How many plug-ins can you add to your personal computer?

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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