The original Ascilloscope was a monstrous creation, built on a breadboard with discrete components. Connected to the parallell port of a computer running FreeBSD, it would sample voltages by integration and display the resulting waveform with ASCII graphics using ncurses.
The original schematics is lost, but supposedly there are pictures floating around on the internet.
2009-01-04: Jan_Goofy have search his pc and his Amiga, no pics avialable of the asciiscope :-(
2009-02-14: Rax posted some more pics, here's how it lookes like in action: