PURPOSE: To read coordinates at low cost with a small error by arranging electrodes at four sides of a plane resistor, applying out-of-phase rectangular wave singals to two facing sides from a tri-state buffer driver etc., and disconnecting other sides and detecting a signal.
CONSTITUTION: When an X coordinate is detected, the buffer drivers 12 and 13 connecting with the X-directional electrode D 1 of the plane resistor 1 are closed, and the buffer drives 14 and 15 connecting with the Y-directional electrode D 2 are opened to apply a signal f 1 (t) from the left and a signal f 2 (t) from the right. When an input pen 6 is put closer, a signal Vp detected electrostatically is passed through an amplifier 9 and a band-pass filter 10 to extract only a basic- frequency component Vf. A counter 21 measures the time difference between zero-crossing points of a driving rectangular wave f 1 (t) and a signal Vc obtained by converting the Vf into the binary signal through a comparator 20 to calculate the phase difference θ, which is converted into a position in a memory ROM22 and outputted. When an Y coordinate is detected, the buffer drivers 12 and 13 are opened and the drivers 14 and 15 are closed to calculate the coordinate similarly.