Theoretically, one personal computer can be connected to 127
USB (Universal Serial Bus) devices. But practically it is somewhat different. Let us have a look into
it. As per the USB specifications, all devices are controlled by a host
controller, & this chip has a 7-bit wide address field, & thus it
should be able to recognize 128 addresses. Since the controller itself uses the
address ‘0’, a total of 127 addresses remain open for plug-in of other USB
devices. The USB architecture comprises of the host controller, hubs, &
actual USB peripherals, & host controllers are designed to be connected to
hubs, & other USB devices. Each device has at least one function, &
each function is assigned an individual address. In order to connect 127
devices to a personal computer, we will need a lot of hubs, & each of which
will be connected to these available addresses.
A USB pen drive only needs one address for mass storage
functions, while, multifunction printers require several addresses, one each
for print, scan, fax, & other features.
Further, each device is allowed to pull current between
100mA and 500mA at about 5V from the controller, which will cut off, if it
reaches its maximum capacity. Hence for example, the power consumption of 127
pen drives will be around 60W in toto. A hub that wants to host many USB
devices has to feed current from its own power supply. Thus, with common
bus-powered 4-port hubs, one will never be able to connect 127 devices
together.
Windows has a maximum of 25 drive letters available for new
drives/devices to be assigned. Over and above this, drives will remain
unrecognized even in the Disk Manager. In Windows 7, one can easily mount new
disks as NTFS (New Technology File System) folders rather than as a logical drive.
In a laboratory experiment, one personal computer was
connected to 97 pen drives through 15 hubs, where each hub had two controllers,
& each controller received one address. Here all the 127 addresses were
consumed, but the experiment was fruitless as it did not yield any desired
practical result.
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