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a (..'onlrol character. If it is, the character is not
t ransi'l'rred to the
2ll
re~ister,
hut instead performs
the ncccssan° control operation that is required by
th('
charal'tt'~'.
Once the character reaches the 2D
r('~ist('r,
it is
ched~edto
determ inc
if
it is a print
or function character.
If
a print character, the
character is gated to the print dccode circuit and
then to the I/O to be printed.
If
the character is a
function character, the charactcr is sent to the func-
tion decod(' circuit and from there to the I/O to per-
form the function.
The 2B register is used only on
r('L'cin' operations.
In summary then, the recehoed scrial character
is deserialized by the
S
register under control of the
Serdes
Clnck.
Tlwn, under control of the control
clock,
the
l'haraetl'l' is transferred from the
IB
reg-
ister to the 2B register and then on to the I/O printer.
In transmit mode, the parallel character is loaded
directly into the
1ll
rr'!';stt.', transferred to the
S
register, 'serialized, and sent out on the line.
CODES
• Data is represented in the standard
2740
by PTTC/
ncn
or PTTC/E13(,D code.
• The standard
2741
uses correspOl. ,encc (Standarti
Selectric typewriter prillt element) code.
• The check-hit mairLains odd parity for checking.
The standard
2,,10
uses hinary coded c\e,'imal (BCD)
code consisting of six information bits and a check
bit
arrang-ed:
ll, A, S, I, 2,
t,
C.
The
six infor-
mation bits identify all alphanumcric characters for
the
I/O
Selectric as
\\'ell
as
all
<..'lll1tro1 codes ne: rled
for semiautomatic opel':.ltion while communi<.:ation is
in process. The chcck bit maintains odd parity.
A
\Oalid character must contain an odd number of bits
and the code for an eyen-bit chaloactCl" will contain a
check hit.
A 27 -lO
character
is
generated by the
I/o
trans-
mitting contacts and · :anslated to BCD in a diode
matrix before
lea\'in~
the I/O. BCD from the line is
used throughout the
2740.
Figure 11 shows the BCD
code chart. The standard 27
41
uses corr(>spondence
code (Figure
12).
Characters are transmitted and re-
ceived by the
2741
in the same bit order as for BCD
code but the bit configuration for the same character
may be different. For example, the letter A in BCn
code is B, A,
J;
in correspondence code an A is B,
4, 2, I, plus the check
),:t.
Which of the two codes is
used in a terminal is
lp:~l'mini,rf
hy mechanical
changes within the
I/().
No CirC(llt changes are re-
quired. Either the 2740 or th"
2741
can be supplied
with any of the three cou\:!;·.:
,,:lJ.,
espondence, BCD,
or extended BCD
(Fi.:~Jrc
• ). I' 'wever, all inter-
connected terminals
i
:1
),stCD'
hOllrl usp the same
, code.
Character Format
• Start
of
a character identified hy a start (ST) hi t.
• Eml
ora
character identified by a stop (SP) hit.
• Start hi
L
is always a O-hit
(no-bit).
• Stop
bi
tis always a I-bit
(bit).
A
nCD character consists of
iJ~ven
bits. In llrder to
lise this character as the input or output of the term-
inal, two bit positions are added. These arc the start
bit
(ST) and stop bit (SP); the S1' hit is always a O-bit
and the SP
bit
is always a
I-hit.
Thc start
hit
cnahles
t he
terminal to recoj.,rnize the start of a new character.
(The line is maintained in a
markin~
condition between
characters.) The stop· hit separates the characters.
Without
a
stop bit, the two BCD characters "('" and
"A",
for example, would run together as follows:
ST
B A
~
l
~
1 C
ST
B A 8 4 2 1 C
o
1 1 0 0 0
11
0
Because the arrival of a start bit is not easily iden-
tified, a stop bit is added at the end of the
character~
The stop bit allows the interval between characters
to reset the terminal in preparation for the next
character.
Control Codes, 2740/27
-u
The
cont rol codes along with the shorthand repre-
sentation of the codes are shown in Fig1.lres
11. 12,
anJ 13.
When the terminals are in control mode,
the codes do not print.
®
and
©
are not printable
in any mode. When the terminals are in recei\'e or
transmit mode, other codes will print. Thus, for
the most part, the machine mode determines the in-
terpretation of the control c(ldes.
POWEH HEQUmEMENTS
The power circuit should be a separate three-wire,
single-phase branch circuit from the power distri-
bution panel. The green wire should be connected to
ground, not to current neutral.
Voltage
11j
or 208/230
±
10
1
~
at
15
amperes
Frequency GO
±
1/2
Hz*
Phase
1
KVA
o.
F)
*Other voltages and GO Hz are available for World
Trade Corporation terminals. See "Power
Supply. "
11
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