Search This command is provided to allow the processor to
ignore terminal-to-terminal messages on telegraph
lines when using Telegraph Terminal Control Type I
(only). That is, a selected (polled) teletypewriter
station may address another teletypewriter station on
the same line without going through the 2703's pro­
cessor and without causing channel interference due
to data servicing during the body of the message.
The Search command is valid for only Telegraph
Terminal Control Type I lines. On acceptance of Search, the 2703 scans the first
two non-letters characters received from the line.
II the first such charadeI' received is "V" (the ll(;gati \ (; l'(: sfJvlJ..:3C tv l;clling fc r <J 2B3 and Plan 115 stations), the "V" is transferred to the I/O channe 1. Timeout Sequence is set in the addressed MC\V-l, and the command ends with Channel End
and Device End status. If the first tvv'O such charac-
ters comprise the sequence "AZ" (the processor's
own address), then "AZ" is transferred to the I/O channel and the command ends immediately with
Channel End and Device End status.
If any other non-letters chal.'acter is received at
the beginning of the message, the MCW-l monitors
the received data for FIGS-H, LTRS sequence (EOT).
If this sequence is received, the "H" character is
transferred to the processor, Timeout Sequence is
set, and the command ends with Channel End,
Device End, and Unit Exception status. NOTE: If the first non-letters character is "A", but "Z" does
not follow immediately because another station is being
addressed, tbe "A" is transferred to the I/O channel.
Control Commands
Enable When command is accepted, the 'enable' latch
is set within the line adapter of the addressed
communications line. No data transfer occurs. A
28-second timeout is initiated while waiting for the
line to become operational, except on a data set that
has automatic-answering capabilities.
Disable On acceptance of this command, the 2703 resets the
'enable' latch within the line adapter of the addre;ssed communications line. No data transfer occurs. 28 Sense On acceptance of this command, the 2703 returns a
single byte to the channel from the sense field in the MCW of the addressed communications line. This
byte defines the condition of the addressed communi­
cations line. I/O No-Op
This command is treated as a "control immediate"
and no operation is performed by the 2703. Channel
End and Device End are transferred in the status
byte. No access is made to the 2703 storage unless
the multiplexcr channel stacks the status response.
This command does not reset the c::enc::e fieli1 Reserve
The Reserve command is valid only when the Two­ Processor Switch (TPS) feature is installed. This
command is used to achieve attachment to the channel
via the TPS feature. The Two-Processor Switch
must be in a neutral state at this time. Once attach­
ment to a channel is achieved, the Reserve command
is treated as an I/O No-Op, with the setting of
Channel End and Device End status.
Release
This command is valid only when the TPS feature is
installed. If a Release command is issued by the
attached channel to any of the valid 2703 addresses
for that channel, and the command is honored, then
the TPS returns to the neutral state. The 2703 must
be command-free to honor this command.
Exception Commands Sadzer, Sad one , Sadtwo, Sadthree These are standard 2702 commands, each of which is
treated as an I/O No-Op when decoded by the 2703.
These commands ar8 accepted by the 2703 to permit
programming compatibility between the 2702 and the
2703. STATUS-BYTE CONDITIONS A status byte provides an indication to the processor of the specific condition of the 2703. The status byte
is automatically sent to the channel: (1) as a status
response by the 2703 each time a command is received
and before use of this command by the 2703; and
(2) following the completion of each command. The
status byte informs the CPU program of the 2703 status for the initial-selection sequence as well as
following the termination of each 2703 command
operation.
Initial-Status Conditions
The 2703 responds to initial command selection by
always sending a status byte to the multiplexer
channel with the following exceptions:
1. Power is down on the 2703. 2. The 2703 is off-line for testing.
3. The 2703 is equipped with the '!\vo-Processor Switch feature and selection is attempted by
the inactive processor when the TPS is not in
the neutral state.
4. The 2703 machine clock has failed.
5. The 2703 is not metering because the 2703' s
Meter switch on the operator's panel is in the OFF position.
6. The address coming from the multiplexer
channel has incorrect parity.
Any of the following status conditions can be
presented to the multiplexer channel during the
initial-selection sequence.
All Zeros (Normal)
This status is set in response to operations initiated
by a Start I/O instruction or command chaining,
and indicates that the 2703 has accepted a command.
Busy, Status Modifier, and Control Unit End
This status indicates a temporary 2703 Control Unit
Busy. The 2703 is also busy for 5.1 ms when per­
forming a general reset. This status is also indica­
ted when the 2703 is updating the MCW-l and MCW-2 after Start I/O. This status will not be indicated by the 2703 if it
is possible for the channel to command chain. Unit Check
This status is set in response to operations initiated
by a Start I/O instruction; it indicates that the
command issued to the 2703 will not be executed
because it is invalid or has improper parity. This
condition is further defined by the sense byte. Status Modifier
This status serves an an unconditional response to a
Test 1/0 instruction.
Channel End and Device End
This status serves an an unconditional response to
the I/O No-Op, Sad one , Sadtwo, and Sad­
three commands.
Ending-Status Conditions
The following status conditions can be presented to
the channel at the end of a 2703 command execution.
Channel End and Device End
This status indicates that the current command has
been brought to a normal end and the 2703 is free to
accept another command. NOTE: These two conditions always appear together in the 2703
as a single status condition.
Channel End and Device End status has special
meaning for the following commands:
Enable When either of these commands is
or Dial Prepare Search
Read
issued to a line in a switched network
(attached to the Western Electric Data Set 103Al, 103A2, or equivalent, this
status indicates that the connection to
the terminal has been established
through the network. This status is
indicated if a Halt I/O is issued to a
line that is executing Enable and the
data set is "off hook" before the
'enable' latch can be reset.
Here this status indicates that the ad­
dressed line has started to receive;
the line went to Space long enough to be
recognized as a true start bit and then
returned to Mark (the normal state of
the line). However, if this command
had been terminated by Halt I/O, this
status would indicate that the command
was successfully aborted and the line
never went to Space.
Here this status indicates that the
first non-letters character(s) received
over the addressed line (using Tele­
graph Terminal Control Type I) was
either "V", negative polling response,
or "AZ", the processor's address.
When this command is issued to a line
with the Telegraph Terminal Control
Type I, this status indicates that the
first non -letters character received
was "V". "V" is the negative response
to polling and the positive response to
addressing for AT&T 83B2/83B3 and Western Union Plan 115A terminals.
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