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NO_MODEM.TXT
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1995-10-02
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EZ_Online NO_MODEM.TXT
─────────────────────────────────────────────────────────────────────
No modem ... no phone connection?
Going online means, connecting your computer to another
computer using the telephone lines.
Computers deal in digital data, which is represented by only
two signal states ... 0 and 1, high and low, on and off.
Conventional telephone systems are designed for voice
communications, which involve audible, continuously variable,
electrical signals, and cannot transmit two-state digital
information.
The problem of transmitting digital information was easily
and intuitively solved, by simply converting the two states
of the transmitting computer's signals to different sounds,
passing them into the phone system and sending them to a
computer on the other end of the line, where they're
converted from sounds back to two-state digital pulses.
The tool that does this is called a "MODEM", an acronym for
MOdulate/DEModulate.
Modems can be internal or external. If you have an external
modem (not common), you'll find a separate box with several
little lights plugged into the back of your computer. More
likely, your modem will be internal ... a board plugged into
one of the slots inside your computer, with modular telephone
connectors protruding through openings in the back of its
case.
If you find phone connectors on the back of your computer's
case, simply connect the one marked "Line" to the nearest
wall connector. If needed, standard telephone cables,
extensions, and couplers are available at Radio Shack stores,
hardware stores and discount department stores.
Naturally, this assumes that your phone system is
properly wired. If there's any doubt about it ...
you'll notice that your phone cable has four wires:
RED, GREEN, YELLOW and BLACK. Also notice that the
phone connectors have four pins, then assure that
the RED and GREEN wires are connected to the middle
two pins.
If your computer is not modem-equipped ... not to worry! You
can probably afford, and install, an internal modem.
What to buy? Modems are also classed by transfer speed, and
at the present time 28,800 "baud" (bits per second) units are
the creme of the crop. If you can afford it, spending $180
now will surely save you much more that that in phone bills
over the life of the equipment ... and while you're at it,
make it a FAX/Modem ... it won't cost any more, and you'll be
able to send FAX's directly from your computer, should you
ever wish to do so. If you'd rather not open your computer's
case, another $30 will get you the external version of any
internal fax/modem.
For $100 less, about $96, you can purchase a 14,400 baud
unit, which is obviously slower, but still quite adequate.
If you look hard enough, you'll be able to find a 9,600 baud
internal modem for about $45, and this may be good enough for
now. If you have only pocket change, a 2,400 baud modem is
certainly better than nothing, and shouldn't cost more than
about $15 ... and you might even be able to find one of these
older, but still perfectly functional units, available as a
gift from a friend who upgraded to a faster unit.
To install an internal modem, you need only open the case of
your computer and plug it in at any available "slot". You
will also probably need to "tell" the board which "COM port"
and "Interrupt" it may use, and this is usually done either
by moving jumpers on the board, or setting its "dip switches"
according to directions provided with the unit.
What's a "COM port"? What's an "Interrupt"? Internally,
your computer moves data around in eight-bit "bytes" on a
wiring system composed of parallel lines. Moving all these
bits simultaneously on parallel lines greatly speeds things
up, compared to transmitting eight-bit "bytes" one bit at a
time on a single line. But since telephone lines can only
carry one bit at a time, the computer has to convert its
"parallel" data to "serial" (one bit at a time).
This task is performed by a chip known as a Universal
Asynchronous Receiver/Transmitter (UART). The computer sends
this chip 8-bit bytes on its parallel wires, and the chip
passes them on to your modem as a stream of bits on a single
line. The UART works in reverse also, converting serial bit
streams received from your modem to 8-bit parallel bytes and
sending them on to your computer. The UART is also referred
to as a "serial port", aka "COM port".
Your computer probably came with a single UART, which serves
your mouse. Your modem contains its own UART, so now you'll
have two. DOS can handle up to four COM ports, and you can
designate your UART's as you wish, as COM1, COM2, COM3 or
COM4 ... the name doesn't have any relation to the actual
physical connection of the UART, its simply a handle that DOS
uses to identify the serial ports. By convention, COM1 is
now usually assigned to the UART that serves your mouse, and
COM2 to your modem.
Since handling data serially is a much slower process than
moving the same data on a "parallel bus", the computer is
able to move data much faster than the UART. A system of
"handshaking" has thereby been devised to let the UART "talk
to" the computer, telling it when to stop sending data, when
it has data available, and so on. To permit the computer to
do other work while the UART is slowly doing its work, this
"interrupt" technique is employed, whereby the UART is able
to send a signal on a special line ... the IRQ line ... to
get the computer's attention when it's finally ready to
receive more data, or to send data to the computer.
If your setup is typical, you have a mouse on "COM1", and
you'll want your modem to use "COM2". The default interrupt
for COM2 is "IRQ3" ... and that's all there is to that. A
dip switch setup on the back of your modem (near the phone
connectors) is pretty much standard; if necessary, just
change the switch settings to the proper positions shown for
COM2.
Naturally, you can't have two devices assigned to the same
port at the same time. If you're not sure about your mouse,
use its diagnostic software (usually found in a "C:\MOUSE"
directory) to see if its been assigned COM1 or COM2.
Beyond that, installing an internal modem is probably the
easiest add-on project you'll ever do. So approach the
project with confidence. You can do it!
Evil-Crypt BBS
Tony Zarzecki, SysOp
Redford, MI usa
313-538=3762
(to exit, touch Esc ... then "Y")