NATIONAL INSTRUMENTS NI 9421 is C Series 8-channel sinking digital input module for CompactDAQ and CompactRIO. It supports 24VDC logic signal, max input delay 100µs, optional screw/spring terminal or 25pin D-SUB, IP40 rating and -40~+70℃ wide operating temperature. It reads signals from NPN sensors and limit switches with isolation protection and channel LED indicators, configurable in LabVIEW and NI-DAQmx. Optional conformal coated version available, widely used for automated test and industrial discrete signal monitoring system retrofit and maintenance.