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SUB (Substitute)
The SUB end-to-end control character is not used with
the 2780 terminal.
DEL (Delete)
The DEL end-to-end control character is not used
with the 2780 terminal.
TRANSMISSION CHECKING
Redundancy Check
A redundancy check is performed on all data. A
check character is accumulated for each record of
data at both the transmitting and receiving terminal.
The check-character accumulation is initiated by,
but does not include, the STX character. All char-
acters--except SYN--following the STX,
to
and in-
cluding the end-of-record character, are part of the
accumulation. An STX character following a US
sequence is optional.
If
STX is used following a US
check sequence, it is included in the redundancy-
check accumulation. The receiving terminal com-
pares the received check character that follows the
end-of-record character with the one it has accumu-
lated.
If
the redundancy accumulations are different,
an error has occurred.
The method of accumulating the check character
varies depending on the code being used:
34
EBCDIC--A 16-bit cyclic accumulation using the
polynomial x 16
+
x 15
+
x 2
+ 1.
This check
character is sent as two eight-bit bytes and is
referred to as the C RC (cyclic redundancy
check) character
0
Six- Bit Transcode--
A
12-bit cyclic accumulation
using the polynomial x 12
+
x 11
+
x 3
+
x 2
+
x
+ 1.
This check character is sent as two six-bit
bytes and is referred to as the CRC character.
USASCII--Odd-parity VRC (vertical redundancy
check) on each character, and an eight-bit
cyclic accumulation for the record using the
polynomial x
8
+
1 (LR
C).
The LR C (longitudinal
redundancy check) character is sent as one eight-
bit character. A VR C check is performed on
the LRC character.
The redundancy-check character is accumulated
serially by bit. A pad character is required follow-
ing the block-check characters to cause insertion of
the last byte or character in the redundancy-check
register.
For correct transmission of a record, the
redundancy accumulation must be all zero bits fol-
lowing complete insertion of the block-check charac-
ters.
Therefore. the end-of-block-checking sequences
are as follows:
Six-Bit Transcode
E C C p
ECCp
or EBCDIC
TRRa
TRRa
B C Cd
XC Cd
USASCII
E L P
E L P
TRa
TRa
BCd
XC d
NOTE: The two checking characters (CRC and LRC)
are shown in examples throughout the remainder
of the manual as bcc (block-check character).
The LRC check character is transmitted as a single
block-check character and the CRC check charac-
ters are transmitted as two bcc characters.
The
block-check character is shown in lower case to
distinguish it from text and data-link control charac-
ters.
A pad character must always be transmitted at the
end of any transmission. The pad character trans-
mitted by the 2780 consists of all "1" bits. The first
character received by the 2780 after a US sequence
will be treated as data unless
it
is a defined control
character.
The end-of-record characters are:
US-Unit Separator: Encoded after a record of
text. This character does not cause a line
turnaround, but it does cause checking to be
performed at the receiver. This character may
also be called ITB (Intermediate Block) or IUS
(Information Unit Separator).
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