It must not be assumed that if a PC has 2 USB ports, you could connect 254 devices (127 x 2). This is not possible since each USB controller can only support a maximum of 127 devices. To install more than the maximum, you will have to install additional USB ports and bus controller hardware via the use of adapter cards.
USB signals normally travel approximately 16 feet before they begin to degrade.
USB is an external bus standard designed for enhancing plug and play capabilities for attaching peripherals to your PC.
A very important feature of this bus is the elimination of the need for special-purpose ports and special-purpose I/O cards.
USB ports also save important system resources such as interrupts (IRQs) and, regardless of the number of devices attached to them, only one IRQ is required.
Intel first introduced the USB architecture and all its motherboard chipsets since the PIIX3 South Bridge chipset (February 1996) have USB as a standard feature.
Other chipset manufacturers have also followed and made USB ports a standard feature of today's PCs - just like the serial and parallel ports.
The USB 1.0 standard was released in January 1996, followed by USB 1.1 in September 1998. It was designed to run at 12Mbit/sec (1.5MB/sec) over a four-wire connection.
The USB 1.1 was upgraded to USB 2.0 which ran 40 times faster and was fully backward compatible.
Older computers without built-in USB ports can be upgraded by installing an add-on PCI card (desktop computers) or a PC card on a Cardbus-compatible laptop computer.
USB 2.0 (Hi-Speed) Features
As mentioned earlier, your computer is the host and up to 127 devices can be connected directly or indirectly with the use of hubs.
Individual cables can be a maximum of 5 meters long and, if using hubs, devices can be as much as 30 meters from the host PC.
USB 2.0 bus has a maximum data rate of 480 Mb/s.
A USB cable consists of 4 wires - 2 for power (+5 volts and ground) and a twisted pair for data.
The computer can provide up to 500 mA of power at 5 volts to the power wires.
Devices such as mice that are low-power can get power directly from the bus meanwhile high-powered devices such as printers use minimal power from the bus because of having their own power supplies. There are also powered hubs that provide power for connected devices.
Most USB devices are hot-swappable which means they can be plugged and unplugged when the computer is powered on.
Any device connected to the USB port receives power and data through the USB cable.
USB 3.0 (Superspeed)
This version is 10 times faster than USB 2.0 which means it has a data rate of 4.8 Gb/s.
Even though the USB 3.0 cable at first glance may appear different, it will be backward compatible with USB 2.0.
Advantages of USB 3.0 (Superspeed)
Downloads and uploads will be kept in separate data lanes. With this dual-lane configuration, one lane will receive data while the other transmits which enables a significant speed boost while at the same time allowing read and write operations. Compare this with USB 2.0 which allowed data traffic in only one direction at a time.
USB 3.0 will be more power-efficient.
USB 3.0 will charge more devices faster. Power output was increased to approximately 900 mA which means you can power more than 4 devices from a single hub.
Inside a Typical USB Cable
There are two distinct types of USB connectors; namely Series A and Series B. The Series A connector is made for devices in which the cable remains permanently attached such as hubs, keyboards, mice, etc.
Series B connectors are made for devices that require detachable cables such as printers, scanners, modems, telephones, speakers, etc.
Diagram of Series A and B Male USB Connectors
Unlike serial or parallel connectors, USB connectors are small and the plug is not attached by screws. This makes USB devices very easy to install and uninstall.
Final Thoughts .......
USB ports are a standard on virtually all PCs today and is used for general purpose external high-speed interfacing.
If your present system does not have any of these ports, you can install a USB card in a PCI slot on your motherboard.