... 712 Adapter 2
M E odels with
_xponder feature 0 1
expander ') .. expander [ )( w "'0 C o N 3
expander
4
expander 0 0 8
14
?? I I 28 I 36 -+2 50 56
Model 1 ana Expander Feature
1 expander 2 expander 3 expander
8 10 18 20 28 30 16 18 26 28 36 I 38
22 24 32 34 42 44 3? 4f) 42 - - I I 36 38 46 48
44 46 50 52
Figure 10. Total Low-Speed Lines Available by 2712 Model Configurations 40 4 expander
38 40 Note:
l. The total low s available for a
peed line terminations
ny given 2712
configuration is determined by the
the Modell and
nates. A" i ndi cated
cumulative, thus the
intersection of
Model 2 coordi
features are ac
second model 1 adapter feature
assumes prior attachment of the first
model I teature with the hrst expander
feature, etc.
2. The Expander features require a block
of eight addresses, while providing
two line terminations for the Model I, or six line terminations for the
Model 2.

count of the character is even, a logical zero if the
bit count of the character is odd.
Vertical redundancy check (VRC) and longitudinal
redundancy check (LRC) are provided. Any error
detected sets the Data Check bit within the sense
byte. A Data Check does not cause immediate
termination of the current command.
Polling and Addressing
Polling and addressing of the 1050, 1060, 1070, or 2740 terminals are performed by a Write and a Read
command or by a Poll command preceded by a
Start I/O instruction. When polling, command
chaining is utilized so that the Read command and
the allocated storage are ready to receive the in­
coming data. In the first case (a Write and a Read
command), on sending out polling characters (pro­
vided by the program), chaining occurs to a Read
command, and a timeout begins. At this point, the
IBM Terminal Control Type I pre-empts the 28-
second timeout provided by the common controls and
storage with its two-second short timeout. The 2703 interrupts the CPU program if a ® character
is received or if a timeout occurs. Unit Exception
is set in the unit status byte if ® is received. Unit Check is set in the unit status byte if a timeout occurs;
the Timeout bit is set in the sense field. Using the
Poll command, the 2703 will end the command with
the Status Modifier if a positive response is received
from the terminal. If no response, or an incorrect
response, is received, the command is ended with
Unit Check status and the appropriate sense indica­
tion. If all terminals have been polled and no posi­
tive response has been received, the command will
end normally. The program is required to follow
each address with an index character, which will be
returned to the processor upon a positive response
from the terminal. To initiate a receive operation
from a 2740 or 2741, a Prepare command is followed
by a Read command; then, data transmission is
started.
During addressing, command chaining to a Read
command is used to provide for receiving the ® or ® character. ® sets Unit Exception in the status
byte, as in polling, to interrupt the CPU program. ! causes a normal end, which can be command
chained to the output message.
Character-Recognition Summary
The following characters are recognized during
transmit operations:
1. ©, which sets control mode and resets the
check-character (LRC) accumulator.
2. @' which sets text-out mode and initiates LRC accumulation. @ is recognized if the
line is in transmit direction and in control
mode or text-in mode. This character has no
control effect if transmitted while the line is in
text-out mode. (It is treated as a normal data
character. )
3. @ , which sets LRC sequence. Termination
of the Write command is initiated by ® .
4. In text-out mode and in downshift ( a 0 in the
shift bit), any character with the S bit ON causes
the line to send the upshift character and sets
a 1 in the S bit before being transmitted.
5. In text-out mode and upshift ( a 1 in the shift
bit), any character with the S bit OFF causes
the line to send the downshift character and
sets a 0 in the S bit before being transmitted.
6. A pad character from the channel causes all
marks to be transmitted for one complete
character time.
PROGRAMMING NOTE: For the ending sequence of any text
message, the ® and © must be in the same case. If not,
the 2703 will detect this as a change of case and will insert
a shift character ahead of the ©.
The following characters are rec ":gnized during read
operations:
1. ©, as end of transmission, which sets
control mode and End and Unit Exception status
in the address lines.
2. @, which is recognized when the subject
line is in control mode or poll mode and causes
text-in mode to be set and LRC accumulation
to be initiated. When in the receive direction,
and when not in control mode, @ is treated
as a normal data character.
3. ®, which sets the LRC control sequence.
The next received character will be checked
against the accumulated LRC before the Read
command is terminated.
4. ®, which sets End and Unit Exception when
in control mode. End and Data Check are set
when ® is received and the operation is in
text-out mode.
5. <l?, which sets End during control mode anc1 text-out mode.
6. Upshift, which sets upshift (1 in the S bit) if the
line was in downshift (0 in the S bit); if the line
was already in upshift or in control mode, this
character has no effect. In any case, the
terminal control inhibits transfer of this
character to the data word, removing it from
the incoming data stream, although it is
included in the accumulated LRC character
if the line is in text mode while receiving.
41