UPGRADING REPAIRING PCs

Chapter 6
Serial Ports and
Modems
6
Understanding Serial Ports
The asynchronous serial interface was designed as a system-tosystem
communications port. Asynchronous means that no synchronization
or clocking signal is present, so characters can be sent
with any arbitrary time spacing.
Each character that is sent over a serial connection is framed by a
standard start-and-stop signal. A single 0 bit, called the start bit,
precedes each character to tell the receiving system that the next
eight bits constitute a byte of data. One or two stop bits follow the
character to signal that the character has been sent. At the receiving
end of the communication, characters are recognized by the
start-and-stop signals instead of by the timing of their arrival.
Serial refers to data that is sent over a single wire, with each bit lining
up in a series as the bits are sent. This type of communication
is used over the phone system because this system provides one
wire for data in each direction. Compared to parallel ports, serial
ports are very slow, but their signals can be transmitted a greater
distance. The other wires in the serial port are used to control the
flow of data to or from the port.
Serial ports are also referred to as COM ports because they are used
to communicate between devices.
Physically, serial ports come in two forms, although through
adapters or specially-wired cable, they have no problems communicating
with each other. The following figures show the standard
9-pin (see Figure 6.1) and 25-pin (see Figure 6.2) serial ports. The
25-pin serial port has pins sticking out, as opposed to the 25-pin
parallel port, which has holes for pins.