I don't understand, a rotary encoder is likely an incremental quadrature encoder, youcount pulses, not time them. I'd still do the standard thing of using interrupt drivenquadrature decoder, then in the main code you can do what you like with the encoderoutput (detect change and turn motor on/off, implement a PID loop to turn motor intoservo motor, measure speed, whatever you like).
My project is identical with thread http://forum.arduino.cc/index.php?topic=234667.0
My impression is that the project in that Thread fizzled out because it was too complicated for the OP.Is there a stepper motor in the printer you want to convert? Many (most?) new printers don't have stepper motors.What specific printer do you have?How many pulses per revolution does the encoder produce?Can you post a photo of the encoder?...R
Sory, encoder has 360 pulses per revolution, I haven't a osciloscope, this number is from other forum.
PF DC motor work immediately when printhead is in correct position, that I can use this signal from DC motor to input of Arduino. I must use a encoder, becouse from this, I can use signal for DIR (is very important in project).