Radio Shack Modem I TRS-80 Owner's Manual page 10

General Operation
The
Modem
I
accepts digital
data
from your computer and generates tones
which
can be sent
over
the
telephone lines to
another
Modem.
The
Modem
I
can
also
receive tones
from
the
telephone
line
and change
them back
to digital data.
(The Frequency
Shift
Keying method
is
used.
For
details,
see Specifications.)
Baud
Rate
The
speed
at
which
the
Modem
receives
and
transmits these tones
is
called
the
baud
rate.
The
Modem
I
can send and receive
at
baud
rates
from 0-300.
300
baud
is
standard for
most
time-shared systems, but
some
may
use
1 10,
150, or
other rates.
Before using
the
Modem,
be sure
your
computer and the host
computer
are set to
the
same baud
rate (0-300).
Originate
vs.
Answer
The
Modem
1
can operate
in
either originate or
answer mode.
Electrically,
the
mode
switch determines
which
set
of receive
and
transmit tones the
Modem
will
respond
to.
For
communications
to
take place,
one
modem
must be
in the
originate
mode
and
the
other
in
the
answer mode.
Normally,
when
you
dial a
"host" computer, your
Modem
should be
in
the
originate
mode.
When
you
receive
a call
from
a
"remote"
computer, your
Modem
should be
in
the
answer mode. See Figure
5.
RS-232 Communications
For
communication between computers and
computer-related equipment, the
most
widely used standard
is
the rs~232~c.
This allows use
of cables of
up
to
50
feet
in
length
between
the
Modem
and
the
computer.
The
Modem
has
two
rs~232-c
connectors:
a
standard db-25
connector
(labeled RS-232C)
which
is
used
with the
majority of
computers, and the four-pin
din connector (labeled rs-232),
for
use with the
trs~80
Color Computer.
Cassette
Communications
The
Modem
I
contains special circuitry for
communications with the cassette
interface
of
Model
I
Level
II
computers.
To
use
this
method,
you'll
need
the
"Cassette-Comm" software
and
a
special
cable
(see Installation).