USB tutorial

- a tutorial giving the essential technical details about USB, the Universal Serial Bus a widely used computer interface.

USB, or the Universal Serial Bus Interface is now well established as an interface for computer communications. In many areas it has completely overtaken RS232 and the parallel or Centronics interface for printers, and it is also widely used for memory sticks, computer mice, keyboards and for many other functions. One of the advantages of USB is its flexibility: another is the speed that USB provides.

USB provides a sufficiently fast serial data transfer mechanism for data communications, however it is also possible to obtain power through the connector and this has further added to the popularity of USB as many small computer peripherals may be powered via this. From memory and disk drives to other applications such as small fans and coffee cup warmers, the USB port on computers can be used for a variety of tasks.

USB evolution

The USB interface was developed as a result of the need for a communications interface that was convenient to use and one that would support the higher data rates being required within the computer and peripherals industries.

The first proper release of a USB specification was Version 0.7 of the specification. This occurred in November 1994. This was followed in January 1996 by USB 1.0. USB 1.0 was widely adopted and became the standard on many PCs as well as many printers using the standard. In addition to this a variety of other peripherals adopted the USB interface, with small memory sticks starting to appear as a convenient way for transferring or temporarily storing data.

With USB 1.0 well established, faster data transfer rates were required, and accordingly a new specification, USB 2 was released. Wit the importance of USB already established it did not take long for the new standard to be adopted.

With USB defining its place in the market, other developments of the standard were investigated. With the need for mobility in many areas of the electronics industry taking off, the next obvious move for USB was to adopt a wireless interface. In doing this wireless USB would need to retain the same flexible approach that provided the success for the wired interface. In addition to this the wireless USB interface needs to be able to transfer data at rates which will be higher than those currently attainable with the wired USB 2 connections. To achieve this ultra-wideband UWB technology is used.

USB capabilities

The basic concept of USB was for an interface that would be able to connect a variety of computer peripheral devices, such as keyboards and mice, to PCs. However, since its introduction, the applications for USB have widened and it has been used for many other purposes including, including measurement and automation.

In terms of performance, USB 1.1 enabled a maximum throughput of 12 Mbps, but with the introduction of USB 2.0 the maximum speed is 480 Mbps.

In operation, the USB host automatically detects when a new device has been added. It then requests identification from the device and appropriately configures the drivers. The bus topology allows up to 127 devices to run concurrently on one port. Conversely, the classic serial port supports a single device on each port. By adding hubs, more ports can be added to a USB host, creating connections for more peripherals.

USB Standards

USB is a standard that is being updated. Since its first introduction, the standard has been improved to meet the increasing needs of the user community. As a result there are a number of different USB standards, but fortunately these are backwards compatible.

1. USB1.1: This was the original version of the USB, Universal Serial Bus and was released in September 1998 after a few problems with the USB 1.0 specification released in January 1996 were resolved.. It provided a Master / Slave interface and a tiered star topology which was capable of supporting up to 127 devices and a maximum of six tiers or hubs. The master or "Host" device is normally a PC with the slaves or "Devices" linked via the cable.

One of the aims of the USB standard was to minimise the complexity within the Device by enabling the Host to perform the processing. This meant that devices would be cheap and readily accessible.

The data transfer rates of USB 1.1 are defined as:

* Low speed: 1.5 Mbps
* Full speed: 12 Mbps
The cable length for USB 1.1 is limited to 5 metres, and the power consumption specification allows each device to take up to 500mA, although this is limited to 100mA during start-up.

USB 1.1 does not allow extension cables or the inclusion of pass-through monitors (due to timing and power limitations).

2. USB 2.0: The USB 2.0 standard is a development of USB 1.1 which was released in April 2000. The main difference when compared to USB 1.1 was the data transfer speed increase up to a "High Speed" rate of 480 Mbps. However it should be noted that even though devices are labelled USB 2.0, they may not be able to meet the full transfer speed.

3. USB 3.0 : This improved USB standard which was first demonstrated at the Intel Developer Forum in September 2007. The major feature is what is termed the SuperSpeed bus, which provides a fourth transfer mode which gives data transfer rates of 4.8 Gbit/s. Although the raw throughput is 4 Gbit/s, data transfer rates of 3.2 Gbit/s, i.e.0.4 GByte/s more after protocol overhead are deemed acceptable within the standard. The standard is also backwards compatible with USB 2.0

USB connections and cables

The USB connector is remarkably simple having just four main connections for the data and power. In addition to this it is also possible to use extender cables. The maximum allowable length for an individual cable is 5 metres (3 metres for slow devices) and this allows the USB data acquisition module to be located remotely from the computer.

A USB cable has two forms of connector. These are designated the "A" and "B" connectors. The connections to the connectors are given below:

Pin Function
1 Vbus 4.75 - 5.25 V
2 Data -
3 Data +
4 Ground
Shell Screen

USB cable pin assignments

The connectors used for USB are designed to enable the power and ground connections to be made first applying power to the device before the signal lines are connected.

USB tutorial summary

With USB in almost universal use in new computers, a host of peripherals using the USB standard, its use is set to continue for many years to come. With the USB standard being updated to enable it to keep pace with technology, it could run like a similar story to Ethernet, where it will be in use for many years, but still at the forefront of technology..