A signal transfer system. A first device operates with a first voltage and outputs a first signal and a second signal. A protection circuit receives the first and second signals and outputs the first and second signals when the first voltage is greater than or equal to a predetermined voltage, and provides a third signal and a fourth signal when the first voltage is smaller than the predetermined voltage. A delay circuit delays the second and fourth signals to generate a first delay signal and a second delay signal, respectively. A second device operates with the first signal and the first delay signal when the first voltage is greater than or equal to the predetermined voltage, and operates with the third signal and the second delay signal when the first voltage is smaller than the predetermined voltage.