Radio Shack TRS-80 Model III - Page 261

APPENDIX
I /
rs-232-c
Technical
Information
Transmission
of
Digital Data
The
transfer of
digital data
over
relatively
long
distances
is
generally
accomplished
by
sending data
in serial
form
using
a
single
twisted
wire pair to
connect the
transmitting
and
receiving
devices. One
of two
general
transmission
techniques is
commonly
used,
asynchronous
or
synchronous.
The
transmission
technique
used
in the
Radio Shack
system
is
asynchronous-bit-serial.
Since we
don't use the
synchronous
technique,
we'll not
mention
it
again.
Asynchronous
transmission
does not
require a
synchronizing
clock
to
be
transmitted
with the data
and, the
characters
need not
be
contiguous.
This
means that
gaps
of varying
lengths may
be
present between
transmission
of
individual
characters.
The bits
which
comprise a
data
character
(generally
from five to
eight bits
in length)
and
synchronizing start
and stop
elements are
added
to
each
character as
shown
below.
The start
element is a
single
logic zero
(0)
data bit
that is
added
to
the
front
character.
The stop
element
is
maintained
until the
start
element of the
next
character is
transmitted.
There
is no
upper
limit
to
the
length
of the stop
element.
However,
there
is a
lower limit
that
depends
on system
characteristics.
Typical
lower
limits
are 1
.0,
1 .42
or
2.0 data-bit
intervals
(although
most
modern
systems use
1 .0
or
2.0 stop bits) .
The
negative-going
transition
of the
start
element defines
the
location
of the data
bits in the
character being
transmitted. A
clock
source at the
receiver is
reset by
this
transition
and is used to
locate
the
center of
each data
bit.
There
are
several good
reasons
for using
the
asynchronous data
transmission
system. A
clock
signal
does not
need to be
transmitted
with the
data, thus,
equipment
is
simpler.
Also, the
characters
don't
need to
be sent
all
at
one
time; they
can be
transmitted
as they
become
available.
This is
particularly
useful
when
transmitting
data
from
manual-entry
input
devices
(e.g. a
keyboard).
The
major
disadvantage
of
asynchronous
transmission
is that it
requires a
significant portion
of the
communications
bandwidth
for start
and stop
elements.
The rate at
which
asynchronous
data
is transmitted
is
defined
as the
baud
rate.
Baud rate
is the
inverse
of the
time duration
of the
shortest
signal
element.
Normally ,
this
is one data
bit
interval .
The baud
rate is
equal
to
the
bit rate
if one
stop
bit is
used; but
for
systems
which use
more than
one stop
bit, the
baud rate
does
not
equal the
bit rate.
STOP
ELEMENT
I
/
START
ELEMENT
STOP
ELEMENT
ONE 8
BIT
CHARACTER
(11001000)
START
ELEMENT
ONE 8
BIT CHARACTER
(00100000)
Asynchronous
Data
251