Radio Shack TRS-80 Model III

TRS-80
MODEL
Asynchronous
transmission
over
a simple
twisted wire
pair can
be
accomplished
at
moderately
high baud
rates
(10K baud or higher,
depending
on the length
of wire,
type of drivers,
etc.).
Transmission
over
the telephone
network
is
generally
limited
to
approximately
2K baud
and a modem
is required
to convert
the
data pulses
to
tones that
can
be transmitted
through
the telephone
network.
Radio
Shack's
Telephone
Interface
is the ideal
modem
for this
RS-232-C
Interface.
Signal
Conventions
The
E.I. A.
RS-232-C
electrical
specification
defines
voltage
levels
and
corresponding
logic
conventions
associated
with
data
and control
information
transmitted
between
equipment.
For
data
interchange,
the
signal is
considered
in
the
marking
condition
when the
voltage
measured
at the
interface
point is
more
negative
than
-
3
Volts(with
respect
to signal
ground).
The
signal is
considered
in the
spacing
condition
when the
voltage
is
more
positive than
+3
Volts(with
respect
to signal
ground).
The marking
condition
corresponds
to
a
logic
one
(l) and
the
space
condition
corresponds
to a logic
zero
(0).
For timing
and
control
interchange
circuits,
the
function
is
considered
to be
"on" when
the
voltage
on the
interchange
circuit
is
more
positive
than
+
3 Volts(with
respect
to signal
ground);
and is
considered
to
be
'
'off
'
when
the
voltage
is more
negative
than
-
3 Volts(with
respect
to signal
ground)
.
The
"
on
"
condition
corresponds
to
a
logic
zero
(0)
and
the
'
'
off
'
condition
corresponds
to a logic
one
(
1
) . The
following
table
summarizes
this
information.
NOTATION
INTERCHANGE
VOLTAGE
Negative
Positive
Binary
State
Signal
Condition
Function
1
Marking
OFF
Spacing
ON
Table
1. On/Off
Condition
252