1. Positive response in the form of a @ followed
bv text.
2. Negative response-- ® .
3. No response.
4. The received character not recognized as
either a @ or (!) .
The action taken with each response is as follows:
1. Status Modifier is set in the status byte and
the command is ended by the 2703. The
channel then chains to a Read command and
the count character, instead of the @ charac­
ter, is returned to the channel.
2. The 2703 requests three more characters from
the channel and transmits only the first two
characters.
3. A three-second timeout is completed and the 2703 ends the command with the Unit Check and Timeout bits set in the status and sense
bytes.
4. The command ends with Unit Check and Data Check set in the status and sense byies. In
some cases Intervention Required is set;
refer to "Sense-Byte Conditions" later in
this portion of the manual.
Dial NOTE: Any time tbe 2703 receives a negative response to a Poll command, the polling operation is continued without interrclpting the CPU, On aeeeptance of the Dial command by the 2703, bytes are transferred through the Autocall Adapter
to the automatic-calling equipment. They are,
however, transferred from the channel to the 2703 up to four digits at a time, in keeping with the
normal transfer of data. This command is very
similar to a Write command, except that the charac­
ters are transferred to the Automatic Calling Unit rather than a data set. The error conditions are
identical to the Write command except that Equip­
ment Check indicates a failure of the 'call request'
latch rather than a transfer check of the 'transmit data' latch.
Receive--Type Commands
Read
Bytes are transferred from the line to the channel
each time the MDW-l is filled with four characters,
or at the end of a message. The terminal controls provide for deletion of conil'ol characters (UlJ- shift, d(w.n --shift, idle, and delete) from the in­
coming message. This command is terminated at
the end of a message by setting of Channel End,
Device End, and Unit Exception bits in the status
field. These bits are set when the 2703 receives
a © from the communications line while in text
mode. When in control mode, either a <0 , or (only for IBM Line Sets), causes the Channel End,
Device End, and Unit Exception bits to be set. Otherwise, the command is ended normally with Channel End and Device End. For other ending­
status conditions, refer to "Status-Byte Conditions. " Prepare This command is normally used in a contention-
type communications system to notify the processor
when data is arriving. It is also useful to check for
the Break signal, to indicate when a Break or
elongated Space signal ends (the line returns to Mark). When a valid start bit is detected by a line instructed
to Prepare, a character is strobed off. If at stop-
bit time the line is at Mark (which indicates the line
has returned to the normal idle condition or that a
character has been received), the command is ter­
minated with Channel End and Device End status.
The character assembled is not transferred to the
multiplexer channel. If the line was at Space at
stop-bit time, the 28-second line timeout is started.
If the line returns to Mark before the timeout is
complete, this command is terminated with Channel End and Device End status. If the line does not
return to Mark before completion of this timeout,
then the Prepare command is terminated (indicating
an open-line condition) with Channel End, Device
End, and Unit Check status, and the Timeout sense
bit is set in the addressed MCW-2. No data transfer occurs under Prepare, and no
characters received during its execution are trans­
ferred to the processor (they cannot be recovered).
However, line-control characters received under
this command continue to be recognized (as they
are under Read) and they perform their normal
function. NOTE: To avoid a hang-up condition when operating with the
2741 Break feature or with any telegraph device, a Halt I/O command should be issued immediately following a Prepare command. The Halt I/O command is essentially ignored
while the line is still at Space. However, when the line is
at Mark, the Prepare command is ended immediately.
Inhibit On acceptance, the 2703 performs normal read
operations except that idle-line timeouts are inhibited
(except during Timeout Sequence, which is per­
formed just before the command ends).
27

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