The DD 501 Digital Delay is an events counting plug-in unit designed for use in a TM 500-Series Power Module mainframe. Separate external trigger signals are connected to the EVENTS and START INPUT connectors which allows up to 99999 events to be counted. The plug-in unit counts a predetermined number of events, from 0 to 99999, which are selected by the front-panel thumb-wheel switches. The last event counted generates a trigger pulse to the DLY’D TRIG OUT connector on the front panel.