A125 speed feedback alarm in ABB DCS800 drive indicates an issue with the speed feedback signal from the motor. This means the comparison of the speed feedback from pulse encoder or analog tacho has failed.

Troubleshooting Steps:

1. Verify that the sensor is securely mounted and aligned.
2. Visually inspect the tachometer sensor for damage or debris.
3. Check the wiring connections between the tachometer sensor and the drive.
4. Review the tachometer parameter settings, including scaling factors, filter settings, and input type.