Section 18.4 The RS-232 standard
The UART protocol is sometimes called “RS-232”. While they are closely related historically, they are not synonymous and should not be confused. RS-232 is a standard for the wiring of a serial interface and the voltages on those wires. It is independent of the way the bits are formatted, which is the UART protocol. RS-232 specifies, at a minimum, one wire for the transmitted signal, one for the received signal, and a ground. Data can move in both directions at the same time, so called “full duplex” operation. The voltage levels on the data wires are allowed to be between +3V and +15V for logical ’0’, and between -3V and -15V for logical ’1’. Special interface chips, such as the MAX232, have been developed which convert the RS-232 voltages to normal logic levels. While the standard also defines additional wires for special control features, many UART protocol applications today, in microcontrollers, MIDI, etc, use only the transmit, receive, and ground wires. If you ever need to work with an actual RS-232 interface, you can find ample information about it online.
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www.ti.com/lit/ds/symlink/max232.pdf
