The invention provides a digital quantity output unit and a digital quantity output board card. The digital quantity output unit comprises a first power generating coupler, a light emitting diode, a relay, a second power generating coupler, and a phase inverter. When the port voltage is zero, the first power generating coupler can be conducted, and the light emitting diode can be switched on, and thereafter the current flows through the relay coil, and then the second power generating coupler can be conducted, and then after the current flows through the phase inverter, the feedback signal pin FBI end voltage is 0. When the port D1 end voltage is 3.3 V, the first power generating coupler is cut off, and the light emitting diode can be switched off, and at the same time, the current in the relay coil is 0, and at the same time, the second power generating coupler is cut off, and after the current flows through the phase inverter, the feedback signal pin FBI end voltage is 1. The control unit can be used to determine whether the fault of the digital quantity output occurs according to the FBI end voltage, and therefore the stability and the reliability of the digital quantity output board card can be improved, and the safety operation of the train can be guaranteed.