Ascii Communication Control Characters; 2845 Display Control Modes Of Operation; Control Mode; Text Mode - IBM 2265 Manual

Seven Bits of USASCII CharaCter
.
Figure 5. Ten-Bit Format of TransmittedUSASCII Characters
facilities. The basic function of the data sets is modula-
tion and demodulation of 2701 and 2845 signals for data
transmission.
USASCII COMMUNICA1.ON,COHTROLCHARACTERS
To establish and maintain an
·orderlY
flOw of traffic over
the communications channel, particularly ,in multidrop ap-
plications (where more than one 2845 is attached directly
to the communications channel), a method of controlling
line traffic is necessary. Line traffic -between the System/
360 I/O channel and the remote display system is con-
trolled by six of the US ASCII communication control char-
acters. The six codes, used singly or in sequences, perform
all the necessary control fUrictioos.'
In
addition to the six
USASCII communication control-characters, a USASCII
format effector (line feed, LF, character) and a USASCII
control character (cancel, CAN) are used in d'ata transmis-
sions between a remote display system and the I/O channel.
Each of these eight USASCn characters is defined below.
Their code structure is shown in Figure 4. For a detailed
description of each character (when it is used, its signifi-
cance, and interpretation), see "Sutnnlary of Sequences
and Responses".
STX (Start of Text) - A communication control character
which precedes a sequence of characters (text) that is to be
treated as an entity. This entity is usually terminated by
ETX.
ETX (End of Text) - A communication control character
used to terminate a $equence of characters (text) started
with STX and transmitted
asanert~ity:
CAN (Cancel) - A control
cnuractet'llSea;to
indicate that
the data with which it is sent
mayie.irt·,error.
The CAN
control character is transmitted~
fol1owihg
text and before
ETX, if an internal parity error has been detected by the
2845 Display Control. CAN is transmitted only by the
2845.
ACK (Positive Acknowledge) -:-
4!.~omn)~nication
control
character transmitted
byat~ei*tj$".;,a~f~mative
re-
sponse to a sender
.:~~\':::c',:;,;~~,;~'::{:.,:,:,i~;;>,;
. ,' ; ,'
NAK (Negative
ACkft(JWIiJ~iA,::':"'~>'~:·'":~"",~~",·'~:'~;ct}ntrol
character transmitted by at~_'iS.'a:~~~Uve response to
the sender.
SOH (Start of Heading) - Acomtbttftication control charac-
ter used at the beginning of a sequence: of characters (head-
ing) which constitute a tna _ _ _
~fldress.
, . ' .. ~'":"} {< '
EOT (End of Transmission) - A communication control
character used to indicate the conclusion of a transmission
which may have contained one or more texts and any asso-
ciated heading (addressing sequence). EOT may also be
used, instead of SOH, as the first byte in the addressing
sequence.
LF (Line Feed) - A format effector which should be
included only as part of text. When included in text ad-
dressed to a 2265 Display Station, the LF character
causes the NL symbol to be displayed and the cursor of
the selected 2265 Display Station to move to the first char-
acter position in the next display line. If the cursor is in
the bottom line, it will move to the first character position
of the top line. The LF character received by the I/O
channel from the 2845 Display Control indicates that the
following data byte is the first character of the next 2265
display line.
When included in text addressed to a 1053 Printer, the
LF format effector causes the Printer to perform a carriage
.return/line feed (CR/LF) operation.
PROGRAMMING NOTE: Use of any control character other
than LF within the body of a text transmission is invalid. Receipt
of a control character other than LF within the body of text has
the following results:
1.
SOH, EOT, or ETX causes immediate return of the 2845 to
control mode (see below).
1
2. ACK, NAK, or STX has no effect on the 2845 mode. ACK is
displayed as an F; NAK is displayed as an E; STX is displayed as
aBo
2845 DISPLAY CONTROL MODES OF OPERATION
A remote 2845 Display Control operates in one of two
modes: Control or Text.
Control Mode
Control
mod~
is the mode in which the 2845 Display Con-
trol is placed preparatory to receiving an addressing sequence
or control
char~cters.
The 2845 enters control mode, while
in selected or non-selected status, on receipt of an SOH,
EaT, or ETX control character. Receipt of an STX con-
trol character by a 2845 in selected status switches that
2845 from conttol mode to text mode. (The 2845 is con-
sidered selected when it has responded to an addressing se-
quence and is in, communication with the I/O channel.)
Text Mode
Text mode isth¢ mode in which the 2845 Display Control
, operates
while
receiving or transmitting text. The 2845
is placed in text mode, while in selected status only, upon
receipt of an STX control character. Receipt of an ETX,
EOT, or SOH control character switches the 2845 from
text mode to control mode. In a multidrop application,
only one 2845 can be in text mode at anyone time.
Principles of Operation 27