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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-seriai.
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
in
Figure
1
.
The
start
element
is
a single
logic
zero
(0)
data
bit
that
is
added
to
the front
of each
character.
The
stop
element
is
a logic
one
(1) that
is
assed to the
end of each
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
modem
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.
STOP
ELEMENT
STOP
ELEMENT
,
1
/
^T^zzO^—
L
-^D==
=
r
rL
—
START ELEMENT
ONE
8 BIT
CHARACTER
START ELEMENT
ONE
8 BIT
CHARACTER
(11001000)
(00100000)
FIGURE
1.
ASYNCHRONOUS DATA
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