What causes Yaskawa error SToF?

One Safe Disable input channel wire (H1 or H2) is short-circuited to common (HC) while the other channel's wire is open-circuited, indicating a wiring fault.. Internal failure within an external safety device (e.g., safety relay, E-stop contactor) causing one output contact to stick open and another to stick closed.. Damage to the drive's internal Safe Torque Off (STO) input circuitry for H1-HC or H2-HC, potentially from overvoltage or incorrect wiring during commissioning.. Contamination, moisture ingress, or corrosion on the drive's H1, H2, or HC terminals creating an unintended short or open circuit.. Incorrect initial wiring where the Safe Disable circuit has an accidental short (e.g., to HC) on one channel and an accidental open on the other.

How do you fix Yaskawa fault code SToF?

1. Power off the drive. Disconnect all wiring from drive terminals H1, H2, and HC. 2. Using a multimeter, measure the resistance between drive terminal H1 and HC. Then measure the resistance between H2 and HC. Verify both show an open circuit (infinite resistance) when no external wiring is connected. 3. Inspect the removed cables for physical damage (e.g., pinched wires, insulation breaks). Use a multimeter to check continuity for each wire from the safety device to the drive's connector, and verify no short circuits exist between H1-HC, H2-HC, H1-H2, or to earth (PE). 4. Reconnect wiring. With the external safety function enabled (e.g., E-stop released, gate closed), measure the DC voltage between H1 and HC. Then measure the DC voltage between H2 and HC. Both should typically show 24VDC (refer to drive manual for expected voltage). 5. If voltages are inconsistent, isolate the external safety device (e.g., safety relay). Test its output contacts for continuity/resistance (e.g., between its designated terminals for H1 and HC, and H2 and HC) when the device is active and inactive to confirm proper switching behavior. 6. If all external wiring and safety device tests pass, perform a factory reset of STO-related parameters (e.g., P7-03 "STO Enable" if applicable) and re-enable the STO function. If the fault persists, arrange for drive service or replacement.

SToF

Safe Disable Signal Fault

Yaskawa · GA700

What does SToF mean?

This fault indicates an inconsistency in the Safe Disable input channels (H1-HC and H2-HC), where one channel is open and the other is short-circuited. This condition signals a potential issue or damage within either the Safe Disable circuit or the drive itself, which compromises the 'Safe Torque Off' safety function. It requires immediate troubleshooting to restore the safety integrity of the system.

Common Causes

One Safe Disable input channel wire (H1 or H2) is short-circuited to common (HC) while the other channel's wire is open-circuited, indicating a wiring fault.
Internal failure within an external safety device (e.g., safety relay, E-stop contactor) causing one output contact to stick open and another to stick closed.
Damage to the drive's internal Safe Torque Off (STO) input circuitry for H1-HC or H2-HC, potentially from overvoltage or incorrect wiring during commissioning.
Contamination, moisture ingress, or corrosion on the drive's H1, H2, or HC terminals creating an unintended short or open circuit.
Incorrect initial wiring where the Safe Disable circuit has an accidental short (e.g., to HC) on one channel and an accidental open on the other.

Repair Steps & Checklist

Click steps to track your progress.

1
Power off the drive. Disconnect all wiring from drive terminals H1, H2, and HC.
2
Using a multimeter, measure the resistance between drive terminal H1 and HC. Then measure the resistance between H2 and HC. Verify both show an open circuit (infinite resistance) when no external wiring is connected.
3
Inspect the removed cables for physical damage (e.g., pinched wires, insulation breaks). Use a multimeter to check continuity for each wire from the safety device to the drive's connector, and verify no short circuits exist between H1-HC, H2-HC, H1-H2, or to earth (PE).
4
Reconnect wiring. With the external safety function enabled (e.g., E-stop released, gate closed), measure the DC voltage between H1 and HC. Then measure the DC voltage between H2 and HC. Both should typically show 24VDC (refer to drive manual for expected voltage).
5
If voltages are inconsistent, isolate the external safety device (e.g., safety relay). Test its output contacts for continuity/resistance (e.g., between its designated terminals for H1 and HC, and H2 and HC) when the device is active and inactive to confirm proper switching behavior.
6
If all external wiring and safety device tests pass, perform a factory reset of STO-related parameters (e.g., P7-03 "STO Enable" if applicable) and re-enable the STO function. If the fault persists, arrange for drive service or replacement.