Generally, a computer system includes a computer main body (hereinafter referred to as a ‘computer’) for handling information, and various peripheral devices for processing the information. 
The construction of such a computer system is described with reference to FIG.  1. 
As an input means for inputting information to a computer main body 1, there are a keyboard 2, having a plurality of keys, and a mouse 3, which is usefully used for graphic design and the like.  Furthermore, a monitor 4, which is a display device for displaying information transmitted from the computer main body 1 on a display screen, is used as an output means. 
As peripheral devices of the computer system, a modem, a printer, a sound device and a scanner are used.  Each of the peripheral devices must occupy a port acquired through a slot available on the mother board of the computer main body 1.  In this case, a user must open the case of the computer main body 1 and insert an interface card for a device, which is to be assigned to a slot, into the board.  It is occasionally required to manipulate switches, set jumpers, or set the type of connector, for example, a serial type or a parallel type.  Due to such a process, the case where a general user abandons the installation of his or her new devices occurs frequently.  Furthermore, the number of slots in the computer is limited, and thus additional peripheral devices cannot be used once all of the slots are occupied by peripheral devices. 
In response to a demand for support of a large number of peripheral devices, which are more convenient to use, the specification of Universal Serial Bus (hereinafter referred to as ‘USB’) has been developed.  If such USB is used, it is not necessary for the user to open the case of his or her computer in order to connect peripheral devices or related cards to the computer.  Accordingly, providing additional peripheral devices becomes as easy as inserting a desk lamp. 
A device such as a keyboard or a monitor is directly connected to the computer, but other peripheral devices can easily be connected to the computer via an extension hub provided in the keyboard or the monitor, or via an independent USB device.  Such extension hubs may provide additional connection sockets and may be connected to each other in the form of a hierarchical tree.  Respective peripheral devices may be spaced apart from each other or from extension hubs by several meters. 
Such USB enables a total of 127 different devices to be connected to a single computer.  The USB enables the transmission of an operational voltage of 5V, so that it can eliminate a problem that a large number of currently used peripheral devices consume a large amount of AC power.  Furthermore, a fast data transmission rate of 480 Mbits/sec provides extensive processing capability to most peripheral devices having a high bandwidth at only the cost required for current connector technology. 
Based on the fact that system and peripheral device manufacturers are adopting such USB, the USB will be a major feature of computer systems in the future.  As examples of devices to be connected using the USB, there are a telephone network, a modem, a printer, a microphone, a mouse, a scanner and a digital camera. 
The chief effects of such USB are simplicity and convenience.  The USB receives information from the computer, and detects the information when devices are additionally provided or removed.  Unlike a different typical internal slot, such tasks can be performed in a state in which power is turned on, and do not require the system to be rebooted. 
Furthermore, real plug-and-play operation is supported.  The USB automatically determines information about resources, such as driver software and bus bandwidth, which are necessary for each peripheral device, and acquires the information so as to be used without the user's interference. 
FIG.  2 shows the construction of a computer system to which a USB device is applied. 
Referring to FIG.  2, a computer main body 1, a monitor 4, and peripheral devices, such as a printer 5, a scanner 6 and an external modem 7, are disposed around the USB device 10. 
FIG.  3 shows the USB device in greater detail. 
Referring to FIG.  3, the USB device 10 includes a USB control circuit 11 for controlling the transmission and reception of information between the computer 1 and the peripheral devices using data and a clock signal, which are provided from the monitor 4, a DC-DC converter 12 for providing operational power to a USB control circuit 11 and downstream ports 14 to 16, which are connected to the respective peripheral devices 5 to 7, by processing power supplied from the monitor 4, and an overcurrent protection circuit 13 for preventing damage caused by the occurrence of overcurrent, by detecting the output current of the DC-DC converter 12. 
The DC-DC converter 12, which receives a power of 14 V from the monitor 4, is used to supply operational power (5 V) to the USB control circuit 11.  In this case, the monitor 4 and the USB control circuit 11 are configured such that the data and the clock signal can be transmitted and received therebetween. 
The power of 5V, which is processed through the DC-DC converter 12, is provided to each of the peripheral device 5 to 7 of FIG.  2 via the USB control circuit 11 and the downstream port 14 to 16. 
Meanwhile, the computer main body is connected with the upstream port (not shown) of the USB device 10.  In this case, it is required that a USB use environment for the computer main body 1 be set.  For this purpose, the computer main body 1 automatically supports a USB control environment. 
When the peripheral devices 5 to 7 of FIG.  2 are connected to the downstream ports 14 to 16 of the USB device 10, IDentifications (IDs) that are registered in the computer main body 1 are checked.  When no error occurs, automatic installation is performed.  Accordingly, the user makes use of the peripheral devices without any additional action. 
The above-described peripheral devices can operate while transmitting and receiving data to and from the computer main body only when they are plugged in the downstream ports of the USB device 10 and are in a preparation completion state, in which their application programs can be used. 
However, in the case where the peripheral devices are plugged into the USB device 10 but are not in a preparation completion state, in which their internal application programs can be used, the respective peripheral devices are displayed on the screen of the computer in the form of icons, but the actual operations corresponding thereto cannot be performed.  This is misunderstood by the user as faulty operation and, furthermore, power is consumed in the case.  Additionally, in the case where the peripheral devices are not plugged into the USB device 10 but the peripheral devices are in a preparation completion state, in which their internal application programs can be used, power is consumed even when such actual operations are not performed.