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Line Control; Timeouts - IBM 2702 Information Manual

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LTRS (Letters shift). The LTRS character sets
LTRS mode if the terminal control is in FIGS mode.
If
the terminal control is already in LTRS mode,
this character has no effect.
In any case, the 2702
inhibits transfer of this character to main storage,
removing it from the incoming data stream.
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).
FIGS (Figures shift). The FIGS character sets FIGS
mode if the terminal control is in LTRS mode; if
the terminal control is already in FIGS mode, this
character has no effect. In any case, the 2702
inhibits transfer of this character to the I/O
channel, removing it from the incoming data stream.
EaT (End of Transmission). The format for EaT is
FIGS X, LTRS, where X is a character assigned by
the customer. This character is assigned on a per-
system basis and must be different from the charac-
ter chosen for the EOB sequence. EaT sets Channel
End, Device End, and Unit Exception status when
received during a read-type operation. (Its recog-
nition is similar to the EaT sequence of Telegraph
Terminal Control Type I, except the EaT sequence
for Type I is fixed at FIGS H, LTRS.)
EOB (End of Block). The format for EOB is FIGS Y,
where Y is a character chosen by the customer and
assigned on a per-system basis. It must be differ-
ent from the character chosen for the EaT sequence.
Recognition of the EOB sequence during a read-type
operation terminates the command with Channel End
and Device End.
Line Control
Attachment to the
\VT
Telegraph Terminal Control
is point-to-point (one terminal per line); thus, the
line-control method used is the contention type rather
than the polling type.
Processor to Terminal. When the processor begins
transmission, the program must insert from 4 to 20
LTRS characters before the Start of Message
character (Line Feed) to ensure that the terminal
is able to receive properly. Should the terminal bid
for the line at the same time as the 2702, an Echo
Check occurs. The write operation is ended with
Channel End, Device End, and Unit Check status.
A Read command must be issued to receive the
message from the terminal.
Terminal to Processor. The processor normally
monitors the inactive communications line by issuing
the Prepare command. Timeout is inhibited under
this command until the line receives a Space signal.
The Prepare command is normally terminated in
one of three ways:
1.
When a Space signal is detected--usually, this
signal is the start bit of the first of a series of
LTRS characters that is sent ahead of a message
or as a result of a motor-on operation at the
sending terminal.
If
the line returns to Mark
at stop-bit time of the first character, the
Prepare command ends with Channel End and
Device End.
Command chaining to a Read com-
mand must be utilized to receive the incoming
message.
2. The line does not return to Mark within 28
seconds after the first start bit is received--
the Prepare command ends with Channel End,
Device End, and Unit Check status and the
Timeout bit is set in the sense byte.
3. A Halt I/O instruction is issued by the program--
if
no true start bit has been detected before the
Halt
I/o
is issued, the command is terminated
with Channel End, Device End, and Unit Excep-
tion status.
If
a true start bit has been detected
(data has started to arrive) and Halt
I/o
is
issued, the command is ended with Channel End
and Device End.
Timeouts
Both the line timeout immediately following initial
selection and before arrival of the start bit of the
first character and the line timeout between received
characters are 28 seconds in duration.
The WT
Telegraph Terminal Control does not'pre-empt the
28 second timeout with one of shorter duration.
With this terminal control, read-type commands
always perform an idle-line timeout for one charac-
ter time before ending, following recognition of the
EaT sequence. This is done to allow the line time
to clear any superfluous character sent by the ter-
minal following the EaT. It also prevents initiations
of a write-type command, thus preventing sending
onto a busy line. This idle-line timeout is not per-
formed
if
a read-type command is ended by Halt
I/O or Interface Stop from the I/O channel.
These
signals cause the command to end immediately.
Operati ons
2 -4 7

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