Data
Communications
INTRODUCTION
7
Data
communications (datacomm) refers to
the
transfer
of
data
between
the
terminal
and
a
host computer.
The
datacomm
system
can
be of
either
the
point-to-point or
multipoint
type. Considerations
involved
in
selecting
the
type
are
discussed
early
in
this
section.
In
addition, for
each
type
of
system,
this
section discusses
the
following items:
• Considerations involved
in
selecting
either
a modem
or
hardwired
connection.
•
Installation.
• Configuration.
•
Considerations
involved
in
selecting
either
an
asynchronous
or
synchronous
system
(multipoint systems only).
• Considerations involved
in
selecting
either
a daisy
chain
or
DSN
data
link
type
connection
(multipoint systems only).
• Information
required
for
writing
programs which interface
with
the
datacomm
system.
Following is a
list
of
datacomm
terms
with
their
definitions,
as
used
in
this
discussion.
Data
Link
The
means
by which a
terminal
is connected to a
host
computer.
This
always
includes some
type
ofcommunications line (a coaxial cable,
the
public tele-
phone network,
or
a leased telephone line),
and
it
may
also include a
pair
of
modems (one
at
each
end
of
the
line).
Point-to-Point A
data
communications configuration
in
which a single
terminal
is
con-
nected to a
host
computer over a
data
link.
Multipoint A
data
communications configuration
in
which two or more
terminals
are
((chained"
together
so
as
to
share
a
data
link
to a
host
computer.
Asynchronous A mode
of
transmission
in
which each
data
character
is
framed
by a ((start
bit"
and
one
or
more
((stop
bits".The
interval
between
successive
data
charac-
ters
is random.
7-1
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