Universal Asynchronous Receiver/Transmitter

Universal Asynchronous Receive and Transmit

<hardware> A UART with a one-byte FIFO buffer. The 16450 is a higher speed, fixed version of the 8250. It was superseded by the 16550. The 16450 was used for the IBM PC AT and PS/2 but will not work in a IBM PC XT. (2004-03-21)

<communications, hardware> (UART) An integrated circuit used for serial communications, containing a transmitter (parallel-to-serial converter) and a receiver (serial-to-parallel converter), each clocked separately. The parallel side of a UART is usually connected to the bus of a computer. When the computer writes a byte to the UART's transmit data register (TDR), the UART will start to transmit it on the serial line. The UART's status register contains a flag bit which the computer can read to see if the UART is ready to transmit another byte. Another status register bit says whether the UART has received a byte from the {serial line}, in which case the computer should read it from the receive data register (RDR). If another byte is received before the previous one is read, the UART will signal an "overrun" error via another status bit. The UART may be set up to interrupt the computer when data is received or when ready to transmit more data. The UART's serial connections usually go via separate {line driver} and line receiver integrated circuits which provide the power and voltages required to drive the serial line and give some protection against noise on the line. Data on the serial line is formatted by the UART according to the setting of the UART's control register. This may also determine the transmit and receive baud rates if the UART contains its own clock circuits or "baud rate generators". If incorrectly formated data is received the UART may signal a "framing error" or "parity error". Often the clock will run at 16 times the baud rate (bits per second) to allow the receiver to do centre sampling - i.e. to read each bit in the middle of its allotted time period. This makes the UART more tolerant to variations in the clock rate ("jitter") of the incoming data. An example of a late 1980s UART was the Intel 8450. In the 1990s, newer UARTs were developed with on-chip buffers. This allowed higher transmission speed without data loss and without requiring such frequent attention from the computer. For example, the Intel 16550 has a 16 byte FIFO. Variants include the 16C550, 16C650, 16C750, and 16C850. The term "Serial Communications Interface" (SCI) was first used at Motorola around 1975 to refer to their start-stop asyncronous serial interface device, which others were calling a UART. See also bit bang. [Is this the same as an ACIA?] (2003-07-13)

A universal asynchronous receiver-transmitter (UART ) is a computer hardware device for asynchronous serial communication in which the data format and transmission speeds are configurable. It sends data bits one by one, from the least significant to the most significant, framed by start and stop bits so that precise timing is handled by the communication channel.

The 16550 UART (universal asynchronous receiver/transmitter) is an integrated circuit designed for implementing the interface for serial communications. The corrected -A version was released in 1987 by National Semiconductor. It is frequently used to implement the serial port for IBM PC compatible personal computers, where it is often connected to an RS-232 interface for modems, serial mice, printers, and similar peripherals. It was the first serial chip used in the IBM PS/2 line, which were introduced in 1987.

The part was originally made by National Semiconductor. Similarly numbered devices, with varying levels of compatibility with the original National Semiconductor part, are made by other manufacturers. A UART function that is register-compatible with the 16550 is usually a feature of multifunction I/O cards for IBM PC-compatible computers and may be integrated on the motherboard of other compatible computers.

Replacement of the factory-installed 8250 UART was a common upgrade for owners of IBM PC, XT, and compatible computers when high-speed modems became available. Above 9600 baud, owners discovered that the serial ports of the computers were not able to handle a continuous flow of data without losing characters. Exchange of the 8250 (having only a one-byte received data buffer) with a 16550, and occasionally patching or setting system software to be aware of the FIFO feature of the new chip, improved the reliability and stability of high-speed connections.

16550

<hardware> A version of the 16450 UART with a 16-byte FIFO. Superseded by the 16550A. This chip might not operate correctly with all software. The 16C550 is a CMOS version. (2004-03-24)

<hardware> A version of the 16550 UART. Superseded by the 16650. (2003-07-05)

<hardware> A UART that can operate at a maximum of 9600 baud. The 8250 is used in IBM PC XT computers. It works in an IBM PC AT under DOS but generates unwanted interrupts when used at 9600 baud. The IBM PC BIOS has a bug fix for this chip. (2004-03-21)