UART stands for universal asynchronous receiver transmitter. UART is one of the simplest method of digital communication between two devices. Devices connected on UART communicate using two separate wires and a common ground. Hence three wires are required between transmitter and receiver in UART communication.
When microprocessor or simply processor communicates with the outside world it provides the data in byte-sized chunks. A device can transfer data to another device in serial form or parallel form. In serial communication a single data line is used where as 8-bit data bus is used in parallel communication. This makes serial communication not only much cheaper but also enables two computers located in two different cities to communicate over telephone. In some cases such as printers the information is taken from the 8-bit data bus and presented to the 8-bit data bus of the printer or hard disk. In parallel communication data transfer can happen at a faster rate using many wires in parallel, but distances between devices cannot be great, because long cables diminish or even distort signals. Also bits might be lost in long wire and many parallel wires might be expensive.
For serial communication to work the byte of data must be converted to serial bits using a parallel in serial out shift register and then it can be transmitted over a single data line. Also at the receiving side there must be a serial-in-paralle-out shift register to receive the serial data and pack them into a byte. If data is to be transferred over a telephone line it must be converted to audio tones which is performed by using a modem(modulation/demodulation). But for short distance like keyboard on a PC no modulation demodulation is required.
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