RS-232 is a short range connection between a single host
and a single device (such as a PC to a modem) or another host (such
as a PC to another PC). The standard uses a single TX line, a single RX
line, numerous modem handshaking lines and a ground line with the
option of DB9 and DB25 connectors. A minimal 3-wire RS-232 connection
consists only the TX, RX, and ground lines, but if flow control
is required a minimal 5-wire RS-232 is used adding the RTS and CTS
lines. The RS-232 standard has been commonly used in computer serial
ports and is still widely used in industrial communication devices.