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| Intel 8251 | |
|---|---|
| Name | Intel 8251 |
| Caption | Intel 8251 USART (Universal Synchronous/Asynchronous Receiver/Transmitter) |
| Developer | Intel Corporation |
| Introduced | 1970s |
| Type | USART |
| Data bandwidth | serial |
| Package | DIP |
Intel 8251
The Intel 8251 is a Universal Synchronous/Asynchronous Receiver/Transmitter introduced by Intel Corporation in the 1970s as part of a family of peripheral devices for microprocessor systems. It served as an intermediary between microprocessors and serial communication lines, supporting both synchronous protocols and asynchronous formats used by terminals and modems. The 8251 influenced serial interface design alongside contemporaries from Motorola, National Semiconductor, and other semiconductor firms.
The 8251 was released during a period notable for the rise of microprocessors such as the Intel 8080, MOS Technology 6502, and Zilog Z80. It complemented chipset components like the Intel 8253 Programmable Interval Timer and the Intel 8255 Programmable Peripheral Interface used in systems including the Altair 8800, IMSAI 8080, and early IBM-compatible designs. The device appears in the historical context of the 1970s energy crisis era semiconductor expansion and the development of serial standards later formalized by bodies such as the Electronic Industries Association and the Institute of Electrical and Electronics Engineers.
The 8251 implements transmitter and receiver logic with internal control, status, and buffer registers designed to interface with an 8-bit processor data bus such as that used by the Intel 8085 and Motorola 6800. Its architecture provides separate clock inputs for transmitter and receiver, allowing compatibility with clocking schemes used by synchronous links like those in Bell 103 and Bell 202 modem standards. The chip's control word format and status reporting mirror conventions also used by companion devices from National Semiconductor and Texas Instruments during the microcomputer era. Packaging options like dual in-line packages facilitated socketed use in hobbyist boards such as the Altair 8800 and commercial terminals from companies like DEC and Xerox.
The 8251 supports asynchronous modes with programmable word length (5 to 8 bits), parity selection, and stop-bit configuration; synchronous modes handle character framing aligned to external clocks used in systems from Bell Labs research and telecommunications equipment by Western Electric. It offers command and status registers that reflect line conditions comparable to signal indicators in the RS-232 specification, used by terminals such as the Teletype Model 33 and communications devices from Hayes Microcomputer Products. Flow control and interrupt signaling integrate with CPU interrupt controllers like the Intel 8259 Programmable Interrupt Controller to coordinate DMA solutions such as those found in IBM PC-era architectures.
Software initializes the 8251 by writing control words to configure mode, baud rate sources, parity, and stop bits—procedures similar to programming routines for UARTs in operating systems like CP/M, MS-DOS, and embedded firmware in systems from DEC. The device provides status bits for overrun, parity error, framing error, and transmit-ready conditions; these status flags inform interrupt-driven or polled I/O strategies used by device drivers in early personal computers from Commodore and Apple Computer. Timing considerations require coordination with programmable timers such as the Intel 8253 or external baud generators like the UART 8250 successors in the IBM PC/AT.
Although the core 8251 design remained, manufacturers and system designers offered implementations in ceramic and plastic DIP packages, with vendor-specific ROMs and adapter boards from firms like S-100 bus vendors and third-party suppliers. Successor and related devices from Intel include USART and UART chips that evolved into the 8250 family and later integrated serial controllers embedded in southbridge chips from companies such as Intel Corporation and AMD. Military and industrial variants were available with extended temperature ranges for use in platforms from Boeing avionics programs and industrial controllers by Siemens.
The 8251 found use in minicomputers, microcomputers, terminal controllers, and embedded equipment. It interfaced with teleprinter devices like the ASR-33 and networked modems conforming to standards developed at CCITT/ITU-T. Commercial systems from Digital Equipment Corporation, Xerox, HP, and telecommunications equipment from AT&T included 8251-based serial interfaces in console and data communication roles. Hobbyist projects and educational kits in the 1970s microcomputer revolution era often featured the 8251 for teaching serial communications and interfacing with peripherals such as printers and teletype units.
Interfacing the 8251 required attention to signal standards and voltage levels such as those later standardized by RS-232; level translation was often performed using driver chips like those from Maxim Integrated and Texas Instruments line drivers (e.g., the MAX232 descendant families). Bus integration demanded glue logic compatible with bus standards like the S-100 and processor buses for the Intel 8080, Z80, and Motorola 6800. Systems employing direct memory access used controllers and arbitration schemes similar to designs adopted in IBM PC-class motherboards, while modem and multiplexer setups paralleled developments in bellcore and telecommunication switching centers.
Category:Intel chips