BCD
Code Type of Control
1 BA 8421
OO,
TiN Go Ahead
10 Hi Change
Interchange Address;
also TIC2
(ICA and OCA)
11.1111 First Character
of Character
Synchronization
11.1110 Second
Character of Character
Synchronization
11.1010 -----
10 1010 — Lights Start
10 1110 = TIC] (ICA and OCA)
1100 Type-Head
Carrier Return
(I-O Device); also
DI0(A Ticket-
01 1100 Space
(I-O Device)
ing Pushbutton)
10 1100 Tab
(I-O Device)(Available
only
upon request)
11 1100 Backspace (I-O Device)
00 1110 Type-Head
Carrier Return (I-O
Device)(Output Only)
01 1110 Segment Identifier
Ol 1111 -----
00 1101 End-of-Message
Incomplete
01 1101 End-of-Message
Complete
10 1101 End-of-Message
Unsolicited (Answer-Back,
OCA)
11. 110) End-of-Message,
for Pushbuttons
and Air Information
Device
(Signal for OCA
Start or Request
Repeat)
—-OQ-—-0—-4—-=c00-jo—-0—-0N0
(2)
°
Neven parity is used
for low-speed lines.
This check bit
is dropped before
high-speed transmission
and added
for low-speed
BCD output.
Figure 20. scp Control Code Summary
Message Format
Messages exchanged between the interchange and the
computer center are in this format: bit synchronization
group, character synchronization group, interchange
address, terminal address,
message, end of message,
and check character. If more
than one message is to be
sent, only the first requires
the synchronization group;
the others may then
be chained and
transmitted with-
out interruption.
On output messages, each
new 100-character (or
smaller) group is preceded
by the interchange
address,
includes the terminal address
and end-of-message char-
acters, and is, therefore,
received as
a new message.
Figure 21 shows the data
message format for both
input and output
messages on the high-speed
line to
the data processing center.
The first message trans-
mitted
after the interchange receives the go-ahead
message
from the preceding
terminal interchange is
prefaced
by two synchronization
characters. The first
of
these consists of six 1 bits; the second, of five 1’s
and
a 0 bit. These are followed by a one-character
interchange
address, followed
by a one-character ter-
minal
address. These
two characters
are added by the
terminal
interchange
to input messages;
they are en-
tered
by the program on
output. The interchange
address
is stripped
off (on output) as
soon as the inter-
change
receives the
message. The body
of the message
may
contain up to
98 characters followed
by an end-
of-message
character
that may be an
end-of-message-
complete,
end-of-message-incomplete,
end-of-message-
unsolicited
(output),
or end-of-message
from the push-
buttons
(and air information
device).
If the body of
the
message contains
98 characters, the
terminal inter-
change
address may be thought
of as a prefix to the
message,
because
it is not stored in
a terminal inter-
change
buffer. The
segment identifier
is a character
that
is added by the
terminal interchange
to all follow-
on
segments of long
input messages.
The segment
identifier
is not used in output messages.
The
last character
of the message
is a check char-
acter,
a polynomial character
of extremely high check-
ing
ability. This character
is added by
the interchange
to
input just before
sending the message
to the high-
speed
line. It is compared
with a check
character inde-
pendently
generated
by the computer.
In the output
message,
the check
character is used to
compare against
a
check character
independently generated
in the ter-
minal
interchange. On output
messages, the inter-
change
drops this
check character
after making the
comparison.
It also drops the two synchronization char-
acters
received at the beginning
of the first message.
Every
output data message
that is either incomplete
or
unsolicited elicits a return
message from the inter-
change
confirming its receipt.
The format of this return
Note 1 Note 2 Note 3 Note 2
A. wv: ‘A.
Syne Syne Inter- Terminal Segment
End of Check
I
I change Address Identifica- Message Char
Char Char Address (Stored in | tion BODY OF MESSAGE (Stored in
Buffer)
(Stored
in Buffer)
Buffer)
1111
111110 1 Char 1_Char 1 Char
As Many as 98 Characters 1 Char 1 Char
Note 1. Interchange adds these characters
to first input message after receiving
the go-ahead and drops them
from output messages, upon receipt.
Note 2. Interchange adds these characters
just before an input message goes
to the high-speed data set; on output
messages, it uses these characters
but does not transmit them
to the terminals.
Note 3. Interchange adds this character to all follow-on segments of a long input message.
Figure 21.
High-Speed
Data Message
Format
(Input and
Output)
22
J
message is: interchange address,
terminal address, end-
of-message-incomplete
character, and
check character.
The go-ahead message (Figure
22) differs somewhat
from the data message format.
It begins with the two
synchronization characters
and continues with the one
go-ahead character followed
by the one-character in-
terchange address and ends with the end-of-message
character followed by the
check character. The go-
ahead message format does not contain the end-of-
message or check character when it originates in a
terminal
interchange.
Only if
First Message Note 1
ro ‘A \ AN.
Syne Syne Go
Inter- End of Check
| iT] Ahead
change Message Char
Char Char Address
BRRRA 111110 1 Char 1 Char 1 Char 1 Char
Note 1. These characters are used only in the message from the Data
Processing
Center
Figure 22. Go-Ahead Message Format
The third unique type of
format is the change-
interchange-address
message. This message
is used to
modify the go-ahead message sent by the interchange
and keeps the line scan going
whenever an interchange
is removed from service.
Figure 23 shows
that this type
of message
consists of:
1. The one-character terminal
interchange address.
2. A character meaning
“change interchange ad-
dress.”
3. The next interchange address (one-character).
4, End-of-message character.
5. Check character.
Only if
First Message
a
Syne
Syne Inter- |Change
Next End of Check
|
I change | Inter-
Inter- Message} Char
Char
Char Address | change change
Address Address
WAI
Ao
1 Char 1 Char
1 Char |
1 Char 1 Char
Figure 23. Change-Interchange-Address
Message Format
Modes and Data Flow
The four modes of operation for the terminal inter-
change—receive low speed, send high speed, receive
high speed, and send low speed—have been defined.
To clarify what is going on inside of the terminal inter-
change, remember that each
100-character buffer of the
interchange storage is being operated upon in only one
mode
at a time. Also note that the expression “simul-
taneous” when applied to handling of messages within
the terminal interchange means the following (some
aspects of simultaneity have already been touched
upon but are repeated here to show the interleaving
of activities that permit simultaneous operation in up
to four modes):
1. On input from the low-speed line: The scanning
of
terminal input lines is so rapid that all bits on all
lines
are scanned many times during the period that
they
are present on the line. Interchange design in-
cludes
provisions for maximum possible rejection of
noise
and distortion. Random inputs from all terminals
are accepted simultaneously.
2. On output to the low-speed lines: Characters are
clocked
out in such a way that any number of messages
can
proceed at any time. Output messages to the ter-
minals
are released immediately on receipt from the
high-speed
line and processing in the interchange (if
the terminal is not occupied in handling another mes-
sage),
but without any type of synchronization link to
any other messages. Consequently, all messages can be
progressing simultaneously.
3. On output from high-speed lines: Messages are
received
and processed one at a time. On input to high-
speed lines: Messages are sent one at a time. However,
the two processes can go on simultaneously without
interfering
with each other or with the operation of the
terminals.
4.
Within the interchange: All characters are actu-
ally processed one bit at a time, under the direction of
the appropriate control word. What gives the practical
result of simultaneous processing of all inputs and out-
puts
is the speed of the internal operations of the inter-
change. These operations are so rapid that all possible
character activity can be handled during the real-time
life of any character that is coming from, or going to,
any line. The interchange processes a character bit of
one message, then a character bit of another message,
and so forth—dealing with each character or demand
for a character as it presents itself, regardless of what
message it is associated with. The automatic controls
call out the proper control word for each high-speed
or low-speed line and thereby
process the character
correctly.
Terminal Interchange 23
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