Gadgets Mechanical Digitizing Tablet 1984 One of the problems both the pen and the ruler had was that there was no way to do irregular shaped perimeters. Everything had to be broken up into squares. It consisted of 4 phototransistors wired to a parallel port. a computer program could read when light shined on them. It worked on a pretty simple principle. a spinning disk with a slot could pinpoint the location of the arms of a pantograph that lay on the blueprint. The first model worked well for 18x24 blueprints. The resolution was about 1/16 of an inch. A large advance with this was the ability to have menus you could click on to bring information from the blueprint onto the computer. For example after I drew the location of a set of joist, I could click on a phrase like M/BEDROOM which would annotate the material list with the location of its use.