Data Communications
For
point-to-point operation,
the
driver
program
must
also
support
the
type
of
datacomm
handshaking
selected on
the
datacomm
configuration
menu
(ENQ/ACK
and
XON/XOFF).
Refer to
((Pacing
Mechanisms",
later
in
this
section, for
information
on
both
handshaking
types.
The
operation
of
multicharacter
transfers
is
described
in
Section 9.
Note:
The
computer
should
not
be allowed to echo back
information
that
has
been
transmitted
as
a block from
the
terminal.
Start And Stop Bits
These
hardware-generated
bits
are
used for synchronizing
the
transmit
and
receive devices
in
an
asynchronous
environment.
A
start
bit
is a
((zero"
line
state
that
lasts
for 1.0
bit
time;
it
is
affixed to
the
beginning
of
a
serial
character
bit
stream
(which
may
also include a
parity
bit).A
stop
bit
is a
mark
or
a
((one"
line
state
that
lasts
for 1.0,
or
2.0
bit
times;
it
is
appended
to
the
end
ofeach
serial
character
bit
stream.
Mter
the
stop bit,
the
line
remains
in
the
mark
state
until
the
next
character, signified by a
start
bit, is
transmitted.
Parity Checking
In
an
asynchronous point-to-point
environment,
this
terminal
provides a
vertical
redundancy
check (VRC), which
is
a character-based
error
checking
mechanism
for
non-binary
data.
With
VRC,
an
additional
bit
is
affixed to each
character
to
provide
an
expected
high-order
bit
state
for
each
character.
This
type
of
parity
generation
and
checking
is
a
means
of
determining
the
validity
of
data
transfer
on a character-by-character basis.
When ((NONE"
parity
is
selected,
eight
data
bits
are
exchanged.
Otherwise,
seven
data
bits
and
the
selected
parity
bit
are
exchanged for a
total
of
eight
bits.
This
terminal
offers
the
following five
types
of
parity:
1. 0'8.
The
high-order
bit
is
always
a zero.
2. 1'8.
The
high-order
bit
is
always
a one.
3. ODD.
The
high-order
bit
is
set
to a zero
or
a one, whichever produces
an
odd
number
ofone
bits
in
the
overall
character
representation
(the
seven
data
bits
plus
the
eighth
parity
bit).
4. EVEN.
The
high-order
bit
is
set
to a zero
or
a one, whichever produces
an
even
number
of
one
bits
in
the
overall
character
representation
(the
seven
data
bits
plus
the
eighth
parity
bit).
5. NONE. No
parity
bit
is
sent,
all
eight
bits
are
significant
data
bits.
7-11
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