The 8250 UART (

universal asynchronous receiver/transmitter 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 significa ...

) is an integrated circuit designed for implementing the interface for

serial communications In telecommunication and data transmission, serial communication is the process of sending data one bit at a time, sequentially, over a communication channel or computer bus. This is in contrast to parallel communication, where several bits are ...

. The part was originally manufactured by the

National Semiconductor National Semiconductor was an American semiconductor manufacturer which specialized in analog devices and subsystems, formerly with headquarters in Santa Clara, California. The company produced power management integrated circuits, display dr ...

Corporation. It was commonly used in PCs and related equipment such as

printers Printer may refer to: Technology * Printer (publishing), a person or a company * Printer (computing), a hardware device * Optical printer for motion picture films People * Nariman Printer ( fl. c. 1940), Indian journalist and activist * Jam ...

modem A modulator-demodulator or modem is a computer hardware device that converts data from a digital format into a format suitable for an analog transmission medium such as telephone or radio. A modem transmits data by Modulation#Digital modulati ...

s. The 8250 included an on-chip programmable bit rate generator, allowing use for both common and special-purpose bit rates which could be accurately derived from an arbitrary

crystal oscillator A crystal oscillator is an electronic oscillator circuit that uses a piezoelectric crystal as a frequency-selective element. The oscillator frequency is often used to keep track of time, as in quartz wristwatches, to provide a stable clock ...

reference frequency. The chip designations carry suffix letters for later versions of the same chip series. For example, the original 8250 was soon followed by the 8250A and 8250B versions that corrected some bugs. In particular, the original 8250 could repeat transmission of a character if the

CTS Cts or CTS may refer to: Arts and entertainment Television * Chinese Television System, a Taiwanese broadcast television station, including: ** CTS Main Channel () ** CTS Education and Culture () ** CTS Recreation () ** CTS News and Info () ...

line was asserted asynchronously during the first transmission attempt. Due to the high demand, other manufacturers soon began offering compatible chips.

Western Digital Western Digital Corporation (WDC, commonly known as Western Digital or WD) is an American computer drive manufacturer and data storage company, headquartered in San Jose, California. It designs, manufactures and sells data technology produc ...

offered WD8250 chip under Async Communications Interface Adapter (ACIA) and Async Communications Element (ACE) names. The 16450(A) UART, commonly used in

IBM PC/AT The IBM Personal Computer/AT (model 5170, abbreviated as IBM AT or PC/AT) was released in 1984 as the fourth model in the IBM Personal Computer line, following the IBM PC/XT and its IBM Portable PC variant. It was designed around the Intel 80 ...

-series computers, improved on the 8250 by permitting higher serial line speeds. With the introduction of multitasking operating systems on PC hardware, such as

OS/2 OS/2 (Operating System/2) is a series of computer operating systems, initially created by Microsoft and IBM under the leadership of IBM software designer Ed Iacobucci. As a result of a feud between the two companies over how to position OS/2 r ...

Windows NT Windows NT is a proprietary graphical operating system produced by Microsoft, the first version of which was released on July 27, 1993. It is a processor-independent, multiprocessing and multi-user operating system. The first version of Win ...

or various flavours of

UNIX Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, an ...

, the short time available to serve character-by-character

interrupt In digital computers, an interrupt (sometimes referred to as a trap) is a request for the processor to ''interrupt'' currently executing code (when permitted), so that the event can be processed in a timely manner. If the request is accepted, ...

requests became a problem, therefore the

IBM PS/2 The Personal System/2 or PS/2 is IBM's second generation of personal computers. Released in 1987, it officially replaced the IBM PC, XT, AT, and PC Convertible in IBM's lineup. Many of the PS/2's innovations, such as the 16550 UART (serial p ...

serial ports introduced the 16550(A) UARTs that had a built-in 16 byte FIFO or buffer memory to collect incoming characters. Later models added larger memories, supported higher speeds, combined multiple ports on one chip and finally became part of the now-common

Super I/O Super I/O is a class of I/O controller integrated circuits that began to be used on personal computer motherboards in the late 1980s, originally as add-in cards, later embedded on the motherboards. A super I/O chip combines interfaces for a vari ...

circuits combining most input/output logic on a PC motherboard.

Blocks

The line interface consists of: SOUT, SIN, /RTS, /DTR, DSR, /DCD, /CTS, /RI Clock interface: XIN, XOUT, /BAUDOUT, RCLK Computer interface: D0..D7, /RD, /WR, INTRPT, MR, A0, A1, A2, ADS, WR, RD, /CS2, CS1, CS0 The interrupt line will (when the IER bit has enabled it) be triggered to go high when one of the following events occur: Receiver line status, Received data available, Transmitter holding register empty, and MODEM status. The interrupt signal is reset to low level upon the appropriate interrupt service or a reset operation (via MR).

History

8250 UART was introduced with the IBM PC (1981). The 8250A and 8250B revisions were later released, and the

16450 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 significa ...

was introduced with the

IBM Personal Computer/AT The IBM Personal Computer/AT (model 5170, abbreviated as IBM AT or PC/AT) was released in 1984 as the fourth model in the IBM Personal Computer line, following the IBM PC/XT and its IBM Portable PC variant. It was designed around the Intel 80 ...

(1984). The main difference between releases was the maximum communication speed. IBM refused to use

Intel 8251 The 8251 is a Universal Synchronous/Asynchronous Receiver/Transmitter (USART) packaged in a 28-pin DIP made by Intel. It is typically used for serial communication and was rated for per second signalling rate. It is commonly confused with the mu ...

in the serial port adapter because Intel chip had no internal baudrate generator, and the adapter had to be more complex and expensive. Later IBM implemented Syncronous Communication Adapter, but it was not popular.

References

{{Reflist, refs= {{cite book , author-first=Frank , author-last=Van Gilluwe , title=Undocumented PC , edition=2 , date=1997 , publisher= Addison Wesley , isbn=0-20147950-8 {{cite web , title=Re: d-devANNOUNCE: CuteMouse 2.0 alpha 1 , author-first=Matthias R. , author-last=Paul , work=freedos-dev , date=2002-04-06 , url=https://marc.info/?l=freedos-dev&m=101807226917577 , access-date=2020-02-07 , url-status=live , archive-url=https://web.archive.org/web/20200207130948/https://marc.info/?l=freedos-dev&m=101807226917577&w=2 , archive-date=2020-02-07 (NB. Has various information on 8250 chip bugs.)

PC16450C/NS16450, PC8250A/INS8250A Universal Asynchronous Receiver Transmitter; datasheet 1990 {{cite web , url=http://www.lammertbies.nl/comm/info/serial-uart.html , title=Serial UART, an in-depth tutorial /NB. Focus on 16550 primarily.)

External links

Serial and UART Tutorial

archive.pcjs.org: National Semiconductor PC16450C/NS16450, PC8250A/INS8250A Universal Asynchronous Receiver Transmitter
(datasheet, dated 1990) Input/output integrated circuits