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.

BSC (BINARY SYNCHRONOUS COMMUNICATIONS) CAPABILITIES INTRODUCTION TO BSC The Binary Synchronous Communications (BSC) features provide the 2703 with the ability to operate
with other BSC-adapted stations and terminals, thus
providing the following System/360-based communi­
cations capabilities: • Attachment of up to 48 lines at speeds up to 2400 bps or up to 24 lines at 4800 bps, coupled
with powerful error-detection capabilities. • Versatility of transmission code used: EBCDIC, USASCll, SBT. EBCDIC--Extended Binary-Coded-Decimal
Interchange Code. USASCII--United States of America Standard
Code for Information Interchange (formerly
called ASCll, American Standard Code for
Information Interchange). SBT--Six- Bit Transcode. • Transmission of a full range of bit patterns
w hen operating in transparent mode. • Increased throughput by ability to transmit
two packed-decimal digits in a given character­ ti'me frame, as well as ability to transmit
unedited information. • Multipoint operation, on leased communications
networks with station-selection features. • Programmed dialing of remote stations (via
Autocall feature) and automatic answering of "calls" originated by a remote station. • Operation on point-to-point leased communi­
cations networks using a contention system. • Reduced (or eliminated) code-translation
operations in CPU before and/or after trans­
mission. • Incorporation of flexible line-control procedures
adaptable to the user's requirements. • Auto-polling capabilities as standard. • Attachment of the 2703, with the Synchronous
Base and Synchronous Features, to the multi­
plexer channel of the System/360 Models 25, 30, 40, or 50; or to the System/360 Models 65 and
75 via the IBM 2870 Multiplexer Channel. • Provision by the Synchronous Features of
communications attachments with another
Synchronous-Feature-equipped IBM 2703, to
an SDA-ll equipped IBM 2701 Data Adapter Unit, to an IBM 2780 Data Transmission Terminal,
to an IBM System/360 Model 20 with BSC adapter, to an IBM System/360 Model 25
with ICA Feature, or to an IBM 1130 Com­
puting System equipped with a BSC adapter.
GENERAL DESCRIPTION OF BSC OPERA nONS The IBM 2703 Binary Synchronous Communications
operations use circuitry provided by the basic 2703 as well as by the Synchronous Base and Synchronous
Features. For reading ease, therefore, references
in this publication will be to Synchronous Features (SF) rather than to specific circuit areas.
The BSC uses a data-link control procedure and
its associated control-character repertoire.
(Understanding the difference between a "code" and
a "data-link control" procedure is important. A
code--such as EBCDIC, USASCll, SBT--consists of
bit configurations having meaning to a circuit, or
a program. As such, it can be compared to a word
in the English language. On the other hand, a "data­ link control" procedure is one that determines the
sequencing of the coded characters. This sequencing
can be compared to the English grammar that deter­
mines the sequencing of words in the language. )
This data-link procedure (and associated repertoire)
provides a "common communications-control
language, " and is applicable to EBCDIC, USASCll, and SBT. It implements the control of the following
aspects of data transmission:
Establishment of communications through-­
Contention (point-to-point system control);
Multipoint control operations (selection and
polling);
Switched-network operations (automatic answer­
ing and automatic disconnect).
Message transmission--
Message-exchange operation (heading and text
transmission);
Data blocking;
Transmission-error checking;
Station-status replies;
Enquiry functions and alternating replies;
Transparent-data transmission.
The basic control of the transmission link between
two BSC-adapted items of equipment is accomplished
by the recognition of the data-link-control character
in conjunction with established equipment-generated
timeouts. All transmission over the data link is a
binary-bit stream and is synchronous by bit and by
character. Bit synchronism is established by the
modem (data set), or by an optional internal clock
47