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The 16550 UART (
One drawback of the earlier 8250 UARTs and 16450 UARTs was that interrupts were generated for each byte received. This generated high rates of interrupts as transfer speeds increased. More critically, with only a 1-byte buffer there is a genuine risk that a received byte will be overwritten if interrupt service delays occur. To overcome these shortcomings, the 16550 series UARTs incorporated a 16-byte FIFO buffer with a programmable interrupt trigger of 1, 4, 8, or 14 bytes.
The original 16550 had a bug that prevented this FIFO from being used. National Semiconductor later released the 16550A which corrected this issue. Not all manufacturers adopted this nomenclature, however, continuing to refer to the fixed chip as a 16550.
According to another source, the FIFO issue was corrected only in the 16550AF model, with the A model still being buggy. (The C and CF models are okay too, according to this source.) The 16550AFN model added DMA transfers.
The 16550 also incorporates a transmit FIFO, though this feature is less critical as delays in interrupt service would only result in sub-optimal transmission speeds and not actual data loss.
The 16550A(F) version was a must-have to use modems with a data transmit rate of 9600 baud. Dropouts occurred with 14.4 kbit/s (
Serial Programming/8250 UART Programming
{{Computer-bus Input/output integrated circuits
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 corrected -A version was released in 1987 by 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 ...
. It is frequently used to implement the serial port
In computing, a serial port is a serial communication interface through which information transfers in or out sequentially one bit at a time. This is in contrast to a parallel port, which communicates multiple bits simultaneously in parallel. ...
for IBM PC compatible personal computer
A personal computer (PC) is a multi-purpose microcomputer whose size, capabilities, and price make it feasible for individual use. Personal computers are intended to be operated directly by an end user, rather than by a computer expert or tec ...
s, where it is often connected to an RS-232
In telecommunications, RS-232 or Recommended Standard 232 is a standard originally introduced in 1960 for serial communication transmission of data. It formally defines signals connecting between a ''DTE'' (''data terminal equipment'') such ...
interface for modems, serial mice, printers, and similar peripherals. It was the first serial chip used in 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 ...
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.
Features
Main features of the 16550 include: * The ability to convert data from serial to parallel, and from parallel to serial, usingshift register
A shift register is a type of digital circuit using a cascade of flip-flops where the output of one flip-flop is connected to the input of the next. They share a single clock signal, which causes the data stored in the system to shift from one loc ...
s.
* An on-chip bit rate ( baud rate) generator to control transmit and receive data rate.
* Handshake lines for control of an external 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 ...
, controllable by software.
* An 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, ...
function to the host microprocessor
A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circ ...
.
* An on-chip FIFO buffer
Buffer may refer to:
Science
* Buffer gas, an inert or nonflammable gas
* Buffer solution, a solution used to prevent changes in pH
* Buffering agent, the weak acid or base in a buffer solution
* Lysis buffer, in cell biology
* Metal ion buffer
* ...
for both incoming and outgoing data; this gives the host system more time to respond to an interrupt generated by the UART, without loss of data.
Both the computer hardware and software
Software is a set of computer programs and associated software documentation, documentation and data (computing), data. This is in contrast to Computer hardware, hardware, from which the system is built and which actually performs the work.
...
interface of the 16550 are backward compatible with the earlier 8250 UART and 16450 UART. The current version (since 1995) by Texas Instruments
Texas Instruments Incorporated (TI) is an American technology company headquartered in Dallas, Texas, that designs and manufactures semiconductors and various integrated circuits, which it sells to electronics designers and manufacturers globa ...
which bought National Semiconductor is called the 16550D.
The 16550A and newer is pin-compatible with the 16450, but the Microsoft diagnostics program ( MSD) supplied with MS-DOS 6.x, Windows 9x, Windows Me, and Windows 2000 often report the 16450 chip as an 8250 chip.
The 16550 FIFO
One drawback of the earlier 8250 UARTs and 16450 UARTs was that interrupts were generated for each byte received. This generated high rates of interrupts as transfer speeds increased. More critically, with only a 1-byte buffer there is a genuine risk that a received byte will be overwritten if interrupt service delays occur. To overcome these shortcomings, the 16550 series UARTs incorporated a 16-byte FIFO buffer with a programmable interrupt trigger of 1, 4, 8, or 14 bytes.
The original 16550 had a bug that prevented this FIFO from being used. National Semiconductor later released the 16550A which corrected this issue. Not all manufacturers adopted this nomenclature, however, continuing to refer to the fixed chip as a 16550.
According to another source, the FIFO issue was corrected only in the 16550AF model, with the A model still being buggy. (The C and CF models are okay too, according to this source.) The 16550AFN model added DMA transfers.
The 16550 also incorporates a transmit FIFO, though this feature is less critical as delays in interrupt service would only result in sub-optimal transmission speeds and not actual data loss.
The 16550A(F) version was a must-have to use modems with a data transmit rate of 9600 baud. Dropouts occurred with 14.4 kbit/s (v.32bis V3 or V03 may refer to:
Medicine
* Mandibular nerve, (V3),division of the trigeminal nerve
* ATC code V03, a subgroup of the Anatomical Therapeutic Chemical Classification System
* Area V3 of the visual cortex
* V3, one of six precordial leads in ...
and higher) units and as compression was added with v.42
The ITU-T V-Series Recommendations on Data communication over the telephone network specify the protocols that govern approved modem communication standards and interfaces.
''Note:'' the '' bis'' and ''ter'' suffixes are ITU-T standard designato ...
getting more data per interrupt was critical as data speed continued to increase.
See also
*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 ...
(UART)
References
Further reading
* *External links
Serial Programming/8250 UART Programming
{{Computer-bus Input/output integrated circuits