USB

Developed jointly by Compaq, Digital, IBM, Intel, Microsoft, NEC and Northern Telecom, the Universal Serial Bus (USB) standard offers a new standardised connector for attaching all the common I/O devices to a single port, simplifying today's multiplicity of ports and connectors. Significant impetus behind the USB standard was created in September of 1995 with the announcement of a broad industry initiative to create an open host controller interface (HCI) standard for USB. Backed by 25 companies, the aim of this initiative was to make it easier for companies - including PC manufacturers, component vendors and peripheral suppliers - to more quickly develop USB-compliant products. Key to this was the definition of a non-proprietary host interface - left undefined by the USB specification itself - which enabled connection to the USB bus. The first USB specification was published a year later, with version 1.1 being released in the autumn of 1998.

Up to 127 devices can be connected, by daisy-chaining or by using a USB hub which itself has a number of USB sockets and plugs into a PC or other device. Seven peripherals can be attached to each USB hub device. This can include a second hub to which up to another seven peripherals can be connected, and so on. Along with the signal USB carries a 5v power supply so small devices, such as hand held scanners or speakers, do not have to have their own power cable.

Devices are plugged directly into a four-pin socket on the PC or hub using a rectangular Type A socket. All cables that are permanently attached to the device have a Type A plug. Devices that use a separate cable have a square Type B socket, and the cable that connects them has a Type A and Type B plug.

USB 1.1 overcame the speed limitations of UART-based serial ports, running at 12 Mbit/s - at the time, on a par with networking technologies such as Ethernet and Token Ring - and provided more than enough bandwidth for the type of peripheral device is was designed to handle. For example, the bandwidth was capable of supporting devices such as external CD-ROM drives and tape units as well as ISDN and PABX interfaces. It was also sufficient to carry digital audio directly to loudspeakers equipped with digital-to-analogue converters, eliminating the need for a soundcard. However, USB wasn't intended to replace networks. To keep costs down its range is limited to 5 metres between devices. A lower communication rate of 1.5 Mbit/s can be set-up for lower-bit-rate devices like keyboards and mice, saving space for those things which really need it.

USB was designed to be user-friendly and is truly plug-and-play. It eliminates the need to install expansion cards inside the PC and then reconfigure the system. Instead, the bus allows peripherals to be attached, configured, used, and detached while the host and other peripherals are in operation. There's no need to install drivers, figure out which serial or parallel port to choose or worry about IRQ settings, DMA channels and I/O addresses. USB achieves this by managing connected peripherals in a host controller mounted on the PC's motherboard or on a PCI add-in card. The host controller and subsidiary controllers in hubs manage USB peripherals, helping to reduce the load on the PC's CPU time and improving overall system performance. In turn, USB system software installed in the operating system manages the host controller.

Data on the USB flows through a bi-directional pipe regulated by the host controller and by subsidiary hub controllers. An improved version of bus mastering allows portions of the total bus bandwidth to be permanently reserved for specific peripherals, a technique called isochronous data transfer. The USB interface contains two main modules: the Serial Interface Engine (SIE), responsible for the bus protocol, and the Root Hub, used to expand the number of USB ports.

The USB bus distributes 0.5 amps (500 milliamps) of power through each port. Thus, low-power devices that might normally require a separate AC adapter can be powered through the cable - USB lets the PC automatically sense the power that's required and deliver it to the device. Hubs may derive all power from the USB bus (bus powered), or they may be powered from their own AC adapter. Powered hubs with at least 0.5 amps per port provide the most flexibility for future downstream devices. Port switching hubs isolate all ports from each other so that one shorted device will not bring down the others.

The promise of USB was a PC with a single USB port onto which would be connected one large, powered device - like a monitor or a printer - which would act as a hub, linking up all the other smaller devices such as mouse, keyboard, modem, document scanner, digital camera and so on. Since many USB device drivers did not become available until after its release, this promise was never going to be realised before the availability of Windows 98. However, even post-Windows 98 its take-up was initially disappointing.

There were a number of reasons for this. Some had complained that the USB architecture was too complex and that a consequence of having to support so many different types of peripheral was an unwieldy protocol stack. Others argued that the hub concept merely shifts expense and complexity from the system unit to the keyboard or monitor. However, probably the biggest impediment to USB's acceptance was the IEEE 1394 FireWire standard.

Developed by Apple Computer, Texas Instruments and Sony and backed by Microsoft and SCSI specialist Adaptec, amongst others, IEEE 1394 was another high-speed peripheral bus standard. It was supposed to be complementary to USB, rather than an alternative, since it's possible for the two buses to coexist in a single system, in a manner similar to today's parallel and serial ports. However, the fact that digital cameras were far more likely to sport an IEEE 1394 socket than a USB port gave other peripheral manufacturers pause for thought.