The following introduction to start/stop (asynchro­
nous) type operations uses as examples IBM type terminals, such as the 1030, 1050, 1060, and 2740. The transmission of data by means of start/stop
type communications involves, for example, the
coding of each character with a start bit and a stop
bit, in addition to the data bits and possibly a check
bit for odd parity such as shown in Figure 5. Thus,
the transmission of characters can occur at an
irregular rate, since each character contains its own
sync information (character timing). This is parti­
cularly useful for transmission from unbuffered
units such as keyboards and devices requiring the
manual insertion of input documents (badge readers,
card readers, etc.). Start/ stop communications require the use of a
unique set of line-control characters to provide for
communications-line discipline (polling and addres­
sing) and identification of the various portions of the
message (station identification, text blocks, etc.).
All messages are transmitted during text mode,
after certain control operations are first performed
in control mode. All characters transmitted during
text mode are either printable data characters or
functional characters (not printable). The functional
characters consist of such codes as CR/LF, delete,
and idle. Codes transmitted during control mode provide
terminal control, station identification, and component
selection for the remote terminal. For example, the 1050 uses a line-control signal, © --EOT, and
an alphabetic station-identification character and a
numeric component-select code (together with the
appropriate response) to maintain communications­
line discipline. These polling and addressing pro­
cedures allow the 2703 to control the communications
line at all times. Messages transmitted from the 2703 are preceded by an address. Remote stations
have the opportunity to transmit only when polled
from the 2703. *8 Indicates no 8-bit
Figure 5. Bit Configuration of a "G" Character
(Serialized) START/STOP COMMUNICATIONS CAPABILITIES LINE CONTROL The following are transmitted during control mode: • Control signals--EOT, EOA, EOB, Yes, No, SOA, and Inquiry. • Polling characters--These consist of an alpha­
betic station -identification character, A - Z ,
followed by a numeric component-select
character (used by the 1050), 5, 6, 7, or o.
The polled terminal is requested to transmit,
if the polled component is ready. • Addressing characters--These also consist of
an alphabetic station-identification character, A-Z, followed by a numeric component-select
character (used by the 1050), 1,2,3,4, or9.
The addressed terminal is requested to receive,
if the addressed component is ready.
The line-control signals--EOT, EOA, EOB, Yes,
No, SOA, and Inquiry--are represented in a short­
hand form--© ' CD , @ , (i) , ® , ® ' and @ , respectively (Figure 6). form is used in
programming as well as in communications and line­
control discussions. Also, the terms "response" and "answer" are used in the following restricted manner:
1. Answcr--The negative or positive reply, or no
rep.1y at all, to an LRC compare.
2. Response--The negative or positive reply, or
no reply at all, to a component-select charac­
ter (addressing or polling).
Description Symbol Processor Bi t Processor Configuration Character End of Transaction (EaT)
© C-8-4-2-1 r(TapeMark) End of Address (EOA) @ 8-2-1
# (Pound Sign)
End of Block (EOB) ® C-A-8-4-2 't (Record IV Jrk) Positive Response (YES) G) B-A-8-2-1 (Period) Negative ResponsE' (NO) @ B -(Hyphen)
Address Select (SOA) CD C-A-8-2-1 , (Comma) Positive Response (Inquiry) 0 8-2-1 it (Pound Sign)
Figure 6. Line-Control Characters
23

EOT (End of Transmission)-- <0 • Indicates an end of message transmission, and
resets the LRC counters at both sending and
receiving stations. NOTE: LRC does not apply to the 1030 or to the 2740 (without
the checking feature installed). EOA (End of Address)-- @ • Indicates the transmission of text data is to
follow. • Indicates an end of address, with text data to
follow. This signal starts the LRC counter at
both the sending and receiving terminals. The EOA signal is not included in the following
LRC check. • Indicates positive response to a poll from the
2703. EOB (End of Block) -- ® • Indicates the end of a unit block of text. This
is followed by the LRC charader to provide an
LRC check at the receiving station
with the EOB character included in the check.
Yes (Positive Response)-- Ci? • Indicates a positive response to an address. • Indicates a positive to an EOB when the
checking feature is installed.
No (Negative Response)-- ® • Indicates a negative response to an address. • Indicates a negative response to a poll. • Indicates a negative answer to an EOB; the
hyphen character is printed as an indication
of this condition. SOA (Start of Address) -- ® • Indicates a start-of-station-identification
condition (not used with the 1050). Inquiry (Positive Answer)-- @ • Indicates a positive answer to an EOB from the
terminal. At this time, the terminal switches
to receive status.
TRANSMIT OPERA TION (WRITE COMMAND)
The transmit operation is initiated when the 2703
accepts a Write command from the multiplexer
24
channel. Before transmission to the terminals
occurs, the transmission control requests the first
four bytes of data from the multiplexer channel.
These bytes are then transmitted on a bit-by-bit
basis to the terminal. Additional requests for four
more bytes of data are made each time the last byte
is being transferred, until the complete message has
been transmitted.
The four bytes of data are stored in the main data
word (MDW -1, see Figure 4) located in the 2703
magnetic-core storage. From there they are trans­
ferred byte-by-byte to the main control word (MCW-1) for that line. The transfer from the control word in
core storage to the line base is accomplished on a
bit-by-bit basis until the character has been sent.
Each bit is huffered in the line-base delay line. The
byte, while in magnetic-core storage circuitry, is
shifted one bit position toward the high-order bit
before the delay line is loaded. When a character
has been sent, the next byte is requested from the
data-word location. When the data word is emptied,
a request for the next four bytes is sent to the multi­
plexer channel. This is repeated until the operation
is ended.
RECEIVE OPERATION (READ COMMAND)
The receive operation is initiated when the 2703
accepts a Read command from the multiplexer
channel. On detection of a start bit, the transmission
control prepares to receive a data character. Before
each bit is put into the low-order bit position of the
serial-data field, the field is shifted forward one bit
position in the high-order direction. Each bit of the
character is received in this manner until the terminal
control signals stop time to the common controls.
Stop time occurs when the bit count equals X, which
is a number preset in the terminal control and defined
by the number of bits in a character. For example,
X is 7 for the IBM Terminal Control Type 1. At stop
time, the character stored in the main control word (MCW-1) is transferred to the appropriate main data
word (MDW-1). The 2703 requests data service when
the fourth character has been assembled, and Simultaneously prepares to receive the next character.
- ..... • ,. Jl __________ __ ____ ' ...... DurIng the recelVe operanon Lne and-storage section of the 2703 (see Figure 3) pro­
vides a timeout of 28 seconds or less between data
characters, provided a Read or Search command has
been accepted by the 2703. The timeout process is
interrupted by a received start bit and is reset and
restarted at the following stop time. If the timeout
completes before a start bit is received, the Read or
Search command is tenllinated.
Timeout may also result if a terminal indicates an
intention to send data after being polled and then fails