Transmit Receive Control Commands (
WRITE WRAP I BREAK
\ ---
( - - I READ
\ \ PREPARE I ) I I'JH I BIT ( I ENABLE
\ DISABLE SENSE j I/o NO-OP P o 2 3 4 5 6 7
o o o o o o o o t o o o o o o
o o o o o o
__ 0_ .-2 _0_ 0 _ JJ _ 0 __ [ _0 _ 0 _ _ _ _ 0 _ _1_ [ I 0 II o II t:- : f'\ v
o II : o
o
o
o f'\ v
o
o
o
o
o
o
o 0 o 0 o 0 o 0 o
o
o 0 o o
o o o
o o
o o
o
o o o
o o
Bit Positions Telegraph Terminal Control Type I or Type II World Trade Telegraph Terminal Control IBM Terminal Control Type I (equipped with
Type I Terminal Interrupt feature)
i 8M Term inai Control Type I or Type II Autocall Feature Telegraph Terminal ContiOl Type I RESERVE _I _0 _ 0 0 0 1 \ - - - _ _ _ _ _ _ _ _ _ _ _0_ Two Processor Switch Feature
RELEASE 0 0 0 I SADZER SADONE o o o o
o o o
Exception \ ) _. - -w- ( o o o
o o o o Figure {'. Start/Stop Commands '.hUe buffering the cOlmt character. It then turns "lr('LEld and awaits a response from the terminal. , '1" P t ' l'e four possible responses, each of which ;"j it diffp ],pnt cti nn frnrn the 270:3: o o
o
o

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