Universal Serial Bus, Accelerated Graphics Port

Universal Serial Bus

A relatively new I/O bus is the Universal Serial Bus or USB, originally created by a seven-member consortium including Compaq, Digital Equipment, IBM, Intel, Microsoft, NEC, and Northern Telecom. It is designed to make the installation of slow peripheral devices as effortless as possible.

USB is much faster than regular serial ports and much easier to manage, eliminating the need to manually resolve resource conflicts, since the host controller only uses one set of resources for all devices.

It is expected that USB will ultimately replace both serial and parallel ports as the technology matures and more devices are built to use USB. One or two USB ports are found on most new systemboards today, and older systemboards that don’t have USB ports can be upgraded by adding a PCI-to-USB controller card in a PCI slot to provide a USB port.

USB allows for two speeds, 1.5 MB per second and 12 MB per second, and works well for slow I/O devices. A USB host controller, which is included in the PCI controller chip for the 400 series Intel chip set, manages the USB bus. As many as 127 USB devices can be daisy-chained together. The host controller manages communication to the CPU for all devices, using only a single IRQ, I/O address range, and DMA channel. USB allows for hot-swapping, meaning that a device can be plugged into a USB port while the computer is running, and the host controller will sense the device and configure it without your having to reboot the computer. One USB device, such as a keyboard, can provide a port for another device, or a device can serve as a hub, allowing several devices to connect to it.

There can also be a standalone hub into which several devices can be plugged. In USB technology, the host controller polls each device, asking if data is ready to be sent or requesting to send data to the device. The USB cable has four wires, two for power and two for communication. The two power wires (one carries voltage and the other is ground) allow the host controller to provide power to a device.

I/O devices that are now or are soon to be available with a USB connection are the mouse, joystick, keyboard, printer, scanner, monitor, modem, video camera, fax machine, and digital telephone. USB must be supported by the operating system in order to work. Windows 95 with the USB update, Windows 98, and Windows 2000 support USB, but Windows NT does not.

Accelerated Graphics Port

The Accelerated Graphics Port (AGP) is designed to provide fast access to video. Systemboards have a single AGP slot to support an AGP video card, as shown in the figure below. AGP is more of a port than a bus, since it does not allow for expandability and can only support a single card. The faster AGP bus has a direct connection to the CPU, without having to use the slower PCI bus.

The AGP bus runs at the same speed as the memory bus, connects directly to it, and has a 32-bit-wide data path. AGP runs faster than PCI, running at half the memory bus speed, but also offers additional features that give it overall better performance for video than PCI. AGP offers an improved rendering of 3-D images when software is designed to use it.