The Universal Asynchronous Receiver Transmitter was one of the first really successful integrated circuits. In your PC, the UART is the key component of the serial communications port. Data is tranferred within the computer in units of 1 to 8 bytes, with a separate wire for each bit within the byte. The UART receives a byte and converts it to a sequence of voltage changes to represent the 0s and 1s on a single wire. The modem takes the signal on this wire and converts it into sound. At the other end, a modem converts the sound back to voltages, and another UART converts the stream of 0s and 1s back into bytes of data.

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