Data
Communications
DATA
CHECKING.
There
are
two
types
of
data
checking used
with
the
multipoint
protocol.
The
first
is a check ofeach
character
as
it
is received
and
is called a
vertical
redundancy
check
(VRC)
or
parity.
This
check isonly used for ASCII characters.The second is a check of
an
entire
block
of
data
and
iscalled a block check.Two
types
ofblock checking
are
available.
The
first
is
a
Longitudinal
Redundancy Check (LRC). The second is a more complex method called a Cyclic
Redundancy Check (CRC). Note
that
a CRC is a more thorough form of
data
checking
than
a
LRC
and
that
both
the
HP
1000
and
3000 computers use
the
CRC.
Character
Checking.
The
vertical redundancy check is also known
as
a
parity
check. When
an
ASCII
character
is
transmitted
by
the
computer
or
the
terminal,
the
high
order
(eighth
bit
of
each
character
is
set
to a
cC1"
or
a
cCO"
to
make
the
number
of
CC
1"
bits
in
the
character
either
even
(EVEN
parity)
or
odd (ODD parity).
There
is also a
variety
ofVRC
in
which
the
parity
bit
is
always
set
to a "0"
or
a
"I".
The
parity
must
be
the
same for both
the
computer
and
the
ter-
minal.
For
example, if
even
parity
is used
the
high
order
bit
of each
character
would be
set
to
cause
the
number
of
"I"
bits in
the
character
to
be even.
Character
checkingis
not
done
when
ASCII8 codes
are
used
or
when
operating
in
transparency
mode.
The
types
of\lRC
available
for use
in
a
multipoint
configuration
are
as
follows:
EVEN, ODD,
O's
(parity
bit
always zero)
l's
(parity
bit
always one)
NONE
(no
parity
used - 8-bit
data
only)
Block Checking.
Each
block includes a Block Check
Character
(BCC).
The
BCC
is
in
additionto
the
parity
bit
associated
with
each
character
(VRC).
The
BCC
can
be
either
a
one-character
(LRC) or two-character (CRC) check sum.
To
select which
type
of block checking you
want
performed you
use
the
cCBCC"
field of
the
datacomm configuration
menu
described
earlier
in
this
section.
The
LRC
character
is
a 7-bit check
sum
obtained by exclusive
cCOR"ing
the
low order 7
bits
of
each
character
included
in
the
text
block. A
parity
bit
(VRC) is
then
added to
this
character
when
it
is
transmitted.
For
ASCII8 all 8
bits
are
cCOR'ed"
together
and
no
parity
bit
is
generated
for
the
LRC character.
The
CRe
is a 16-bit (two-character) check
sum
calculated
using
a
formula
that
is compatible
with
that
used by
the
IBMBisynchronouscommunications protocol.VRC
parity
is
never
added
to
these
characters.
TRANSMISSION
CODE
(ASCII7/ASCII8).
The
terminal
can
be
set
to
use
ASCII 7-bit
or
ASCII 8-bit
data
code.
The
uASCII8" designation specifies
that
8-bit codes (with no VRC
conversion) be used. All
terminals
on
the
same
communication
line
must
use
the
same
transmission
code.
You
use
the
CCCode"
field of
the
datacomm
configuration
menu
to select
the
desired
type
of
transmission
code.
7-36
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