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FIGS. FIGS character (figures or upshift) sets
FIGS mode;
if
the terminal control is already in
FIGS mode, this character has no effect. In any
case, the 2702/03 inhibits transfer of this character
to the
I/o
channel, removing it from the incoming
data stream.
LTRS. LTRS character (letters or downshift) sets
LTRS mode if the terminal control is in FIGS mode;
if the terminal control is currently in LTRS mode,
this character has no effect. In any case, the
2702/03 inhibits transfer of this character to the
I/o
channel, removing it from the incoming data
stream.
Space.
If
the Downshift on Space feature is present,
the space character sets LTRS mode when it is
received. The space character is then transferred
to the I/O channel with its shift bit a zero (lower
case).
EOT (FIGS H, LTRS sequence). The FIGS H, LTRS
sequence, when received as EOT, sets Channel
End, Device End, and Unit Exception status.
Example of USing Special Characters
Figure 2-34 show examples of the use of special
characters in messages.
Terminal-to-Terminal Operation with Search
Command
When operation on a multipoint line allows terminal-
to-terminal traffic, a different line control is
required. The sending terminal must be polled in
the conventional manner under a Write command.
If
the terminal has no message to send, it responds
with V (or
M),
and the Search command is termin-
ated with Channel End and Device End. Because
only one byte (Vor
M)
is transferred to the channel,
the channel causes a record-length interrupt to the
program. The program should then proceed to poll
the next terminal.
If
the terminal has a message to send, polling
activates the terminal tape reader and it responds
with a Call Directing Code (CDC) or Station Selection
Code (SSC) specifying the address (es) of the
receiving terminal (s). The processor is considered
a terminal and is always assigned the address AZ.
Several terminal addresses can be sent to permit
parallel message transmission. Each addressed
terminal must reply with V (or M), positive res-
ponse to addressing, to signify its readiness to
receive.
2-50
Note that if the CPU is to process or log the
message, the processor address must be the first
address in the series of addresses sent by the ter-
minal.
If
the first address received is A Z (the
address of the processor), the AZ is transferred
to the processor under the Search command. Since
two characters are transferred to the channel, no
record-length-interrupt condition occurs. The com-
mand is ended with Channel End and Device End
status. The channel may then command chain to a
Write command to send the V (positive answerback)
to the polled terminal. This allows that terminal
to start its tape reader and to continue to send the
message. The Write command must be command
chained to a Read command to receive the message.
If
the first address received under the Search
command is other than AZ, the data exchange
between the polled terminal and the recei ving ter-
minal takes place without any transfer of data to the
processor. However, the 2702/03, still under the
Search command, monitors the communications line
to detect the EOT sequence (FIGS H, LTRS). Under
this condition, the H is transferred to the processor
and ends the command with Channel End, Device End,
and Unit Exception status.
The 28-second line timeout between characters
is active during execution of the Search command.
Note that if the first address on the tape of the polled
terminal begins with A but is not followed by Z, only
the A is transferred to the processor. The H of the
EOT sequence is placed in the next position of main
storage.
Time outs
The normal 28-second line timeout between characters
is pre-empted by a 3-second short timeout following
initial selection and receipt of the first character's
start bit.
Message Restrictions
Terminal-to-Processor
1. All messages transmitted by a Western Union
Plan
115A
terminal from its paper-tape reader
must be preceded with the sequence !fA, V,
space" (assuming that A is the assigned ter-
minal call character).
2. A Space character received by the 2702/03 will
not downshift (FIGS to LTRS) unless the Down-
shift on Space feature has been wired at install-
ation time.
3. Text that immediately follows the sequence FIGS
H, LTRS is lost. This EOT sequence must not
be sent between blocks of data.
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