Data Communications
Asynchronous/Synchronous Decisions
For
eachdesired multipoint
data
link
you
must
firstdecide
whether
toemployasynchronous
or
synchronous transmission. The following considerations
may
be helpful
in
making
this
deci-
sion:
• Synchronous
transmission
is generally more efficient
than
asynchronous
and
may, there-
fore, provide
better
thoughput.
• Synchronousmodemsoperate
at
higher
speeds
than
asynchronous modems (1200-9600
baud
as
compared
with
300-1200 baud).
• Asynchronous modems
are
less expensive
than
synchronous modems.
• With
an
asynchronous
data
link
you can use
the
HP
30037A Asynchronous
Repeater
to
achieve
greater
computer/terminal, modem/terminal,
and
terminal/terminal
distances.
• The
async~ronous
daisy-chain cable provides differential signals which give
better
((noise"
immunity,
thus
reducing
the
number
of retransmissions
in
electrically ((noisy" environ-
ments.
Hardwired/Modem Decisions
Having
chosenbetweenasynchronous
and
synchronous, you
must
then
decide
whether
tousea
hardwired or modem connection.
A hardwired connection, where feasible, is
the
cheaper
alternative
because
it
eliminates
the
use of modems
and
common
carrier
(telephone company) lines.
If
an
RS-232-C network is to be used, a major consideration
in
selecting which
type
of
connectiontouseis
the
anticipateddistancebetween
the
first
terminal
and
the
computer.
If
the
first
terminal
will be located
in
the
vicinity of
the
computer system you
may
use a
hardwired
connection. RS-232-C specifications
limit
cable lengths to a
maximum
of50 feet (15 meters),
although
with
an
asynchronous hardwired configuration you
may
use
the
HP
30037A Asyn-
chronous
Repeater
to extend
this
distance.
Another
consideration is
the
desired usage (terminal load) of
the
particular
computer port.
If
you wish to connect several groups ofterminals
in
geographicallydiverse locations to
the
port,
then
you
must
choose a modem connection with a leased multidrop line.
Figure
7-7
illustrates
such a configuration.
7-16
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