World Trade Telegraph Terminal Control
This terminal control provides controls for opera­
ting WT teleprinter attachments at transmission
speeds of 50, 75, or 100 bps. The transmission
code is ITA2, with the following relation to the I/O interface and the 2703 serial data field.
o 234 567 ShUt 5 234 5
System/360 Byte
Shifted Code S represents the case. A logical one identifies
upper case; logical zero, lower case. The one bit
is the first bit transmitted following the start bit.
Shifted character set conversion, a standard
feature, automatically deletes LTRS and FIGS char­
acters from the received data stream, notes the last
shift character received, and inserts a sixth bit in
the code set to indicate the case to the processor. On outgoing data, the S (case) bit is removed and
noted. A change in the S (case) bit automatically
causes insertion of the appropriate shift character
in the outgoing data stream before sending the data
character.
A ttachment to the World Trade Telegraph Ter­
minal Control is point to point with contention.
For transmitting, the programmer inserts from
4 to 20 LTRS characters before the start of message
character (line feed) to insure that the terminal is
able to receive properly. If the terminal bids for
the line simultaneously with the processor, an echo
check occurs, terminating the write operation at the
CPU.
For receiving, the Prepare command is issued
to the terminal line to wait for the completion of
the first character from the terminal. Command
chaining to a Read command can be used to accept
the message. This terminal control does not have
a short timeout to pre-empt the normal 28-second
timeout.
Character Recognition
End of Transmission (EaT) format is FIGS, CHAR­
A CTER, L TRS, with the character assigned on a
per system basis. EOT sets channel end, device
end, and unit exception status.
End of block (EOB) format is FIGS, CHARAC­
TER, with the character assigned on a per system
basis. EOB sets channel end and device end status.
Message Restriction
Terminal to Processor:
1. The characters used in EOB and EOT must
not be the same.
2. A space character received by the 2703 will
not downshift (FIGS to LTRS) unless the "Un­ Shift on Space" option is installed.
3. A lthough text following an EOT or EOB is not
transferred to the processor, the receive
type command is not ended until the trans­
mission line is quiet for one character time.
Text immediately following FIGS, CHARAC­
TERS, LTRS is lost. EOT must not be sent
between blocks of data.
Processor to Terminal:
The terminal control automatically inserts the
proper shift character in outgoing data when a
change of case is encountered.
Type I Terminal Interrupt Feature
The Type I Terminal Interrupt feature is a modifi­
cation to the IBM Terminal Control Type I. This
feature, in conjunction with the IBM Terminal Con­
trol Type I, is effectively a new terminal control.
It provides the neces sary control to operate either
an IBM 1050 or IBM 2741 equipped with interrupt
features. Terminal Control Type I capabilities are
expanded to re.cognize a 'break' Signal from a termi­
nal and to transmit a 'break' signal to the terminal.
The Interrupt feature is mutually exclusive with
the 2741 Break feature. It is not considered another
terminal control when maximum configuration is
considered. However, when present, 'it prevents
use of the IBM Terminal Control Type I in its
non-modified form. The Type I Terminal Interrupt
feature is program-supported by the Time Sharing Option (TSO) of the System/360 Operating System.
When the Type I Terminal Interrupt feature is
present, each set of lines assigned to IBM Terminal
Control Type I operation must be assigned to one of
the two following types of operation:
1. IBM Terminal Control Type I with Break.
2. IBM Terminal Control Type I with Break
(Inhibit Unit Exception on © ).
Line control assignment is by hard-wire plugging
in the 2703, with each assignment pertaining to an
entire set of eight lines. Line assignment appearf
on the 2703 specification sheet, and lines are plug:;ed accordingly at installation or in the factory. Assign­
ment may be changed later by altering the specifi­
cation sheet and the plugging. Only one Type I
Terminal Interrupt feature may be installed on each 2703. It provides control functions for any number
of line sets.
The IBM Terminal Control Type I equipped with
this feature performs all the functions of the IBM
Terminal Control Type I with the following differences
and similarity.
45

On all IBM Type I lines which are plugged for
Break (Inhibit Unit Exception on © ), the Unit Exception status bit is not set when a © is received
during a Read or Inhibit command. Only Channel End and Device End will be set.
A full duplex communications facility is required
for a terminal to take full advantage of the Type I
Terminal Interrupt feature. On all IBM Type I lines
plugged either for Break or for Break (Inhibit Unit Exception on © ), the 2703 recognizes a 'space'
signal on the receive data lead of a full duplex modem
or data set while the 2703 is transmitting to the
terminal. (The receive data lead, a line connecting
the data set to the 2703, provides the input connection
to the 2703 for data signals from the remote termi­
nal.) Thus whcn a 'space' signal has been present
during the transmission of two characters, any
transmit-type command (except a Break command)
ends with Channel End, Device End, and Unit Check status, and v,dth Intervention Required set in the
sense field. On lines under control of the Type I Terminal
Interrupt feature, break signals are ignored by the
2703 during transmit break operations. Except for
the Break command, any transmit-type command
ends immediately with Channel End, Device End, and Unit Exception status upon recognition of a break
signal on the receive data lead. On half-duplex modems or data sets, Type I
receive data leads are not monitored for break
signals during transmit operations. PROGRAMMING NOTE: After Intervention Required is set during
a Write command, the program may issue a Prepare command,
followed by a Halt I/O command. The 2703 presents Channel
End and Device End status as soon as the line goes back to mark.
This distinguishes the line break from the non-operational subset
that ends the Prepare command immediately with Channel End,
Device End, and Unit Check status, and with the Intervention
Required sense bit on.
For all IBM Type I lines, the 2703 accepts a
Break command and transmits a continuous space
signal to the addressed line for a given number of
character times. The length of the space is deter­
mined by the number of characters transferred to
the 2703 under the Break command. Because the
2703 ignores bit configuration under a Break
command, any character is with one excep­
tion. The first character cannot be a pad character
unless the 2703 is at an engineering chanbe level of
307719 or later.
46 NOTE: While the 2703 is transmitting under any transmit-type
command (except a Break command) on lines under control of
the Type I Terminal Interrupt feature, the receipt of any
characters might be interpreted by the 2703 as a 'break' signal.
This is possible due to the Type I Terminal Interrupt feature's
method of monitoring for a 'break' signal; it monitors for a space
signal on the receive data lead during two consecutive break­
sample times. Break-sample times occur just before transmission
of start bits.
The following function pertinent to time-sharing
operations remains unchanged by this feature. On all IBl\1 Type I lines (with or without this feature),
the 2703 recognizes a 'break' signal on the receive
data lead while the 270:3 is operating under any receive-type cOlI'mand. Aftpr one character tim(' thc command (except a Prepare command) ends
\vith Channel End, Device End, and Unit Check
status, \I.iith Intervention Required and D3.ta Check set in the sense field. The Prepare command ends with Channel End and Device End only \\'h8n thC' 'break' signal has ended.
2741 Break Feature
The 2741 Break feature (in conjunction with IBM
Terminal Control Type I) provides the net:essary control to operate the IBM 2741 Communications
Terminal having an Interrupt feature. The 2741
Break feature modifies the terminal-control opera­
tion in t'vvo ways: (1) Normally, the IBM Terminal Control Type I sets Channel End, Device End, and Unit Exception status upon receiving a © character
during a Read, Inhibit, or Prepare command. With
the 2741 Break feature, only Channel End and Device
End will be set in this situation, thus allowing
command chaining to occur; (2) The IBM Terminal Control Type I does not look at the receive-data lead
from the Western Electric Data Set 103Al, 103A2, or 103F2 while transmitting. When the 2741 Break
feature is present, the receive-data lead is moni­
tored for Space during a transmit operation. If a Space signal is detected for a character time, the
Write command is ended with Channel End, Device
End, and Unit Check in the status byte and Interven­
tion Required in the sense byte.
The Break feature in the 2741 is implemented by
the Attention key on the 2741; when pressed, this
key initiates a Space of 200 ms on the terminal-to­
multiplexer channel. For the appropriate IBM Line
Adapters or data sets available for use with this
Break feature, refer to the special feature section
in the component description manual, IBM 2741
Communication Terminal, Form GA24-3415.
After getting the Intervention Required during a
Write command, the program may give a Prepare
command followed by a Halt I/O command. The
2703 will present Channel End and Device End status
as soon as the line goes back to Mark. This distin­
guishes the line break from the nonoperational
subset that 'NiH end the Prepare command iately with Channel End, Device End, and Unit Check in the status byte and Intervention Required in the sense byte.