After เอลอีดี and pushbuttons, the most basic method for communication
with an embedded processor is asynchronous serial. Asynchronous serial
communication in its most primitive form is implemented over a symmetric
pair of wires connecting two devices – here I’ll refer to them as the host and
target, although those terms are arbitrary. Whenever the host has data to
send to the target, it does so by sending an encoded bit stream over its transmit
(TX) wire; this data is received by the target over its receive (RX) wire.
Similarly, when the target has data to send to the host it transmits the encoded
bit stream over its TX wire and this data is received by the host over its
RX wire. This arrangement is illustrated in Figure 5. This mode of communications
is called “asynchronous” because the host and target share no time
reference. Instead, temporal properties are encoded in the bit stream by the
transmitter and must be decoded by the receiver.