Universal Asynchronous Receiver/Transmitter

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The Universal Asynchronous Receiver/Transmitter (usually initialized as UART) was a functional unit which contained all the circuitry needed to originate and interpret asynchronous serial line signals, including the serial-parallel conversion.

Initially, they were implemented with discrete components. In about 1970, Vince Bastiani of DEC, together with engineers from Solid State Data Systems (in Long Island, New York) produced a complete UART in a single very early MSI chip. This evolved into a standard part, produced in a 40-pin DIP package by many manufacturers. The first, in 1971, was the Western Digital WD1402A; the Fairchild TR1402A, General Instruments AY-5-1013 and Intersil 6402 were others.

The parameters of the line (character size, parity, number of stop bits, etc) are all selected by the inputs to the chip. The circuitry needed to produce the voltages and currents needed by the particular interface type was outside the chip, so a UART could be used with either EIA or 20mA interfaces.

The introduction of UARTs can be seen happening in early DEC PDP-11 serial interfaces; the earliest, the KL11, used discrete components, not a UART chip, as did the DM11; the DL11, and all others after that date, utilized UART chips, though.

Early UART chips only handled a single line; later on, DUARTs appeared, with 2 lines per chip. The QUART (four lines) and OCTART (eight lines) followed. (In general, all the lines in these have to share a singled set of parameters.)


With increased speeds, besides locking the DCE-DTE rate at 2x the communication rate, UART chips would need to increase their local buffer size to be able to send continuously. It has been reported that it did not help that a UART chip was released with a defective buffer, or that in some cases, the buffers would need to be enabled.


The DZV11 technical manual (below) includes a detailed description of the UART in Appendix A, 'IC Descriptions'.

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