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Ports & Connectors (continued...)
Now that we know the basic rules, let's take
a look at some of the connectors or ports you might find on
your computer.
DB Connector
The most common connector is the DB connector. It's sort
of a 'D' shape and is sometimes called a D-shell connector (go
figure). It's designated as DB-x, with 'x' being the number
of pins or holes on the individual connector. So a DB-9 female
connector would be a 'D' shaped connector with 9 holes. This
would receive a cord with a DB-9 male connector (with 9 pins).
If you find a DB male connector port on the
back of your computer, (either a DB-9 male or a DB-25
male) it's going to be a serial port. Serial ports are also
called COM ports or RS-232 ports (Reference Standard #232 as
referenced by IEEE*). Serial ports transmit data one bit at
a time and are relatively slow compared to other ports. However,
they are plenty fast enough for some external devices such as
a mouse, or an external modem. Because only one bit at a time
is passed along a serial cable, it can travel a fair distance
before data integrity is challenged (or errors start to occur).
A serial cable shouldn't be more than 50 feet in length.
Incidentally, in case you were wondering, if
you have a device with only nine holes on its connector, and
only a DB-25 male serial port to connect to, all you need is
a 9 to 25 pin adapter. There's no difference between a 25-pin
serial port and a 9-pin serial port other than the fact that
the DB-25 male has sixteen extra pins that it doesn't use.
If you have an older computer, and see a DB-9
female connector on the back, it's probably a video connector
for an older EGA or CGA monitor. My guess is that you won't
see one on your computer. However, if the question should ever
come up, it could also be a Token Ring network adapter port.
Look on the back of your computer, you may
be able to find two different DB-15 female connectors.
If you see three rows of five holes, then it's your VGA or SVGA
video monitor adapter. If you see only two rows (one of eight
holes and one of seven), then it's probably a joystick adapter.
A DB-25 female connector on the back
of your computer is going to be a parallel port. Parallel ports
can transmit data eight bits at a time which creates a noticeable
speed increase over serial ports. Most commonly used as printer
connections, several other devices now use the parallel port
such as tape backup systems, Zip drives and scanners to name
a few. These devices are generally fitted with what is referred
to as a pass-through port. This means that you can hook up your
scanner to the parallel port (DB-25 female) and then connect
your printer to the DB-25 connector on the back of the scanner
and have access to both devices. This usually works well but
does pose some problems. First off, the device has to be turned
on for the pass-through port to work. To take this one step
further, the device often has to be turned on before the computer
is booted, to be recognized properly and for the right drivers
to be loaded at startup. Also, users tend to think that they
can daisy-chain these devices. In other words, connect their
scanner to the computer, attach their Zip drive to the back
of the scanner, their tape drive to the back of the Zip drive,
and then their printer to the pass-through port on the back
of the tape drive. Believe it or not, I've seen this done and
I've seen it work (more or less). I've also seen it work one
day and not the next. It's a hit and miss sort of thing (more
miss than hit) and I wouldn't trust the integrity of the data
past the second device.
Another thing to keep in mind, is that the
cable on a parallel device shouldn't be more than 10 feet long.
Data errors can occur beyond this distance.
continued...
*Institute of Electrical and Electronic Engineers,
Inc.
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