A digital input interface is provided which can be checked for its reliability. The configuration of the circuit on the input side allows a high impedance for a DC input signal and a low impedance for induced AC noise, naturally attenuating any AC induced noise while maintaining the DC input signal. The interface also provides a latent failure detection engine. The latent failure detection engine can open and close an optocoupler on the input side of the interface, which discharges and charges a capacitor on the input side. The time taken for the capacitor to recharge when the optocoupler is re-opened is used to determine if there has been any threshold decay in the interface.