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.
29

NOTE: For World Trade operation when using Telegraph Term­
inal Control Type 1, this status indicates that the End of
Block (£OB) signal was received--that is, "FIGS-X" (upper­
case X). Channel End, Device End, and Unit Exception
The conditions that cause this status to be signaled
depend on both the type of line and terminal control
involved, as well as the particular command that
was issued. It w ill never be signaled for Disable,
Break, or Sense. For the remaining commands, this
status indication is influenced by the type of terminal
control involved, as follows: 30 Read vi Inhibit Search Write
or Poll Tenllinal C011Lrvi Ty}?c: 1. This
status is signaled if (Q (EaT) or ® (negative reply) is received while the
addressed linc is in control mode and
executing either of these conunands. It is also set any time a © is received,
providing the 2741 Break feature is not
installed.
IBM Terminal Control Type II. This
status is signaled if ® is received
while the addressed line is executing
either of these commands and the line
is in control mode. It is also set any
time a © is received.
Telegraph Terminal Control Type I. Operating with 83B2/83B3 and 115A
terminals, tIlis status is set if the End
of Transmission signal (FIGS- H, LTRS) is received while the addressed
line is executing either a Read or
Inhibit command.
Telegraph Terminal Control Type II.
This status means that the EOT character was received while executing
either of these commands.
World Trade Terminal Control. This status means that the EOT sequence was received while
executing either of tilese commands.
Telegraph Terminal Control Type I
(only). With either 83B2/83B3 or
115A terminals, this status indicates
that the EaT sequence (FIGS-H, LTRS) was received while executing this
command.
This status is signaled if the receive­
control bit in the addressed MCW-l is ON when the command is accepted. The r:om mand' s execution is termina­
ted immediately. Dial
Prepare Enahlp Wrap
If Halt I/O is signaled during the execu­
tion of Dial, before Stop was received
from the I/O channel (the entire
number has not been dialed), the Dial
operation is ended with this status.
If Halt I/O is issued to a line that is
executing Prepare, and the line has
started to receive (a true start bit
was detected) but the ending status
has not yet been signaled, then this
status is signaled at the normal ending
point for this command, after the line
returns to Mark. This statue: is signa1ed if Halt Tla is
issued to a line that is executing
Enable and the call has not yet been
answered (Data Set Ready is OFF). The 'enable' latch is reset.
This status will be signaled if the
receive-control bit in the addressed MCW-l is ON when the Wrap command
is accepted. Channel End, Device End, Unit Exception, and Unit Check The conditions that cause this status to be signaled
are a combination of the conditions that cause the
command to end with Channel End, Device End, and Unit Exception status plus one or more of the error
conditions that cause the sense bits to. be set. A Sense command should be issued to the unit address
.in order to determine the error conditions. The
conditions that can be signaled are listed under "Sense-Byte Conditions." This status can be signaled
only when ending Wrap, Read, Write, Dial, Prepare,
Enable, Inhibit, Search, or Poll. Channel End, Device End, and Unit Check This status indicates that the current command has
been ended by an error or an unusual condition. A Sense command must be issued to the addressed unit
to further define these conditions. Channel End, Device End, and Status Modifier
This status is set for the Poll command when a @ (EOA) character is detected. SENSE - BYTE CONDITIONS The sense byte is transferred to the multiplexer
channel only when a Sense command is issued. The
various bit positions within the byte are set by I