IEEE 1394 apparatus capable of making power class coincide even at the change over of a power supply to be used

An IEEE 1394 apparatus connected to a serial bus and being operable using power supplied from said serial bus or a its own power supply, said apparatus comprising:

• voltage detection means for determining whether or not an output voltage of said power supply of its own is present and outputting the result of the determination;
• code generation means for generating, based on the result of the determination of said voltage detection means, a code indicative of power class information which is information representing which one of the power supplied from said serial bus and said power supply of its own is used for operation of said IEEE 1394 apparatus;
• changing point detection means for detecting a change of the result of the determination output from said voltage detection means;
• and
• a physical layer circuit for being reset with an output signal from said changing point detection means to vary the connection of said serial bus to perform bus resetting and for placing, when self-identification is performed in response to the occurrence of bus resetting, the code generated by said code generation means and indicative of the power class information into a Self-ID packet to be used for transmission of a result of the self-identification to a bus manager and outputting the Self-ID packet.