

   Test the external safety circuit components (e.g., safety relay outputs) for proper and synchronous switching operation using a multimeter or scope.

5. 5

   Perform a safety circuit functional test according to the machine's safety manual, observing the STO activation and deactivation sequence with a stopwatch.