History
Some early telegraph schemes used variable-length pulses (as in Morse code) and rotating clockwork mechanisms to transmit alphabetic characters. The first UART-like devices (with fixed-length pulses) were rotating mechanical switches (commutators). These sent 5-bit Baudot codes for mechanical teletypewriters, and replaced morse code. Later, 6-bit codes became common to avoid the figures vs. character errors that could occur when Baudot's shift character was corrupted. ASCII was adopted by the U.S. so that the military services would all use the same teletypewriter codes. ASCII added upper and lower case, with enough punctuation to print most documents. This required a seven bit code. When U.S. telephone systems became digital in the 1960s, they used an 8-bit data sample size to digitize voice. As a result, most data transmission and computers shifted to 8-bit character sizes to use the lowest cost data transmission system. For example, IBM built computers in the early 1960s with 8-bit characters, and an "upper half" of Latin I characters became common in many data networks that had formerly used 7-bit ASCII. The economic effect of the telephone system is large: It effectively forced character systems with more than 8-bits (e.g. Unicode) back into an 8-bit form (e.g. UTF-8), and most commercially important computers for the last forty years have used internal word sizes that are multiples of 8 bits.
Gordon Bell designed the UART for the PDP series of computers. The ASCII code to a teletypewriter made an excellent general-purpose I/O device for a small computer. To reduce costs, including wiring and back-plane costs, these computers also pioneered flow control using XON and XOFF characters rather than hardware wires.
Western Digital made the first single-chip UART WD1402A around 1971; this was an early example of a medium scale integrated circuit. Another popular chip was a SCN2651 from the Signetics 2650 family.
An example of an early 1980s UART was the National Semiconductor 8250. 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 popular National Semiconductor 16550 has a 16 byte FIFO, and spawned many variants, including the 16C550, 16C650, 16C750, and 16C850.
Depending on the manufacturer, different terms are used to identify devices that perform the UART functions. Intel called their 8251 device a "Programmable Communication Interface". MOS Technology 6551 was known under the name "Asynchronous Communications Interface Adapter" (ACIA). The term "Serial Communications Interface" (SCI) was first used at Motorola around 1975 to refer to their start-stop asynchronous serial interface device, which others were calling a UART.
Read more about this topic: Universal Asynchronous Receiver/transmitter
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