that it should be re-entered manually from the tele-
graph equipment.
Reader Check: This light indicates that an alarm
condition exists in the telegraph equipment.
Output Communication Adapter Functions
To transfer data from the Airlines Reservation System
to a telegraph system, the
oca translates the code,
inserts telegraph-code shift characters, and transmits
the message to the particular telegraph terminal equip-
ment addressed. Associated functions are: checking the
validity of each character received from the data
processing center, sending an
answer-back to the data
processing center if all characters are valid, and alert-
ing an operator to any alarm condition that may arise.
Translating and Validity Checking
The oca translates the six
data bits of the Bcp char-
acters into five-level telegraph code (Figure 27). The
output message from the data processing center arrives
at approximately 207 bits
per second, serially by bit
and serially by character.
The oca checks each scp
character for validity as it
is received. If it finds a
character with an odd number
of one bits (excluding
the start bit), it does
not send an answer-back
to the
data processing center.
Instead, it returns
to wait mode
and awaits a retransmission
from the data processing
center.
Transmitting
The oca transmits
data to telegraph
equipment in
five-level code characters.
The transmission
is serial by
character and
parallel by bit
(Figure 27A).
The rate
of this transmission
is governed by the
demand of the
receiving equipment.
In addition, oca transmits
control
characters to the
terminal interchange. See
“Output Communication
Adapter Message
Formats.”
Inserting
Shift Characters
As the output
message
passes through
the
oca, the
proper shift characters
are
inserted. When
an alpha-
betic character
is followed
by a
numeric character,
a
figures shift is
inserted between
the two. A
letters shift
is inserted
when
an alphabetic
character
follows
a
numeric character.
Special
characters and
punctuation
characters
require the
figures shift
and, therefore,
are
treated as numeric
when inserting
shifts.
Addressing
and Controlling
Telegraph
Equipment
At the beginning
of
an operation
period,
or
to reacti-
vate a telegraph
unit
that has
been
out of service,
the
30
operator
must turn a dial
switch on the oca to the
address
of the telegraph
unit being brought
into service
and
then push the
start key on the
oca. This notifies
the
data processing
center
that it can
communicate
with
this particular
telegraph unit.
The
first character
of
each output
message
as it ar-
rives
at the oca
is the address
of one of
the telegraph
units.
Data and control
functions are
then directed to
that
telegraph
unit. As stated
before,
the speed of
transmission
to
the telegraph
equipment
is governed
by
pulses from
the telegraph
unit. During
alarm con-
ditions,
data
transfer is stopped
and transfer
is not
resumed
until
the alarm condition
is cleared
by the
data
processing center
or by manual
intervention.
Alerting
the
Operator
to Alarm
Conditions
The
oca has
three types of
alarm conditions:
output
check,
input
check, and
telegraph
equipment
check.
The
last two
conditions cause
the audible
alarm to
sound
and
operations
to stop.
The telegraph
equipment
check
alarm
also turns
on the punch
check light
on the
Oca
operator's panel
(Figure 30).
The
input
check alarm
results
when the
terminal
interchange
senses an
invalid character
in
an input
(control)
message
from
the oca.
The
ouput
check alarm
is produced
when
the oca
receives
an
output (data)
message
containing
an in-
valid
character.
The
telegraph
equipment
check
alarm signifies
an
error
condition
in the
telegraph
unit currently
receiving
a message.
Output
Communication
Adapter
Operations
Operations
in
the oca, like
those in the
Input Com-
munication
Adapter,
are divided
into four
phases or
modes.
Wait
Mode: The
oca is normally
in this
mode prior
to
receiving a
message from
the data processing
center.
Output
Mode:
Upon receipt
of an output
message,
oca
transfers
from the
wait mode
to the output
mode.
MAIN] OFF POWER | SYSTEM | REQUEST | PUNCH
LINE
ON |AVAILABLE]
REPEAT | CHECK
TAPE
BLANK
BLANK | START STOP FEED
Figure
30. oca Operator’s Panel
Messages are accepted
into the data
register, are
checked for validity, and are
placed in oca storage.
Input Mode: Upon
detecting a scp
end-of-message
character, the oca
goes into input mode
to send to the
data processing center
the answer-back
signifying re-
ceipt of the message.
After oca sends
the answer-back,
it also sends one more
character (the
hold character)
to the terminal interchange.
This prevents
the inter-
change from sending unsolicited
messages until oca
sends a reset
character.
Buffer-to-Telegraph Mode: As the oca goes into in-
put mode, it simultaneously enters buffer-to-telegraph
mode. In this mode, oca reads characters out of stor-
age, translates them, and transmits them upon demand
from the telegraph equipment. Shift insertion also
occurs during this mode. Upon translating the end-of-
message character in this mode, the oca sends a reset
character to the terminal interchange and then returns
to
wait mode.
Use of the Data Register
and Core Storage
All information that passes through
oca does so by way
of the data register. All information
being received
from the terminal interchange is accepted by the data
register and stored in core storage. Later, the data
register receives this same information
from core stor-
age and transfers it to the
translator. Answer-back
messages to the data processing center enter the data
register before going to the input line.
Like the 1ca, the oca contains
a 96-character core
storage where output messages are stored before their
transfer to the telegraph unit.
Translation and
Shift Insertion
The translator in oca has two sections: the decoder and
the encoder. The decoder
interprets all 64 combina-
tions of the scp code
and presents them
as inputs to the
encoder. Certain special combinations
(punctuation
and special characters) are
programmed through an
IBM customer engineer control
panel to special five-
level code characters. Part
of the encoder is wired to
the decoder and part is wired
to the control panel. The
encoder accepts outputs
from the decoder
and trans-
lates to the corresponding
five-level code.
As each sop character is read
from the core storage,
it is checked to determine the
five-level shift code that
should precede it. The character
is compared with the
shift already set up in the
controls (by the previous
character). If the shift is
the same, the character is
translated into the five-level code with no insertion of
a shift character. If the shift
is different, the correct
shift character is inserted. Immediately following the
insertion, the scp character is translated and transmit-
ted to the telegraph equipment.
Message and Address Controls
Upon receipt of the output message anda signal from
the telegraph equipment that it is ready to receive, the
input message control sends the normal answer-back
message
to the data processing center. When the oca
or one of the telegraph units is being restored to nor-
mal operation, the input message control is used to
send the reactivation message (see “Message Formats”)
to the data processing center.
The first character of the output message, as it is
received at the oca, is the telegraph unit address. The
oca is able to address one of four units:
TELEGRAPH UNIT
scp Address
NUMBER GBA S 421
ik
10000 01
2 10070 0 10
3
O50; 070,07 1a 1
4 1000100
The oca address itself is dropped by the terminal
interchange after it has been used to put the output
message
on the correct output line to the oca indicated
by
the address. The oca and 1ca addresses follow the
pattern of “Terminal Addresses” shown in Figure 14.
Validity Checking Control
As
each output character is received from the terminal
interchange, it is checked for an even number of one
bits (excluding start bit). Whenever an odd number
is found, the oca ceases the transmission, sends a reset
character
to the terminal interchange, and returns to
wait mode.
Message Formats
Although all data messages pass through the oca as
output messages, control characters make up input
messages.
INPUT MESSAGES
Answer-back Message: This message is sent to the
data processing center after
the oca is ready for the
next segment of a message
or a new message. The
answer-back is automatically
sent as soon as the oca
receives the output end-of-message
character and has
found all characters valid. The format is:
TIC;, telegraph unit address,
EOMpy (unsolicited),
hold character
Communication
Adapters 31
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