A logic device for the transformation of the output of the RDC into series A-B pulses according to the present invention comprises: a chip with its two minimal bits (LSB-LSB-1) simulating as the clock source of A-B pulses, a signal-processing pin (BUSY) which output a TTL pulse wave as a control signal resource while LSB shifts from a low level to a high level and contrariwise, and a logic processing circuit as well as a buffer to output the displacement and velocity of a rotor in the form of A-B serial pulses.