IBM 2740-1 User guide

Systems Reference Library
IBM 2740 Communication Terminal Models 1 and 2
Component Description
This publication describes the application areas, principles of oper
ation, line-control signals, and special features of the IB 2740
Communication Terminal odels 1 and 2. The communication
facilities and data sets that can be used with this Tele-processing
terminal are also discussed. Included is a glossary of communi
cation terms used in this publication, and the timing formulas
necessary to calculate the data-handling capability of the terminal.

2740/2741 SRL/ Publications Availability Guide
Use this uide to determine what available publications
will best fulfill your individual requirements
Fourth Edition August, 1968)
This manual, Form A 24-3403-3, is a major revision of, and obsoletes the previous
edition, A 24-3403-2. Significant changes have been made throughout the manual;
therefore, it should be reviewed in its entirety.
Operating procedures for the IBM 2740 Models 1 and 2, and the IBM 2741, are
described in the SRL publication IBM 2740/2741 Communications Terminal—
Operator’s Guide, Form A27-3001. For information on other IBM Tele-processing
products, see IBM Tele-processing Bibliography, Form A 24-3089.
Significant changes or additions to the specifications contained in this publication
are continually being made. When using this publication in connection with the
operation of IBM equipment, check the latest SRL Newsletter for revisions or contact
the local IBM branch office.
This manual has been prepared by the IBM Systems Development Division, Product
Publications, Dept. 860, P. O. Box 12275, Research Triangle Park, N. C. 27709.
Address comments concerning the manual to this address.
li

CONTENTS
IBM 2740 COMMUNICATION TERMINAL, MODEL 1 . 1
Types of O peration ..................................................................................... 2
A p p lica tio n s...............................................................................................3
Communication F acilities............................................................................3
Data Sets ...............................................................................................4
Line-Control S ig n a ls .....................................................................................4
TRANSMISSION CONTROLS AND INDICATORS, MODEL 1 . 7
Local/Communicate Key. ....... 7
Standby S) L i g h t ......................................................................................7
Transmit T) L ig h t......................................................................................7
Receive R) L i g h t ......................................................................................7
Bid Key and L i g h t ......................................................................................7
Restart RST) Key and Light Checking Special F eature) 8
Dial-Disconnect Disc) Key and Dial-Connect Light Dial-
Up Adapter Special F e a t u r e ) ......................................................... 8
EOB End-of-Block) Key Checking Special Feature) . . 9
Automatic EOB Prerequisite, Checking Feature) . . 9
EOT End-of-Transmission) K e y ........................................................10
Transmit-Control Switch MTC, Off) Transmit Control
Special F e a tu r e ....................................................................................10
Test S w i t c h .............................................................................................10
PRINCIPLES OF OPERATION, 2740 MODEL 1 . . . . 11
Point-to-Point Operation Terminai-to-Terminal) . . 11
Broadcast Operation....................................................................................12
Dial-Up O p eration ....................................................................................13
Dial-Up Operation with c h e c k i n g ) .............................................. 13
Point-to-Point Operation--Multiplexer and Terminal . 14
Point-to-Point Operation--Multiplexer and Terminal
with Dial-Up Feature Installed........................................................15
Transmit C ontrol--M u ltiplexer........................................................16
Station Control--Multiplexer Addressing and Polling . 16
SPECIAL FEATURES, MODEL 1 17
Dial-Up A d a p t e r ....................................................................................17
Transmit Control Prerequisite, Dial-Up Adapter Feature) 17
Automatic Disconnect Feature World Trade Only) . . 18
Checking - -Parity VRC) and Block LRC) .... 19
Station Control Multiplexer Addressing and Polling) . 19
Automatic EOB Prerequisite , Checking Feature) . . 21
IBM 2760 A tta c h m e n t..........................................................................22
IBM Line Adapter Limited-Distance Type 1) . . 2 2
IBM Line Adapter Limited-Distance Type 2) . . . 22
IBM Line Adapter Leased L i n e ) .......................................................22
IBM Line Adapter Shared L i n e ) .......................................................22
Pin-Feed P l a t e n s ...................................................................................22
Paper Guide, Tear Bar, and Roll Paper Holder . . . 23
Timing Considerations, 2740 Model 1 ..............................................23
Time-Out C o n d itio n s ..........................................................................23
Delay C o n d itio n s ...................................................................................23
Typewriter Delay C o n d i ti o n s .......................................................23
Data-Handling Capability, Model 1 ..............................................24
Terminal Transmission and Control Speed .... 24
Addressing T i m e ...................................................................................24
Polling Time Negative Answer B a ck )..............................................24
Polling Time Positive Answer B a c k ) ..............................................24
Checking Time Receive Terminal, Positive Answer Back) 24
Checking Time Receive Terminal, Negative Answer Back) 24
Checking Time Transmit Terminal, Positive Answer Back) 24
Checking Time Transmit Terminal, Negative Answer
B a c k ) ......................................................................................................24
IBM 2740 COMMUNICATION TERMINAL, MODEL 2 . . 27
Introduction..............................................................................................27
Communication F acilitie s..................................................................28
Data S e t s ..............................................................................................28
Modes of O p e r a tio n ........................................................................... 28
Local M o d e .....................................................................................28
C om m u nicate.....................................................................................29
Codes, Keyboard Arrangements, and Print Elements . 29
Transmission Controls and Indicators, Model 2 . . . 29
On/Off Power S w i t c h ..................................................................29
Keys ..............................................................................................29
S w itc h e s ..............................................................................................30
Indicator L i g h t s ...........................................................................31
Without Buffered Receive and Without Record Checking
Special Features In sta lled ........................................................ 33
With Buffered Receive and Without Record Checking
Special Features In stalled ........................................................ 33
With Record Checking and Without Buffered Receive
Special Features Installed ....... 33
With Record Checking and With Buffered Receive
Special Features In stalled .........................................................34
PRINCIPLES OF OPERATION, MODEL 2 ......................................35
Local M o d e .....................................................................................35
Communicate M o d e ..................................................................35
Station Control Multiplexer Addressing and Polling) . 36
P o l l i n g .................................................................. ......... 36
Addressing..............................................................................................36
SPECIAL FEATURES, MODEL 2 ............................................... 39
Buffer Expansion . . . ........................................................39
Record C h e c k in g ........................................................................... 39
Operation . ....................................................................................39
Transmitting T erm in al..................................................................39
Receiving Term inal ........................................................................... 39
Buffered R e c e i v e ...................................................................................40
Telegraph Line A d ap te r..........................................................................40
Speed Base—600 b p s ..........................................................................40
Header C o n t r o l ...................................................................................40
Edit 40
Split Friction-Feed P l a t e n .........................................................41
Document I n s e r t i o n ..................................................................41
Ledger-Card S t o c k ........................................................................... 43
Operation . .............................................................................................43
Document D e s ig n ...........................................................................43
Paper Guide, Tear Bar, and Roll Paper Holder . . . 44
Pin-Feed P l a t e n s ...................................................................................44
IBM Line Adapter Limited-Distance Type 1) . . 4 4
IBM Line Adapter Limited-Distance Type 2) . . 4 5
IBM Line Adapter Leased L i n e ) ............................................... 45
IBM Line Adapter Shared L i n e ) ............................................... 45
Timing Considerations, Model 2 .................................................46
Typewriter Delay C o n d i ti o n s ............................................... 46
IBM 2740 Model 2 Data-Handling Capability . . . 46
G LO SSA RY..............................................................................................47
A P P E N D IX ..............................................................................................49
INDEX
Contents iii

Auto Check
and Transmit
Control Switches
Figure 1. IBM 2740 Communication Terminal, Model 1 s
iv

IB 2740 CO UNICATION TER INAL, ODEL 1
The IB 2740 Communication Terminal, odel 1
provides the capabilities of the standard Selectric
typewriter combined with the added convenience and
flexibility of a communications terminal. Thus,
this dual-purpose unit can alternately function as:
• a document writer for correspondence or other
normal secretarial applications (local mode);
• a data sending and receiving unit over com
munication lines (communicate mode), to an
other 2740 terminal (or terminals) or to a
computer using an IB 2701 Data Adapter Unit,
an IB 2702 or 2703 Transmission Control Unit,
or an IB 2712 Remote ultiplexer connected to
a 2702 or 2703. Hereafter the above units will
be referred to as multiplexers for ease of
reading.
This Tele-processing terminal presents an op
portunity for any company to provide quality corres
pondence copy at a customer’s location to complete
an inquiry-reply cycle on a same-day basis. This
means that the 2740 can accomplish a significant
time reduction in the normal cycle time involved in
obtaining vendor information for executive-level
decision making. With this terminal the total cycle
time from the customer-initiated inquiry to receipt
of the vendor reply is reduced from days (or weeks)
to hours (or minutes).
The IB 2740 Communication Terminal odel 1
(Figure 1) is a typing terminal that can be operated
by a typist with a minimum of additional training.
The major advantages of the basic Selectric
typewriter are retained, and added to these is the
additional flexibility provided by the ability to com
municate between remote locations. This typewriter
offers the exclusive Selectric features of:
Stroke Storage. This permits faster keying action
due to the reduction of interlocking action between
the keys. Thus, when two keys are struck in rapid
succession, the second character is stored until
the first is printed.
*Perforated Tape and Transmission Code for Binary Coded
Decimal environments, and Perforated Tape and Trans
mission Code for Extended Binary Coded Decimal environ
ments see code charts in Appendix. )
Typamatic Index Key. Pressing and releasing this
key moves the paper up one line space. Pressing
and holding this key down brings about the typamatic
action, which causes the paper to rapidly line space
until the key is released.
Interchangeable Print Elements. The print elements
available for the 2740 odel 1 (PTTC/BCD* *, and
PTTC/EBCD* and Correspondence) are easily inter
changed. This is accomplished by either pressing
the spring levers or raising the handle (depending
upon the element being used) and lifting it off the unit.
The other element is positioned by snapping it into
place.
NOTE: A choice of print elements PTTC/BCD, PTTC/EBCD,
or standard Selectric typewriter) is available at the time the
2740 Model 1 terminal is ordered.
Although the PTTC/BCD and PTTC/EBCD print
elements are primarily used for terminal-to-
computer applications (such as on-line computation
or programming) they can also be used for terminal-
to-terminal applications (such as inter/intra company
communication or executive correspondence). How
ever, the standard Selectric typewriter print element
can also be used for terminal-to-computer appli
cations (such as text handling) provided the users
program is designed to accept the unique line code
assignments for the associated graphics.
NOTE: The Correspondence code and the PTTC/BCD code with
their associated keyboard arrangements and print elements)
are not System/360 Type 1 Program supported.
The graphics and associated line transmission bit
values for the Correspondence code differ from the
bit values of the PTTC/BCD and PTTC/EBCD codes.
(See code charts and keyboards in Appendix). Al
though a choice of print elements is available, it is
important that all interconnected terminals use the
same type of print element: either all standard
Selectric typewriter, all PTTC/BCD, or all PTTC/
EBCD. Although the PTTC/BCD print element is
manually interchangeable with the PTTC/EBCD
print element (same line bit value assignments), the
standard Selectric typewriter print element is not
interchangeable with either of these print elements.
In all cases, the keyboard interposers must match
the associated print element to be used.
IBM 2740 Communication Terminal, Model 1 1

Types of Operation
Compact Ribbon Cartridge. The ribbon cartridge
provides a clean and efficient approach to ribbon
changing. With this cartridge, ribbon changing con
sists of snapping one cartridge off and the new one on.
The entire process is accomplished in seconds.
aximum Keying Efficiency. The terminal can
operate at speeds up to 14. 8 characters-per-second
(cps), however, the keying rate will vary depending
upon the speed of the operator. The keyboard is
designed to provide maximum keying efficiency and
throughput by retaining the basic Selectric type
writer features combined with easily accessible
communication controls (Figure 2).
The IB 2740 Communication Terminal odel 1
can be specified for any one of three types of opera
tions: point-to-point, between two terminals or
terminal and multiplexer; dial-up, between two ter
minals or terminal and multiplexer over a switched
network; or station control, when two or more ter
minals and a multiplexer communicate over the
same non-switched line and the multiplexer (trans
mission control unit) is the master station. The
particular type of operation is specified when the
terminal is ordered, and thus a customized ter
minal is installed for one of the following types of
system operation.
Figure 2. 2740 Model 1 Keyboard PTTC/BCD)
2
k.

Point-to-Point. Point-to-point operation requires
the use of leased or privately owned private line
communication facilities. It is intended to operate
between two points (terminal-to-terminal or
terminal-to-multiplexer). For communication
between terminals (no multiplexer) more than two
points may be included. However, when more than
two points are included, all stations can receive the
transmitted message and this becomes a broadcast
application. The basic 2740 is capable of either
point-to-point (terminal-to-terminal or terminal-
to-multiplexer), or broadcast operation.
Dial-Up. Dial-up operation allows the user to have
access to common-carrier switched Telephone
or Teletypewriter Exchange (TWX) networks. Any
2740 terminal connected to this network can call
(or be called by) another similarly connected 2740
terminal (or multiplexer). However, the dial-up
adapter special feature must be installed on each
terminal.
Station Control. This operation uses private line
facilities (non-switched) and provides for commu
nication with 2740 terminals on a multipoint basis.
All communications in this type of operation are
centralized (controlled by the multiplexer) and in
dividual station addressing or polling, as well as
group or broadcast addressing, is permitted.
Applications
When operating in local mode, the performance of
the 2740 Communication Terminal is similar to a
typewriter. When assigned to communicate mode,
each message character keyed at the sending terminal
is printed at both the sending and receiving terminals.
This data is transmitted over the communication
lines attached to the terminals. Several of the many
application areas for which the IB 2740 is designed
are:
Intracompany Communication. Internal communi
cation between company departments can be ex
pedited by using the 2740. A quality memorandum
is provided at the receiving end, using the appropri
ate upper and lower case letters plus special sym
bols, as required. An identical monitor copy is
also available at the sending end for filing. For
example, this operation could be between a purchasing
and receiving department, with the receiving ter
minal preparing a memorandum copy for depart
mental distribution, as well as a running log of all
received messages for future reference.
Remote Inquiry and Reply with a Computer. Using
this terminal, individuals having access to a CPU
(Central Processing Unit) can accomplish inquiry
and reply type operations without leaving their
department area. For instance, an engineer using a
2740 can enter desired variables into a Fortran pro
gram (used for processing of scientific data) avail
able in the CPU, and start the program running.
When a program run is completed, a reply can be
returned to the original terminal. This entire opera
tion requires only minutes, and the engineer can
stay close to his work area during this time. In
addition, the engineer can interrogate the program
during its run to determine if the problem is staying
within preset limits.
Intercompany Correspondence. Customer orders
and correspondence are available from key locations
sooner, when using the 2740 terminal. As a sales
aid and to expedite customer orders, a 2740 can be
installed in the purchasing department of your major
customers. Then, when an order is placed with
your company, the customer merely dials your
number and types out his order. This provides both
you and your customer with a completed copy of the
order. Confirmation of the order and availability of
all ordered items is provided by reversing the pro
cedure. Thus, the ordering and confirmation cycle
time is reduced by a significant amount and customer
service can be improved.
Executive Correspondence. Effective two-way com * •
munication is easily maintained between the execu
tive offices and the sales or manufacturing locations
by employing the 2740. All messages are trans
mitted directly to the individual’s secretary
(executive or middle-management level), thereby
eliminating possible costly delays or security expo
sure if processed normally by a communication
center or mail room. The message is handled only
by the executives and their secretaries at the sending
and receiving locations.
CO UNICATIONS FACILITIES
The IB 2740 odel 1 operates in half-duplex mode
only over a:
• Common-carrier switched telephone network,
• Common-carrier switched 150 bits-per-second
(bps) Teletypewriter Exchange (TWX) network.
• Common-carrier leased private line telephone
service.
IBM 2740 Communication Terminal, Model 1 3

• Common-carrier leased private line telegraph
service (Telephone Company 150 Baud Sche
dule 1006 channels),
• Western Union Class D (180 baud) channels,
• Western Union Class E channels, or
• Privately owned communication facilities or
equivalent.
When the 2740 is used on common-carrier
switched networks, common-carrier provided data
sets must be used. IB Line Adapters are avail
able for use on common-carrier leased private line
telephone channels, or privately owned communi
cation networks. Commercially available data sets
can also be used with private line facilities if they
present the proper interface to the 2740.
Point-to-point data communication is possible on
all these networks, however, multipoint communi
cation is possible only on leased private line or
privately owned networks.
Data Sets
The IB 2740 odel 1 operates over communication
facilities using the following data sets or their
equivalent:
• Western Electric Data Set 103A1 or 103A2—for
use in point-to-point dial-up connections.
• Western Electric Data Set 103F2—for use in
multipoint or point-to-point leased private line
telephone service.
• IB Line Adapters (Limited Distance Type 1,
Limited Distance Type 2, Leased Line, and
Shared Line)—for use in multi-terminal or point-
to-point operations on privately owned line con
nections or common-carrier leased private con
nections. Refer to ’’Special Features” in the 2740
odel 1 section for further information.
• An appropriate channel termination for use on
Telephone Company 150 baud (Schedule 3A) chan
nels.
• Western Union Data Loop Transceiver 1183-A for
use on Western Union Class D (180 baud) channels.
LINE-CONTROL SIGNALS
In terminal operations with a multiplexer or other
terminals, a control discipline is required on the
communication line. This discipline is called line
control, and one of its functions is to prevent two or
more stations from attempting to use the line simul
taneously (line contention).
The line-control signals (EOA, EOT, EOB, Yes,
No, SOA, and Inquiry) are represented in a short
hand form (§),©,(§)., ®,(N),(D, and (d)
respectively. This form is used in programming,
as well as in communications and line-control dis
cussions. Also, the terms ’’response” and ’’answer”
are used in a restricted manner. These terms and
signals are defined in the following:
Answer. The negative, positive, or no answer at
all to a check (VRC or LRC) following an EOB.
Response. The negative, positive, or no response
at all to addressing or polling.
(§)—EOA (Pound sign character). Signal indicates:
• the Bid key has been pressed and the transmission
of text data is to follow. This pertains to point-
to-point operation or broadcast operation without
the station control feature installed.
• an end-of-address, with text data to follow. This
is used in multiplexer operations with the station
control feature installed. This signal starts the
LRC counter at both the sending and receiving
terminals. The EOA signal is not included in the
following LRC check.
• positive response to a poll from the multiplexer.
5)—EOT. This signal indicates:
• an end-of-transmission, and resets the LRC
counters at both sending and receiving terminals.
(g)—EOB. This signal indicates the end-of-a-unit-
block of text. This is used when the checking fea
ture is installed and is followed by the LRC charac
ter. This provides an LRC check comparison at
the receiving terminal with the EOB character in
cluded in the check.
© —Positive Response, Yes (Period character).
When this is received, it indicates:
• a positive response to an address.
• a positive answer to an EOB/LRC sequence when
the checking feature is installed.
(N)—Negative Response, No (Hyphen character).
When this is received, it indicates:
4

• a negative response to an address.
• a negative response to a poll.
• a negative answer to an EOB/LRC sequence when
the checking feature is installed. Either the
hyphen or underscore character is printed as an
indication of this condition (a different character
is printed on IB World Trade machines).
(§)—SOA (Comma character). This is used to indi
cate a start-of-address condition, and is used when
the station control feature is installed.
(d)—Positive Answer, Inquiry (Pound sign charac
ter) .. This signal is used in a multiplexer operation
as a positive answer to an EOB/LRC sequence from
the terminal (Figures 11 and 15). At this time,
the terminal switches to a receive status. The
character associated with this signal is not printed.
IBM 2740 Communication Terminal, Model 1 5

TRANS ISSION CONTROLS AND INDICATORS, ODEL 1
These controls and indicators are located directly
adjacent to the left and right of the typing keyboard,
as shown in Figure 2. They provide additional con
trols required for operating in communicate mode.
Certain keys are associated with special features
(as indicated) and are provided only when that fea
ture is installed.
Local/Communicate Key
Press the Local, bottom portion of this two-position
key (Figure 3), for local mode (normal typing). If
your terminal is addressed to receive a message
while in this mode, an alarm sounds automatically.
If the alarm sounds, press the Com (Communicate)
portion of the Local/Communicate key to receive
the incoming message from the calling terminal or
multiplexer.
Pressing the Com (Communicate) portion of this
key places the terminal in communicate mode and the
terminal can alternately transmit and receive from
the communication line. However, the terminal is
switched to a receive status only when signaled to
do so by another terminal or computer.
Standby (S) Light
This light (Figure 3) is on when your terminal is in
the communicate mode, in a ready status, and
neither transmitting nor receiving (standby status).
The typewriter motor is not running when the ter
minal is in this status. If the Bid key is operated,
or the terminal receives a signal to receive, the
terminal automatically shifts to either transmit or
receive status and the typewriter motor is automati
cally turned on. When the terminal changes status,
the Standby light is automatically turned off, and
either the Transmit or Receive light turns on.
Transmit (T) Light
This light (Figure 3) is on during the time one can
transmit information from the keyboard to the com
munications line and consequently to other terminals
or a multiplexer. During this time the keyboard is
unlocked and the terminal is controlling the com
munications line. This light also turns on when the
terminal transmits the answerback to an EOB/LRC
sequence when the Checking special feature is in
stalled.
Receive (R) Light
This light (Figure 3) is on when your terminal is
ready to receive a character from another terminal
on the line, or from a multiplexer in the communi
cations system. During this time your terminal can
receive characters only from the terminal or
multiplexer which initiated the call, and your key
board is locked. When the checking feature is in
stalled, this light also turns on while the terminal
is awaiting an answerback to an EOB/LRC sequence.
Bid Key and Light
Point-To-Point Operation
Press this key to obtain the line to transmit a mes
sage (Figure 3). The Bid key can be operated
only when your terminal is in a standby status
(Standby light on). Operating this key places your
terminal in transmit status (Transmit light on),
turns the typewriter motor on, and causes a signal
to be transmitted to the other terminal on the line.
The other terminal recognizes this signal and auto
matically shifts from standby status to receive
status. If a terminal is without paper, without data
set status, or operating in local mode, it does not
shift to receive status, but does sound an alarm
alerting the operator that corrective action is re
quired. The Bid light (located to the right of the Bid
key) is not operative in a terminal-to-terminal type
of operation.
Broadcast Operation
When operating in broadcast mode (more than one
other terminal on the same communications channel)
press the Bid key to obtain the communications line
to transmit a message. The Bid key can be operated
only when your terminal is in standby status and the
Bid light is on. The Bid light on indicates that the
terminals in the broadcast network are in contention
for the line; that is, no other terminal in the broad
cast network is on the line and sending a message.
Operating the Bid key shifts your terminal to trans
mit status (Transmit light on, Standby light off),
turns the typewriter motor on, and turns the Bid
light off (see Broadcast Operation under " Principles
of Operation, 2740 odel ÜÏ
Transmission Controls and Indicators, Model 1 7

Figure 3. IBM 2740 Model 1 Transmission Controls Right Side of Keyboard)
Check-Loop Test
The Bid key is also used to test a portion of the
electronic circuitry of the terminal (see nTest Pro
cedures” section in the IB 2740/2741 Communication
Terminal—Operator’s Guide, Form A27-3001).
Set the local/communicate switch to the local position
to perform the test. While holding the Bid key down,
a single operation of any one of the character keys
on the keyboard (except Shift key) causes that char
acter to print repeatedly. This indicates that the
electronic circuitry associated with the printing of
the character is functioning properly. Releasing
the Bid key stops the repeat printing.
Station Control (Special Feature)
When the Station Control special feature is installed,
operating the Bid key sets up a circuit to notify the
multiplexer of a message to be transmitted when
the multiplexer polls the terminal. The Bid light
turns on to indicate a request to transmit the next
time your terminal is polled (called) by the multi
plexer. When your terminal is polled and shifts to
transmit status, the Bid light turns off, the Trans
mit light turns on, and an alarm sounds to notify
you that transmission can begin from your terminal
(see Station Control under Special Features, odel
1).
Restart (RST) Key and Light (Checking Special
Feature)
The Restart light is located to the right of the Re
start key (Figure 3). When in transmit status,
the Restart light at your terminal turns on when
you perform a checking sequence (press EOB key)
and remains on if the message you transmitted was
received incorrectly at the receiving terminal.
When in transmit status, pressing the Restart key
turns the light off, unlocks the keyboard, and allows
message transmission to continue, or the error
message to be retransmitted. When in receive
status, the Restart light turns on as soon as an
error is detected and turns off automatically when
your terminal sends a negative answer to the trans
mitting terminal.
Dial-Disconnect (Disc) Key and Dial-Connect Light
(Dial-Up Adapter Special Feature)
Use this key to disconnect the line in a dial-up line
connection (Figure 3). If in transmit status, the
EOT key must be operated before operating this key.
Pressing this key, following the operation of the
EOT key, releases both terminals from the com
munications line (provided both data sets have the
Automatic Disconnect feature installed). Otherwise,
this key must be operated at each terminal following
the operation of the EOT key.
8

The Dial-Connect light on indicates that the
terminal originating the dial connection is connected
to a remote terminal and is in standby status (com
municate mode). The Dial-Connect light at the
remote terminal also turns on when the connection
is established and turns off when the terminal shifts
to transmit or receive status. When the originating
terminal goes into the transmit or receive status,
this light turns off and the Transmit or Receive
light turns on.
EOB (End-of-Block) Key (Checking Special Feature)
This key is used to signal the end of a unit-block
of text. Pressing this key (Figure 4) initiates a
signal that causes the block of text just transmitted
to be checked at the receiving terminal. This check
does not provide an indication of accuracy as a
typist, since typed copy can be checked, but it does
ensure that all characters that were typed on your
keyboard have reached the receiving terminal cor
rectly.
Immediately following the operation of the EOB
key, the Restart light turns on and the status lights
change from transmit to receive momentarily, and
then back to transmit, indicating that checking has
been completed. If an error occurred, the Restart
light remains on and the keyboard locks. Pressing
the Restart key turns the light off, unlocks the key
board, and allows you to retransmit the message (if
necessary), or to continue message transmission.
Pressing the EOT key also turns the light off, causes
a (c) to be transmitted, and places the terminal in
control receive mode. If the message was received
correctly, the Restart light turns off automatically.
Automatic EOB (Prerequisite, Checking Feature)
Included with this feature is a three-position switch
Auto Ck (auto check) located on the left side of the
2740 cabinet (Figure 1). The three positions
of the switch are: Term (terminal), plx (multi
plexer), and Off. Set the switch to Term when
transmitting to another terminal and to plx when
transmitting to a multiplexer. With the switch set
to Term or plx, each operation of the Return key
causes the carrier to return and the EOB/LRC se
quence to be transmitted automatically, thereby
causing the line of characters just transmitted to
be checked at the receiving terminal. Your terminal
shifts momentarily from transmit to receive status
(Transmit light turns off, Receive and Restart lights
turn on). If the receiving terminal responds with a
positive answer, your terminal shifts back to trans
mit status (Transmit light turns on, Receive and Re
start lights turn off, and keyboard unlocks) and you
can continue with message transmission.
If the message is received incorrectly, the re
ceiving terminal prints a hyphen (below the message
block just completed) and a negative response is sent
back to your terminal. This negative response
causes a hyphen to print (below the message block
just completed) at your terminal. The negative re
sponse also locks your keyboard, turns the Transmit
light on, sounds the audible alarm, and causes the
Restart light to remain on. The message can be
retransmitted by pressing the Restart key. Opera
ting this key turns the Restart light off, causes a
CR/LF (carrier return and line feed) to occur at
your terminal, and another CR/LF to occur at the
receiving terminal if the switch is set to the Term
position. If the switch is set to the plx position,
the CR/LF code is not transmitted to the receiving
terminal. The CR/LF operation positions the
carrier on a new line (below the hyphen). The mes
sage can now be retransmitted.
When the switch is set to Off, the EOB key must
be manually operated to initiate a record check (see
Automatic EOB under Special Features, odel 1).
Figure 4. IBM 2740 Model 1 Transmission Controls Left Side of
Keyboard)
Transmission Controls and Indicators, Model 1 9

EOT (End-of-Transmission) Key
After typing the contents of a message on your key
board, signal the other terminals that you have com
pleted your message. Operating the EOT key (Figure
4) transmits a signal to all terminals that have re
ceived your message, causing them to shift to stand
by status (Standby light turned on and Receive light
turned off). Operating the EOT key also causes your
terminal to shift to standby status (Standby light
turned on and Transmit light turned off).
Transmit-Control Switch ( TC, Off) Transmit
Control Special Feature
This switch is located on the left side of the 2740
cabinet (See Figure 1). The two settings of this
switch ( TC— ultiplexer Transmit Control, and
Off) provide or inhibit the proper line-control sig
nals for the trans mi t-control feature when operating
between a 2740 and a multiplexer. With this switch
set to the TC position and in control receive mode,
the terminal shifts to transmit status when the multi
plexer transmits a special two-character code (/
space). The Off position of this switch places the
trans mi t-control feature in an inactive status. (See
Transmit Control under Special Features, odel 1.
Test Switch
The Test switch is present only on terminals using
certain IB Leased or Shared Line odems (special
feature) to connect to the communications line. The
Test switch, labeled od ( odem), Line and Off is
used when communication cannot be established
with a remote terminal. Refer to IB 2740 Terminal
and Line Adapter Test Procedure under Test Pro
cedures in the IB 2740/2741 Communication Ter
minal Operator’s Guide, Form A27-3001 for test
procedure and use of this switch.
10

PRINCIPLES OF OPERATION, 2740 ODEL 1
This section covers the operation of the basic IB
2740 Communication Terminal odel 1, without any
special features, and then with special features in
stalled. The communication line operations required
for each are shown in Figures 5 through 15. A de
tailed description of operating procedures is included
in the IB 2740/2741 Communication Terminal—
Operators Guide, Form A27-3001. * •
The IB 2740 Communication Terminal is a modi
fied Selectric typewriter that retains its basic
typing features for both local and communicate mode
operations (Figure 2). The significant differences
in the typewriter are in the addition of transmission
controls and indicators on either side of the typing
keyboard, and in the standardization of the available
print elements to conform with those available with
other IB terminals (e.g. , IB 1050). This means
that the 2740 can interchangeably use the print ele
ments provided with the PTTC/BCD and the PTTC/
EBCD codes unless the standard Selectric typewriter
print element was specified at the time of ordering.
(Refer to ”Interchangeable Print Elements” in the
introductory section.) See Appendix for a full de
scription of these codes, associated keyboards, and
the various character sets available.
In addition to the Selectric features previously
discussed (strokè storage, typamatic index key,
interchangeable print elements, and the compact
ribbon cartridge) this typewriter has:
• 10 or 12 characters per inch horizontal spacing.
• 15 1/2 inch paper-width capacity.
• 13 inch wide writing line with a standard friction
platen.
• pin-feed platen is available as a special feature.
• 6 or 8 lines per inch vertical spacing (6 lines is
standard).
The standard 2740 odel 1 is designed for point-
to-point operations using a private line (leased
private line or privately owned line) between two
2740 terminals or a 2740 terminal and a multiplexer.
However, the computer program must be designed
to handle the basic line-control signals used with
this type of operation (Bid character, (B) , and End-
of-Trans mis sion, EOT (5) Refer to ”Line Control
Signals” in this manual for a complete explanation
of all signals.
These two line-control signals also permit a
broadcast type of operation. In this operation, a
message transmitted from one terminal is received
by all terminals on this line. Installation of an
available special feature additionally allows
terminal-to-computer operation on a multipoint
basis (more than one 2740 terminal on a communi
cation line).
Point-to-Point Operation (Terminal-to-Terminal)
This operation is performed on a contention basis,
which means that both terminal operators independ
ently attempt to acquire the line by pressing the Bid
key (Figure 5). The first terminal acquiring the
line retains control of this line until its EOT key
is pressed. (Refer to Transmission Controls and
Indicators, odel 1 in this section of manual, for
a complete explanation of these keys.) When a ter
minal acquires the line, the other terminal auto
matically switches to a receive status and its key
board is locked. This terminal remains in the
receive status until an EOT code is received.
Before attempting to acquire the line, each ter
minal is in one of the following conditions:
• Standby status (communicate mode)
• Typing status (local mode)
• Off
Transmit
Before attempting to acquire the line, the terminal
must have status; this requires:
• On-off switch (located on the keyboard) must
be on.
• Paper must be properly inserted in the type
writer to close the paper-presence contact.
(This contact can be deactivated by using the
contact lockout.)
• Local/Communicate switch must be positioned to
Communicate.
• Power must be supplied to data set (if used).
As a result, the Standby light will be on. The follow
ing operation can now be performed:
Principles of Operation, Model 1 11

• Press the Bid key. This transmits a (§) signal
to the other terminal on the line (Figures 5
and 6). Upon receipt of the B) signal, the re
ceiving terminal automatically switches to re
ceive status provided the terminal was originally
in standby status. This change to receive status
causes the keyboard to remain locked and the ter
minal motor to be turned on. If this terminal is
operating in local mode or is not in standby status
when the (B) signal is received, an audible alarm
is automatically sounded at the terminal.
Once the initiating terminal acquires the line
through use of the Bid key, it automatically
switches to transmit status. In conjunction with
this, the Transmit light is turned on, the key
board is unlocked to permit the transmission of
text, and the terminal motor is turned on. This
terminal can now transmit a message (or mes
sages) of any length and retain control of the line
until it signals an End-of-Transmission (EOT).
• Press the EOT key. This transmits an EOT (5)
signal to the receiving terminal. Both terminals
(the transmitting, as well as receiving) switch
to a standby status. The Standby light at both
terminals is turned on, the keyboards are locked
(except for the Bid key and EOT key), and both
terminal motors are turned off.
Receive
The initial requirement for this terminal is that it
be in standby status. In order to have Status, the
terminal and data set must have power supplied, and
the terminal must be in communicate mode with
paper present in the typewriter.
• Receipt of the B) signal switches this terminal
to receive status (Receive light on, keyboard
locked, and motor turned on). The terminal re
mains in this status until an EOT signal is re
ceived (see Figures 5 and 6). However, if this
terminal runs out of paper while receiving, or
drops status for any other reason, it switches to
a no status condition (all status lights off, key
board locked, and still in communicate mode).
Until this condition is corrected (by inserting
paper or deactivating the paper presence contact)
and the line connection is re-established, the
terminal is unable to receive any further trans
missions. Thus, a procedural requirement pro
viding for message retransmission when this
occurs should be established for each system.
• Receipt of the EOT signal switches this terminal
to standby status (Standby light on, keyboard
remains locked except for the Bid and EOT keys,
and the terminal motor is turned off).
• If this terminal is in the local mode or is in
communicate mode but out of paper when the B)
signal is received, an alarm sounds at this ter
minal. The operator should now switch to the
communicate mode (by positioning the
Communicate/Local key to Com), or insert paper
(or deactivate the paper-presence contact).
essage reception is not possible until this is
accomplished.
Broadcast Operation
This operation (without any special features in
stalled) is essentially the same as a point-to-point
Terminal
Bid Key EOT Key
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Bid Key EOT Key
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NON-.SWITCHED
LINE
Terminal Text
or Multiplexor ® ©
Bid Key EOT Key
or or
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from from
Multiplexor Multiplexor
Figure 5. Point-to-Point Operation No Special Features)
12

Bid EOB EOB EOB EOT
Key Key L Key L Key L Key
Terminal © R
Text'—' C © R
T e x tW C ( ? ) R
Text ^ C ©© © ©
NO N-
SWITCHED
LINE
Terminal
o)
V H
0)
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(2)
Text Text Text
or Multiplexor ©©
Figure 6. Point-to-Point Operation With Checking Special Feature)
©<3,® i ‘4)® R ©
Bid EOB c EOB R EOB c EOT
Key Key Key Key Key
®o r © or ©
©o
o
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from from from from from
Mult Mult Mult Mult Mult
operation. The basic difference is that now message
transmission is on a broadcast basis (Figure 7).
Thus, all terminals on this line, in a ready condition
and in a standby status, switch to a receive status
when the (B) signal from the initiating terminal is
received. All terminals now in receive status
accept and print all transmitted messages, until an
EOT signal is sent from the transmitting terminal,
or until terminal runs out of paper (paper presence
contact not locked out). At this time, all terminals
switch to a standby status.
NOTE: The checking special feature is not available for broad
cast operation.
Dial-Up Operation
This type of operation requires that each terminal
have the dial-up adapter special feature installed.
This feature permits the terminal to be inter
connected to common-carrier switched networks by
means of common-carrier provided data sets. Refer
to Dial-Up Adapter in the Special Features, odel 1
section for a detailed explanation.
Dial-up is essentially the same as point-to-point
operation except for the type of communication
facilities employed. Since a dial-up operation
connects the terminal to a line accessible to the
general public, this line connection must be dropped
when message transmission is completed. (For
a private line, however, the connection to the ter
minal is permanently assigned).
The dial-up line connection is dropped by use of
the Dial Disconnect key at one of the terminals (or
if the status is lost). With the automatic disconnect
feature (supplied by the common carrier when the
data set is installed or by a 2740 odel 1 special
feature on IB World Trade machines), operating
this Dial Disconnect key at one terminal disconnects
both ends of the line (Figure 8). Refer to Dial
Disconnect Key and Dial Connect Light under Trans
mission Controls and Indicators, odel 1 in this
section of the manual.
Dial-Up Operation (with checking)
This operation (Figure 9) is similar to the pre
viously described dial-up operation, except that
both terminals must have the checking special
feature (VRC—Vertical Redundancy Checking, and
LRC—Longitudinal Redundancy Checking) in addi
tion to the dial-up adapter special feature installed.
Provided with the checking feature is the EOB key
and the Restart key and light (refer to Transmission
Controls and Indicators, odel 1 in this section of
the manual for a detailed description of these items).
With checking installed, each transmitted char
acter is checked for parity (an odd number of bits)
as it arrives at the receiving terminal (VRC). Any
character in error is not printed. Instead, a hyphen
character (or an underscore if in upshift) is printed
in its place, and message reception continues. A
different character is printed on IB World Trade
machines. Also, at the end of each message block,
the total message block is checked by an LRC com
parison at the receiving terminal.
If either a VRC error or an LRC error is de
tected in a message block, a hyphen character is
printed at both terminals (following the LRC com
parison). Also, the transmitting terminal keyboard
is locked (while the receiving terminal keyboard re
Principles of Operation, Model 1 13

mains locked), the Restart light is turned on, and
the audible alarm sounds at the transmitting termi
nal. The Transmit light will remain on.
The Bid and EOT keys are used the same in this
operation as previously described. However, the
EOB key now permits the transmitting terminal
operator to designate blocks of data by pressing this
key when desired. When the EOB key is pressed,
an LRC comparison occurs automatically following
this action. The transmitting terminal is notified
of the check results by receipt of either a positive
answer, Yes © , or a negative answer, No (n) .
The transmitting terminal is then either available
for further transmissions (Transmit light on), or the
keyboard is locked (with the Restart light and the
Transmit light on). In the second instance, further
transmissions cannot occur until the Restart key is
pressed, turning out the associated light and un
locking the keyboard.
The line connection is dropped by operating the
Dial Disconnect key at one of the terminals (or if
either terminal runs out of paper). Operating this
key has no effect when the terminal is in transmit
status.
Point-to-Point Operation— ultiplexer and Terminal
When operating point-to-point with a multiplexer,
the 2740 odel 1 terminal operates basically the
same as when operating point-to-point with another
2740 odel 1 terminal. See " Point-to-Point Opera
tion (Terminal-to-Terminal)” in this section of the
manual.
Transmit
Press the Bid key. This transmits a 3) signal to
the multiplexer (Figures 5 and 6). This signal
causes the multiplexer to automatically switch to a
receive status. Once the terminal acquires the line
through use of the Bid key, it automatically switches
to a transmit status. In conjunction with this, the
transmit light is turned on, the keyboard is unlocked
to permit the transmission of text, and the terminal
motor is turned on. The terminal can now transmit
a message (or messages) of any length and retain
control of the line until it transmits an EOT © _
signal. Press the EOT key. This transmits a ©
signal to the multiplexer and places the terminal in
a standby status. The Standby light at the terminal
is turned on, the keyboard is locked (except for the
Bid key and EOT key), and the motor is turned off.
Receive
The multiplexer initiates a transmission by placing
a 5) signal on the communication line, which causes
the terminal to switch from a standby status to a
receive status (Figures 5 and 6). The Standby
light turns off, the Receive light turns on, the key
board is locked, and the terminal motor is turned on.
NOTE: One Idle character must follow the 0 to enable the
terminal motor to attain full speed.
The terminal remains in this status until a (c) signal
is received from the multiplexer, or until terminal
Terminal A
Bid Key EOT Key
® Text O
NON-SWITCHED
Terminal B
LINE
Terminal C
© Text ©
Bi^Key EOT Key
Figure 7. Broadcast Operation No Special Features)
® ©
Bid Key EOT Key
14

runs out of paper (paper presence contact not locked
out).
The (c) signal switches the terminal to standby
status (Standby light on, keyboard remains locked,
except for the Bid and EOT keys, and the terminal
motor is turned off).
If the terminal is in local mode (or is in commu
nicate mode but without Status) when the (S) signal
is received, an alarm sounds at the terminal. The
operator may now switch to the communicate mode
(by positioning the Local/Communicate switch to
Com) or insert paper. essage reception from the
multiplexer is not possible until this is accompli
shed.
Point-to-Point Operation— ultiplexer and Terminal
with Dial-Up Feature Installed
The dial-up operation of a terminal and multiplexer
is essentially the same as a point-to-point terminal
and multiplexer operation without the dial-up fea
ture installed. When the dial-up feature is used,
a communication line connection must be established
before the terminal and multiplexer can communi
cate with each other (Figures 8 and 9). The
terminal must be in a standby status before attempting
to make a line connection.
Transmit
Before transmitting from the terminal to the multi
plexer, a communication line connection must be
established in the following manner. Lift receiver,
press Talk button on data set, and dial the multi
plexer. The data set at the multiplexer should be
equipped with the auto-answer feature. This feature
places a high-pitched tone on the line when the
multiplexer is called (provided the Auto button on
the multiplexer data set has been operated pre
viously), which indicates that a line connection has
been established.
Press Data button on terminal data set and place
receiver in cradle. The Dial-Connect light will be
on indicating the line is connected and the terminal
is in a standby status. From this point on, the
operation of the terminal is the same as a terminal-
to-multiplexer operation without the dial-up feature
(see " Point-to-Point Operation— ultiplexer and
Terminal” in this section of manual and Figures
5 and 6). While the terminal is in a receive or
transmit status the Dial-Connect light is off, even
though the line is still connected. The light turns
on again when the terminal switches to a standby
status.
At the completion of transmission, the line must
be disconnected as follows. Press Dial-Disconnect
key; this disconnects the line at both the terminal
and the multiplexer, provided both data sets have
the automatic disconnect feature installed. Auto
disconnect is available as a terminal special feature
on IB World Trade machines.
Receive
To receive from the multiplexer, the terminal must
be in a standby status with the Auto button on the
data set pressed down. The auto-call capability
of the multiplexer data set (special feature) enables
the multiplexer to call the terminal automatically.
The Auto button on the data set (having previously
been operated) enables the terminal to answer the
incoming call automatically, establish a line con
nection, and turn the Dial-Connect light on. Once
the line connection is established, the multiplexer
transmits a ^5) signal, which switches the terminal
to a receive status.
D
I
A
L
Terminal A B
SWITCHED
Bid Key
®Text
EOT Key Bid Key EOT Key
O © Text O Dial Disc Key
A
NETWORK
Terminal B or
Multiplexor
V
Connect
VV
Figure 8. Dial-Up Operation Dial-Up Adapter Special Feature)
YY
©Text ©
Bid EOT
Key Key
©
©o
from from
Mult Mult
Disconnect
Principles of Operation, Model 1 15

Transmit Control— ultiplexer
NOTE: One Idle character must follow the to enable the
terminal motor to attain full speed.
The Standby and Dial-Connect lights turn off, the
Receive light turns on, the terminal motor turns on,
and the keyboard locks. The terminal is now
conditioned to receive the data transmitted by the
multiplexer.
At the completion of transmission the communi
cation line can be disconnected automatically by the
multiplexer (if the Auto-Disconnect feature is in
stalled on the multiplexer data set), or by operating
the Dial-Disconnect key at the terminal.
See Transmit Control Under Special Features, odel
1 in this section of the manual and also 2740 Termi
nal and ultiplexer with Transmit Control Feature
Installed in the IB 2740/2741 Communication
Terminal—Operators Guide, Form A27-3001.
Station Control— ultiplexer Addressing and Polling
See Station Control under Special Features, odel 1
in this section of the manual and also 2740 Termi
nals and ultiplexer—Station Control Special Fea
ture Installed in the IB 2740/2741 Communication
Terminal—Operators Guide, Form A27-3001.
a I A Connect ©©Bid Key
©Text
EOB
Key L EOT Key Dial
® c ©Disc
Key
SWITCHED
NETWORK
2)
t t
Terminal B or
Multiplexor D (d) Text ( T ) L
A Bid Key ' EOB Key c
L or or
A ® ®
from from
Multiplexor Multiplexor
© ; ©
1 c 1
© ©
Text ( B ) L
0) W R
EOB Key q EOT Key
or or
from from
Multiplexor Multiplexor
©Disconnect
Figure 9. Dial-Up Operation with Checking Dial-Up Adapter and Checking Special Features)
16