Although
the path of a data character through the
interchange is not continuous (but is interrupted by
step-by-step
similar processing of all other characters
needing
service), the following simplified description
ignores
interruptions and considers only the successive
operations
on characters of one message. On input,
these
characters come in
from a terminal, proceed
through the interchange, and are sent to the high-speed
input line data set; on output, they follow the reverse
process
and are sent out to a terminal.
Receive
Low-Speed Mode
When
the interchange receives the message from the
terminal,
each character is made up of nine serial bits:
a start bit, a Bcp parity check bit, six Bcp character data
bits, and a stop bit (Figure 24). As each new bit of a
character is shifted into the data register, the preceding
bits are shifted one position farther into the register.
When the stop bit is received, the most remote position
of the register senses the start bit and the register con-
trols send the other eight bits of the character into
storage. The start bit is dropped at this point.
As
these characters are assembled, they are placed
in a buffer. This buffer is assigned and its address is
placed
in the control word at the start of the message
(or message segment). Other portions of the control
word are similarly determined automatically.
Send High-Speed Mode
As soon as the 98 data characters have been received,
the proper end-of-message is inserted (if not received
already) and the message is assigned to the input high-
speed line control. Immediately, a queue counter steps
one position. As other messages are completed, they are
similarly transferred and the queue counter steps ac-
cordingly. Therefore, at the time that the go-ahead
message is received, the interchange has an accurate
count of the number of messages waiting to be trans-
mitted. As each message is transmitted, a message-
transmission counter is stepped 1 and the buffer con-
taining that message is made
available for other mes-
sages. As soon as the message transmission count equals
the queue count, the terminal interchange knows that
it has transmitted all of its messages; it then initiates
a go-ahead message.
Receive High-Speed Mode
At the start of receiving a message from the data
processing center, a buffer
is assigned to the control
word for the output high-speed line. The interchange
receives each character from
the high-speed line and
adds a parity check
bit. The character
then goes into
storage in the position designated
by the control word
storage address. This
process continues
until, at the
24
end of the high-speed message, the check character
accumulated
(within the interchange) for this particu-
lar message is compared against the check character
that is received as the last character of the message.
If the comparison does not agree, the buffer used to
accumulate
the message is made available for service,
thus
discarding the incorrect message. (If this was
an incomplete or unsolicited message, the data process-
ing center, failing to receive an answer-back, will re-
transmit the message. If the erroneous message was a
reply
to an inquiry, the agent, failing to receive the
reply, presses the repeat button.)
If
the message is correct and
accepted by the inter-
change,
the control for that
buffer is transferred to the
control
word for the low-speed
line. When the receive-
high-speed
message is completed,
accepted, and trans-
ferred,
the interchange
is ready to
service the next
message.
This transfer process is so rapid that mes-
sages
may arrive with no delay between them.
Send Low-Speed Mode
Under
control of the interchange, characters are sent
out
to a terminal at the terminal’s correct operating
speed.
Since low-speed output takes place immediately
if
the terminal addressed is
not busy, no queuing con-
trols
are necessary. If the
terminal is occupied, how-
ever,
the output message is
put in a waiting status. As
soon
as the terminal is free,
the message is sent out.
Receipt
of an incomplete or unsolicited message causes
the
interchange to respond
with a confirmation-of-
receipt
message after the output
message itself is sent
out
to the terminal.
Special
Considerations for Telegraph
The terminal interchange can process telegraph mes-
sages
as direct input and output.
During
receive-low-speed mode, the five-bit tele-
graph
characters are assembled in the five high-order
data
bits of each scp interchange-input character.
Zeros
fill out the character (Figure 25). The inter-
change
can recognize either
a three- or four-character
telegraph
end-of-message.
(Any one particular inter-
change
can recognize only
one type of telegraph
end-of-message.)
After recognizing the end-of-message
sequence,
the interchange inserts a 9000 series BcD
end-of-message-complete
character to terminate the
message.
On output, the characters (including the telegraph
end-of-message
sequence) are received from the data
processing
center in the character format described
above.
The 9000 series pcp end-of-message character
follows the telegraph end-of-message sequence but is
not
transmitted beyond the interchange.
Y)
Tt. |—Start Bit
—
BCD
Parity
Check
Bit——
—N
eee
i
—
|. —| +—
BCD Data
Bits ies
os ah
e
BCD Data
Bits aces
>
0
Stop
Bit
0
out 0
Output
Input or Output
Character
Char-
Character
in
acter
(Terminal
«+Interchange)
Interchange
(Computere>
Storage
Interchange)
Figure 24.
Changes
in Character
Composition
between
the
Data Processing
Center
and Terminals
BCD
Character
Format
Telegraph
Code
(A 1 bit indicates
a
hole in punched
paper
tape)
Shift
vas
@
~<
.
oO
CRON
ON
xs
CIID
WO
ZS
AS
Ot
moO
e
>
TWSHYTIAAT
Aaa
aan
aa
Anna
r
ree
eer
ere
Ayer
ere
ere
eeeeee
~S9O-|"“CO-=|--S9]0
0-0
Ol—-
O—-—
colo
—
~9
Of—
—
~o-|—
=
-o
-lo
—-o-clogp
oo
=H
oo
o-|-
~o
=
|p
orm
Oj-
COCO
H|Oo—~
~|Oo
=
=~
~oJ0O
0-0
Ol
~C
0
H|0
—
—
-olo0o
=
=
loo
—-—-
Hlo0oo—--
-o-ocoloo—-o-—|>
~“~—O-|-
-~CO9|0O—~o]}m
CO—
+10
-~O-O]-
=
~D
OOO
|
-O]]
—
O=—
lo
Oo
—
Hlo
m=
Ho
H-lo—--o-loo-ocolea
STO
OF
Ono
SS
Ol
Oe
a
Se
OOO
SIO
O
ONO
Oo
=
Slo
Oo
Ole
so
|i
lo
So
a
|
5
$
&
#
)
(
is
[E
Figures
Shift
Ch)
L/F
Carriage
Return
ea)
L/F
Space
(=)
L/F
Letters
Shift
er)
E bell
=)
LF
Line Feed
(
wa
2
eS
oie
Se
eS
Sle
SH
Slo
OOS
Hoe
So
omic
20
O—
6
Oo
=
Or
lso
=
Slo
—
|
os
5
0
]
0
0
0
0
1
]
0
0
0)
]
]
0
1
]
]
0
0
]
0
i
]
]
1
1
i
1
1
)
0
]
1
0
0
]
]
0
0
0
]
]
0
]
1
]
0
]
]
]
0)
0
i
0
0
0
0)
C900|-00—=|-o
s|—-
=o
=
-|0
00
—
00
=
~ofo
=
|
|
ol
oo
=
Ho
olo
om
aclooo
olny
eesegeeee
eco
oc9
COCO
O|C
OOO
OCOC
CO
O|CCCCO|OCOCOOlOoCDC
OOD
OCC
Oloco
CoO
Oloc
coc
ol
OCS
OO
|=
=
O10.
HO
Of.
Solo
=o
Of
—
i
al
onl
=
Ol
clots
Giotl
O16
oO
ele
io
os
or
Oo-
SCO
H9
=
90
=
—-
-Oo190
-~9O/—
=CO
—|O
—~
=
=
aloo
=
=
H|\oo—-
—-
slo
oOo
—
=|
wos
cloo0-—o
|p
—~—S+—==S9O0O—“Cl=
GO
16-6
Jol]
=
so:dlo.0
=
ol
oO
16
0
=
lo
=
10
lo
so
loos
oO
ole
F |
Note: The content
of the
C position
is disregarded
on output;
on input
no bit is entered into
the C position.
Figure 25.
Direct Input-Output
Telegraph
Code
in scp
Character
Format
Terminal
Interchange
25
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