Data Communications
Synchronous
Half
Duplex
Full
Duplex
A mode
of
transmission
in
which
data
is
sent
in
a continuous
stream
with
no
intervals
between successive characters. When
there
is
no
data
being
sent
the
communications line
is
in
the
cCidle"
or
cC
ones
"
state.
At
the
start
of,
and
during,
each
transmission
the
terminal
and
the
computer
maintain
synchronization
with
one
another
through
the
use
of
SYN
(ASCII decimal
code 22) control characters.
A
data
link
in
which
data
can
be
transmitted
in
only one direction
at
a time.
Each
time
the
direction of
the
data
flow
is
reversed,
the
modems on
each
end
of
the
line
must
switchfrom CCtransmit"state
to
cCreceive"state (or vice versa).
This
state
transition
is called a
CCline
turnaround".
A
data
link
in
which
data
can
be
transmitted
in
both
directions
simulta-
neously.
Character
Mode When
the
terminal
is
operating
in
Character
mode,
it
sends
data
characters
to
the
computer
one
at
a
time
as
they
are
typed
into
the
keyboard.
Block Mode
When
the
terminal
is
operating
in
Blockmode,
data
characters
typed
into
the
keyboard
are
merely stored
in
display memory. When a block
transfer
is
subsequently
triggered
(by
the
host
computer
or
by
pressing
the.
key
or
another
appropriately
defined key), a group
of
data
characters
is
sent
from
the
terminal
to
the
computer
as
a block.
POINT-TO-POINT OR MULTIPOINT
The
first
decision you
must
make
is
whether
to
establish
a point-to-point
or
multipoint
configuration.
The
term
CCmultipoint"
as
used
in
this
manual
refers to a
Hewlett-Packard
multipoint
terminal
configuration
in
which
up
to 32
terminals
may
share
a single
data
link.
This
type
of
configura-
tion
provides more extensive
transmission
error
checking
than
is
performed
in
point-to-point
and
it
provides
an
opportunity
for noticeable cost
savings
through
the
use
of
shared
resources
(modems,
data
lines,
computer
interfacechannels).
Terminals
within
an
HP
multipoint
config-
uration
are
physically organized into groups.
Within
each
group
the
terminals
are
daisy-
chained
to one another,
with
distances
up
to
2000 feet between
terminals.
Each
daisy-chained
group
shares
a single modem
or
hardwired
link
to
the
host
computer.
HP
multipoint
configurations operate only
in
Block mode
and
they
may
only be used
in
conjunction
with
a
host
computer
that
supports
this
capability
both
from a
hardware
and
systems software
standpoint.
A point-to-pointconfiguration, on
the
other
hand, is
the
standard
form
of
data
communications
within
the
industry
(it is sometimesreferred to
as
a cCTeletype-compatible"
data
link). Point-to-
point
is
supported
by
most
computers.
At
any
given
time
it
accomodates only one
terminal
per
data
link;
it
may, however,
operate
in
either
Character
mode
or
Block mode.
7-2
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