o 23456 7 System/360 Byte S 2 3 4 5 Baudot "S" represents the case. A logical one identifies
upper-case; logical zero, lower-case. The 1 bit is
the first bit transmitted after the start bit.
Shifted-character-set conversion is provided as
standard. The shift characters are L TRS for "letters" (or "downshift") and FIGS for "figures" (or "upshift"). In addition, "unshift on space" is
provided as a standard-wiring optional feature of
this terminal control; "unshift on space" means
that the space character causes the line to go to
LTRS mode if transmitted or received. The terminal
control most be wired for "unshift on space" only
if the attached 83B2 or 83B3 and 115A teletypewriter
terminals have this feature installed; a combination
of Type I telegraph lines with and without this
feature is not permitted in a single 2703.
LTRS and FIGS characters are automatically
deleted from the received data stream, and the case
is remembered in the lines-control word. Space
characters are not deleted from the received data
stream (whether with or without the "unshift on space" feature wired in), but the LTRS case is then
remembered. A sixth bit, set by the terminal con­
trol, is added to the code to indicate case to the pro­
cessor. Data bytes transferred from the channel
will be in six-bit form. The terminal control will
remove the sixth bit and remember the case. A
change in case will automatically cause insertion of
the proper shift character (LTRS or FIGS) into the
outgoing stream. The terminal control will send
1. 5 stop bits and check for the presence of one stop
bit on receive. After the 2703 is reset, the line
will be in lower case.
Polling and Addressing
Polling and addressing of 83B2, 83B3, or 115A
terminals are performed by Write or Read commands
issued under Start I/O. When polling, command
chaining should be utilized so that the Read command
and allocated storage is ready for incoming data as
quickly as possible. After the polling characters
(provided by the program) are sent out, chaining
occurs to a Read command, and a two-second time­
out begins. The Telegraph Terminal Control Type I
pre-empts the 28-second timeout provided by the
common controls with its two-second short timeout
while the line is in sequence zero. Interruption of
the program will occur if a timeout completes. Unit Exception will be set in the unit status byte if
the EOT sequence is received. For this terminal
control, the EOT sequence is FIGS-H, LTRS (where
FIGS-H is the H character received while the line is
in upper case; that is, the last shift character
received before the H was "figures-shift"). FIGS-H
must be followed immediately by the letters-shift
character (LTRS) to complete the EOT sequence. Unit Check is set in the unit status byte, and the
Timeout bit will be set in the sense field if a time­
out occurs. However, note that when the program
sends EOT, it should send the three-character
sequence--FIGS-H, LTRS--because some teletype­
wrjter terminals will recognize only this sequence
as EOT. When addressing, command chaining to a Read
command is utilized to provide for receiving the "V" (or "M") response.
A time rut will set Unit Check in the unit status
byte and Timeout in the sense field as when polling,
to interrupt the processor program. A "V" (or "M") answerback will cause a normal end, which may be
used to command chain to the output message.
Terminal-to-Terminal Operation with Search
For terminal-to-terminal operation between two
terminals on the same line, the operation should
proceed as follows:
1. The sending terminal be polled to acti­
vate its tape reader (assuming it has a
message to send). This should be done in the
conventional manner with A Write command.
If the message is to be sent to stations on
other lines or if it is to be logged or processed
by the CPU, the first address on the tape
should be "AZ" , the CPU's own address.
Otherwise, the addre ss on the tape should be
that of the station on the same line to which
the terminal-to-terminal message is to be sent.
2. The Write command that sends the polling
characters should be command chained to the
Search command. The Search command
should have a byte count of two and the SLI (suppress length indication) bit should be OFF in its CCW. If the polled station has no mes­
sage to send, it will respond \vith "V", and the
command will be terminated with Channel End
and Device End status. Because only one b J te
is transferred to the channel (the "V" or "M '
reply), the channel causes a record-length
interrupt to the program. The program should
then proceed to poll the next station.
3. If the station had a message and the first
address is '1AZ", the "AZ" is transferred to
the processor under the Search, and the
command is then ended with Channel End and
Device End status. Because two bytes were
43

transferred, .!lQ record-length interrupt occurs
and the channel can command chain to a Write to send the "V" answerback to the polled
station. This permits the station to start its
tape reader and continue to send the message.
4. If another reply to the poll is received under
the Search, the 2703 will scan the data on the
line until it recognizes the EOT sequence
(FIGS-H, LTRS), at which point it transfers
the "H" to the processor and ends the command
with Channel End, Device End, and Unit
Exception status. No transfer of data occurs
during this scan.
5. The Write command should then be command
chained to a Read to receive the message.
6. Line timcouts are active during execution of
the Search command. If the first address on
the tape begins with "A" but is not "AZ", the "A" will be transferred to the processor, but
otherwise the operation is normal; the "H" of the EOT sequence will be placed in the next
position in core storage.
Character-Recognition Summary The following characters are recognized during
write-type operations:
1. In LTRS mode, any character with the S bit ON causes the line to send the FIGS character
and set the shift bit before it is transmitted.
2. In FIGS mode, any cn'lracter with the S bit OFF causes the line to send the LTRS charac­
ter and reset the shift bit before it is trans­
mitted.
3. If wired for "unshift on space" operation, the
space character sets LTRS mode. In this
case, the S bit accompanying the space charac­
ter is ignored.
The following characters are recognized during
read-type operations:
44
1. A "V" (or "M") received as the first character
w ill set End status.
2. FIGS-H, LTRS received as EOT (end of trans­
mission) sets End and Unit Exception status.
3. The two-character processor address "AZ", if received as the first two non-letters
characters of a message under the Search command, will set End.
4. LTRS (letters shift), which sets downshift (0 in the S bit) if the line was in Ufshift (1 in
the S bit). In any case, this terminal control
inhibits transfer of this character to the I/O channel, removing it from the incoming data
stream.
5. FIGS (figures shift), which sets FIGS mode if
the line was in LTRS mode, and has no effect
if the line was already in FIGS mode. In any
case, this terminal control inhibits transfer of
this character to the I/O channel, removing it
from the incoming data stream.
6. If wired for "unshift on space" operation, the
space character sets downshift (0 in the S bit)
when it is received. The space character is
thcn transferred to the I/O channel with its S bit zero.
Telegraph Terminal Control Type II
This feature provides the controls necessary for the
attachment of Teletype Corporation's Models 33 and
35 Teletypewriter terminals to the 2703. Operation
is point-to-point and, generally, on a dial-up basis.
The transmission speed is 110 bps, with an eight­
bit data code. The code employed is the eight-bit
data-interchange code with the following relationship
to the System/360 interface:
o 234 567 System/360 Byte
2345678 35 Teletype Code One start bit and two stop bits are transmitted and
received by this terminal control.
Identification answerback from the 2703 is handled
by the processor. Output messages may be of any
length or format. Input messages may be of any
length. However, certain format restrictions are
imposed due to the line control adopted by these
terminals. On input, or during read-type operations,
the following characters are recognized by the 2703; they cause termination of the I/O operation, with
subsequent interruption of the Communications IOCS: 1. WRU--"Who are you"--which causes End
status in the LCW. 2. XOn--"Transmitter on"--which causes End
status in the LCW. 3. XOff--"Transmitter off"--which causes End
status in the LCW. 4. EOT--"end of transmission"--which causes End
and Unit Exception status in the LCW. 5. The delete character (all Marks) is recognized
by the terminal control. Transfer of deletes
to the I/O channel is inhibited during all read­
type operations.
The Break signal may be detected during trans­
mission. If the receive-data lead at the data set is
at Space for two successive transmit-start times,
the write operation is term inated with Channe lEnd,
Device End, and Unit Check status, and Intervention
Required is set in the sense field.