In data communication among a plurality of component units in an apparatus, transition
state information corresponding to a state transition which has occurred in a sending
side unit is sent to a receiving side unit in serial communication. In the receiving
side unit, a state control signal is regenerated at the time when a given delay
time has elapsed from the occurrence time of the state transition. Therefore, the
state control signal can be sent in serial communication without any time skew.
Also, in serial communication between the sending side unit and the receiving side
unit, even if a control command signal is to be sent, in addition to the state
control signal, a transition state information can be identified from the received
signal, by means of adding identification information to the sending information.
