USB Ports

A USB Port is a port that supports the Universal Serial Bus (USB) standard, which is a serial bus standard and used to interface devices. It is increasingly being used in desktop computers, notebook computers, video game consoles, cellphones, televisions, mp3 players, portable memory devices, and PDAs. Wireless USB ports support wireless applications like wireless networking in wireless notebooks, etc. The USB Implementers Forum (USB-IF), an industry standards body, has standardized the design of USB. The versions of USB are 0.9, 1.0, 1.1, and the current 2.0. Full backward compatibility is a plus point of all USB versions.

Peripherals that can be connected with USB port

Notebook/ desktop external peripheral devices like mouse, keyboard, joystick, game pad, scanner, printer, digital camera, networking components, and external storage media can be connected to the laptop notebook via the USB port. Non-networked printers can also be connected thereby replacing the parallel ports which printers widely use.

Peripherals that cannot be connected with USB port

Laptop/ desktop external display devices like monitors and high-quality digital video components cannot be connected with the USB due to their higher data rate requirement.

Number of connections created with a USB port

Due to its asymmetric design, a USB system has a host controller that can allow multiple daisy-chained devices. Branching into a tree structure is possible with the addition of USB hubs. A maximum of 5 levels of branching per host controller is allowed. A maximum number of 127 devices inclusive of the USB hubs, can be connected to a single host controller. The host controller is connected with a single USB hub known as the root hub, and thereafter a tree structure from this root hub can be built. No termination of USB cables is required. The maximum length of a USB cable is 5 meters. For greater lengths, USB hubs may be used.

Transfer speed of USB

Three speeds are supported, namely low speed, full speed, and hi-speed of data transfer.

• Low speed is 1.5 Mbit/s and is usually used for keyboards, mouse, game pads, joysticks, and other Human Interface Devices (HID).
• Full speed is 12 Mbit/s and is used by USB hubs.
• Hi-speed is 480 Mbit/s and all devices using this speed are known as USB 2.0 compliant devices. Since USB supports full backward compatability, a hi-speed USB hub has a device known as the Transaction Translator which differentiates the bus traffic into low speed, full speed, and hi-speed. This segregation applies only with respect to the bandwidth. Any number of transaction translators may be present in a USB hub, which varies amongst manufacturers.

Data encoding by USB

USB uses the NRZI (non-return to zero, inverted) encoding method to encode data. In this method, logic 1 signal is sent unchanged as it is, but for each change, the signal level is inverted to at logic 0.

Types of USB connectors

1. Original USB specified:
• Series "A" plug (cables have only plugs, hosts and devices have only receptacles)
• Series "A" receptacle
• Series "B" plug
• Series "B" receptacle
2. USB 2.0 specified:
• Mini-B plug (black)
• Mini-B receptacle (black)
3. On-The-Go supplement to the USB 2.0 specified:
• Mini-A plug (white)
• Mini-A receptacle (white)
• Mini-AB receptacle (grey)
• Mini-A receptacle to Standard-A plug (adapter)
• Standard-A receptacle to Mini-A plug (adapter)

The plastic inside Mini-A plugs and receptacles is always white, in Mini-B plugs and receptacles is always black, and in Mini-AB receptacles is always grey.