Hardware to Interface a Computek GT-50 Graphic Tablet with a Digital Computer

Circuitry has been developed to permit a Computek GT-50 Graphic Tablet to be easily hooked up to any digital computer having an analog-to-digital converter. The Computek Tablet indicates the successive locations of a special pen traveling across the tablet surface as a series of digital coordinates. The circuity described represents each new set of digital coordinates. The circuitry described represents each new set of digital coordinates as a deviation from the previous set. Each deviation is represented as an analog voltage. The resulting analog signal is subsequently converted by an analog-to-digital converter into the binary representation required for use with a digital computer. This method of working with only the deviations of the successive pen positions allows the bandwidth needed to describe the movement of the pen across the table to be reduced. The accuracy of the signal describing the pen movement is assured by maintaining a large analog output signal-to-noise ratio. (Author)