User`s manual
Table
B-1
BSC
Data Channel Control Codes
Control
Code Mnemonic
SYN
SOH
STX
ITB
ETB
ETX
EDT
ENQ
ACKO/ACKI
WACK
NAK
DLE
RVI
TTD
DLE EDT
Meaning
Synchronous Idle
Start
of
Heading
Start
of
Text
End
of
Intermediate Trans-
mission Block
End
of
Transmission Block
End
of
Text
End
of
Transmission
Enquiry
Alternating Affirmative
Acknowledgmen ts
Wait-Before-Transmit
Positive
Acknowledgmen ts
Negative Acknowledgment
Data-Link Escape
Reverse Interrupt
Temporary Text Delay
Disconnect Sequence for a
Switched Line
8.3.4
Data
Transparency
In
BSe,
the
transparent
mode
is
defined
by
starting
the
text field with
DLE
STX.
Once
in
transparency,
the
only
control
character
of
significance
is
OLE.
Any
Data
Link
control
characters
transmitted
during
the
transparent
mode
must
be
preceded
by
a
OLE
B-5
control
character
to
be
recognized
as
a
control
func-
tion.
When
a
bit
pattern
equivalent
to
DLE
appears
within
the
transparent
data,
two
DLEs
are
used
to
permit
transmission
of
DLE
as
data.
When
received,
one
DLE
is
disregarded;
the
other
is
treated
as
data.
This
technique
is
called
"character
stuffing."
B.3.5
Data
Channel Utilization
BSe
transmission
is
half-duplex.
The
line
must
be
turned
around
iwice between each block (once for
the
acknowledgment
sequence
and
once
for
the
data
block).
All fields
are
delimited
by
control
characters,
and
acknowledgments
are
handled
by
separate
con-
trol
sequences.
An
acknowledgment
sequence
is
required
for
each
block
and
for
each
acknowledg-
ment
sequence.
A
minimum
of
two
character
times
is
required
for
each
synchronization.
BSC
supports
both
point-to-point
and
multipoint
lines.
8.3.6
Synchronization
BSC
synchronizes
on
each
block
or
control
sequence
by
preceding
the
formatted
block
with
the
synchro-
nizing
(SYN)
characters.
Two
synchronizing
charac-
ters
are
required,
but
more
(usually
five)
are
sent.
SYN
is
defined
as a Unique
bit
pattern
in
each
of
the
three
information
exchange
codes
available
with
BSC. In
addition,
some
BSC
applications
require
that
all Is
PAD
characters
follow
messages.
B.4
DnCMP
PROTOCOL
DDCMP
(Digital
Data
Communications
Message
Protocol)
was
developed
to
provide
full-duplex
mes-
sage
transfer
over
standard
existing
hardware.
8.4.1 Controlling
Data
Transfers
The
0
OCM
P
message
format
is
shown
in
Figure
8-
4. A
single
control
character
is
used
in a
DOCMP
message,
and
is
the
first
character
in
the
message.
Three
control
characters
are
provided
in
DOCMP
to
differentiate
between
the
three
possible
types
of
messages:
SOH
-
data
message
follows
ENQ
-
control
message
follows
OLE
-
bootstrap
message
follows.
Note
that
the
use
of
a
fixed-length
header
and
mes-
sage
size
declaration
obviates
the
BSC
requirement
for
extensive
message
and
header
delimiter
codes.