(Universal Asynchronous Receiver Transmitter) The electronic circuit that makes up the serial port. Also known as "universal serial asynchronous receiver transmitter" (USART), it converts parallel bytes from the CPU into serial bits for transmission, and vice versa. It generates and strips the start and stop bits appended to each character. Note that in the following paragraphs, dashes have been added after the 16 for readability. Older 8250 and 16-450 UARTs are not fast enough for today's modems. A 16-550 is required for transmission up to 115,200 bps (115 Kbps).

ISDN users running both 64 Kbps channels are losing performance with a 16-550 UART, because the maximum 115 Kbps is reduced further to 92 Kbps when the start/stop bits are removed. Upgrading to a 16-650 or higher UART boosts real data speed from 92 to 128 Kbps. The 16-650 is the more sophisticated UART, providing hardware flow control that reduces the burden on the CPU. See UART overrun.

           Buffer    Maximum
   UART    Size      Speed
   Chip    (bytes)   Bits/sec
   8250     None        9,600
   16450       1        9,600
   16550      16      115,200
   16650      32      430,800
   16750      64      921,600
   16850     128          1.5 Mbps