The TDR command in the SX user guide states that it sets the debounce time for the registration input. This is true but a better way putting it may be:
It specifies a lockout time after the first edge of the input is seen to prevent another registration input from triggering an additional move on top of the original registration. It does not specify the time the input must be active for it to be a recognizable input. Registration inputs can be very short in duration, almost as short as some noise spikes. If your application has a problem with noise triggering registration inputs you should concentrate on eliminating the noise. This can be done with isolating, shielding or filtering. A quick fix might be to add a 0.01 microfarad capacitor between the Registration input and ground.