User`s manual
Bit
Position
P
6 5
4 3 2
0
01aracter 1 0
0
0
Character 2
0
0 0
0 0
Character 3
0 0
0 0 0
Character
4
0
0 0
0 0
LRC-8
BCC
0 0
0 0 0
Figure
B-1
Longitudinal
Redundancy
Checking
An infinite
number
of
constants
may be used
to
per-
form the
CRC
division.
The
DVII
makes available
two
CRC
computations:
CRC-I6
(which uses a poly-
nomial
of
the form X
l6
+ X
l5
+ x
2
+ I),
and
CRC-
CCITT
(which uses a polynomial
of
the form X
l6
+
x
12
+ X 5 + I). Each generates a I6-bit BCC.
B.3
BSC PROTOCOL (BISYNC)
One
0f
the
most
widely used
protocols
is
IBIVi's
Bina-
ry
Synchronous
Communications
(BSC). BSC, also
known as
HIS
Y NC, has been in use since 1968 for
transmission between IBM
computers
and
remote
terminals
of
the batch
and
video display types.
LRC
is
the
modulo
2 sum (exclusive-OR)
of
the
bits
in
each bit position
of
all characters in a message
block
to
produce
a BCC.
The
figure shows
the
BCC
computation
for four 8-bit characters using
LRC.
Each
character
contains seven
data
bits
and
an
odd-
parity bit.
B.3.1 Controlling Data Transfers
The
format
of
a BSC message
is
shown in
Figure
B-2.
BSC uses control characters
to
delimit the fields.
The
header
is
optional;
if
it
is
used, it begins with
SOH
(Start
of
Header)
and
ends with STX
(Start
of
Text).
The
contents
of
the header are defined by
the
user.
Polling
and
addressing
on
multipoint
lines are han-
dled by a
separate
control
message
and
not
by using
the header field.
The
text
portion
of
the
field is vari-
able
in
length
and
may
contain
transparent
data.
If
it
is
defined as
transparent,
it
is
delimited by
DLE
(Data
Line Escape)
STX
and
DLE
ET
(End
of
Text),
or
DLE
ETB
(End
of
Text Block).
The
block
is
termi-
nated by the BCC.
sse
protocol
employs a
ngorous
set
of
ruies for
establishing, maintaining,
and
terminating
a
commu-
nications sequence. A typical exchange between a
data
terminal
and
the DV
II
jPDP-II
on
a
point-to-
point
private line
is
illustrated in
Figur~
6-~
B.3.2 Error Checking and Recovery
To
detect
and
correct transmission errors, BSC uses
either
VRCjLRC
or
CRC,
depending
upon
the
char-
acter code.
If
the code
is
ASCII,
a
VRC
check
is
per-
formed on each
character
and
an
LRC
on
the whole
message.
The
LRC
becomes
one
8-bit BCC.
If
the
code
is
EBCDIC,
CRC-16
(X16 + XiS + x
2
+
1)
is
used, resulting
in
a I6-bit BCC.
I f
the
Bee
transmItted
dOeS
not
agree
with the
Bee
computed
by the receiver,
or
if
there
is
a
VRC
error, a
NAK
sequence (shown in
Figure
B-3) is sent
back
to
the
data
source. BSC calls for the retransmission
of
the block when an
error
occurs.
BSe
will typically
retry three times before concluding
that
the line is in
an
unrecoverable state.
BSe
checks for sequence
errors
by alternating positive acknowledgments
to
successive blocks. ACKO
and
ACK
I
are
the respon-
ses
to
the even-numbered
and
odd-numbered
blocks
in the message, respectively. These
are
sent in sepa-
rate
control
messages.
B.3.3 Character Coding
BSe
supports
ASCII,
EBCDIC,
or
6-bit
Transcode.
Table
D-I
lists
and
describes certain bit
patterns
in
each set
that
have been set aside for
the
required BSC
control
characters. Some BSC
control
codes
are
mul-
ti-character sequences.
11-2898
Figure B-2
BSe
Data
Message
Format
B-3