Pronounced U-Art, and short for Universal Asynchronous Receiver-Transmitter, the UART is a computer component that handles asynchronous serial communication. Every computer contains a UART to manage the serial ports, and all internal modems have their own UART. As modems have become increasingly fast, the UART has come under greater scrutiny as the cause of transmission bottlenecks. If you are purchasing a fast external modem, make sure that the computer's UART can handle the modem's maximum transmission rate. The newer 16550 UART contains a 16-byte buffer, enabling it to support higher transmission rates than the older 8250 UART.

ULTRA ATA

Ultra ATA is similar to Ultra SCSI in the sense that it brings the gap between the current standard (ATA-3) and the next generation, which is not yet completed. Ultra ATA adds a new, high-performance mode: DMA/33 with 33 Mbytes per second bandwidth – twice that of DMA mode 2.

ULTRA SCSI

For faster data-transfer rates, Ultra SCSI enables to double bandwidth and progress to the next performance plateau with existing cables, connections and cabinets. It provides 8-bit data rates of 20 Mbytes per second and 16-bit data rates of 40 Mbytes per second.

ULTRA2 SCSI

Ultra2 SCSI uses the SCSI-3 interface standard to provide greater performance. Ultra2 doubles the interface bandwidth to 80 Mbytes per second for the 16-bit wide 68-pin or 80-pin SCSI II parallel interface connections.

Underscanning
A technique generally used in professional TV and video systems as a way of ensuring that the complete image is always visible within a display area; the opposite of overscanning.

User bits
Undefined bits within the 80-bit SMPTE time code word that are available for uses other than time coding.

USB

Short for Universal Serial Bus, a new external bus standard that supports data transfer rates of 12 Mbps (12 million bits per second). A single USB port can be used to connect up to 127 peripheral devices, such as mice, modems, and keyboards. USB also supports Plug-and-Play installation and hot plugging. Starting in 1996, a few computer manufacturers started including USB support in their new machines. Since the release of Intel's 440LX chipset in 1997, USB has become more widespread. It is expected to eventually completely replace serial and parallel ports.