The following table lists the main I/O definitions of the Cosmac Elf in a simple configuration, using a hex keypad, Pixie (CDP 1861) video and speaker connected to the Q flag. Note that I/O would off course only work if specified HW is connected.
Speaker, where frequency of switching Q gives a square wave tone
CDP 1861 display: display status
If EF4=0 IN button is pressed
CDP 1861 display: display off
TIL 311 data display
CDP 1861 display: display on
Hex panel input