I/O Channel Sequence And Responses; Start Of Text (Stx); End Of Text (Etx); Positive Acknowledgment (Ack) - IBM 2265 Manual

8. The 2845 in Test status.
9. The 2845 in a power-off condition.
I/O Channel Sequences and Responses
The following iine control characters are recognized by the
2845 during execution of an I/O channel Write command:
Start of Text (STX)
STX must be transmitted by the I/O channel as the first
character of a text transmission sequence. Receipt of STX
by the 2845 places the 2845 in text mode and initiates
LRC accumulation. (STX is not included in the accumula-
ted LRC character.) Failure to transmit STX resets the
2845 and no response from the 2845 will be forthcoming.
End of Text (ETX)
ETX indicates the last character of the message has been
transmitted. The LRC character is transmitted immedi-
ately after the ETX character. Upon receiving the LRC
character, the 2845 performs an LRC accumulation check.
Line turnaround also occurs at this time. The ETX charac-
ter is included in the LRC accumulation. Channel End
and Device End are set in the 2701 status byte following
transmission of the LRC character.
Positive Acknowledgment (ACK)
An
ACK response from the I/O channel to a message re-
ceived from the 2845, as a result of a Specific Poll or
General Poll command, is an affirmative response indicat-
ing that the message was received correctly. Upon receipt
of the ACK response at the 2845, the SOM symbol is
deleted, the ENTER indicator is extinguished, and the key-
board- is restored. ACK causes an immediate line turn-
around.
Negative Acknowledgment (NAK)
A NAK response from the I/O channel indicates that an
error was detected in a message sequence transmitted by
the 2845 Display Control. The NAK response from the
I/O channel causes the 2845 to retransmit the message.
NAK causes an immediate line turnaround.
Start of Heading (SOH)
The SOH communication control character is transmitted
only by the I/O channel and is normally used as the first
character of all addressing sequences containing a Write
Addressed DS, Erase/Write Addressed DS, Write Printer,
or Write DS Line Address command. The SOH character
places the 2845 in control mode, and prepares the 2845
to receive an addressing sequence. (In this respect, it is
identical in function to EOT.) LRC accumulation is
inhibited.
44
The SOH control character may also be used as a nega-
tive response to a message sequence from the 2845. When
so used, it terminates the operation but does not restore
the keyboard of the selected 2265.
End of Transmission (EDT)
EOT is normally used as the first character of the Specific
Poll, General Poll, and the Read Addressed Full DS Buffer
addressing sequences. The EOT communication control
character places the 2845 in control mode, and prepares the
2845 to receive an addressing sequence. (In this respect,
it is identical in function to the SOH communication con-
trol character.) LRC accumulation is inhibited. EOT is
also used to conclude an operation.
An EOT response from the I/O channel to a transmis-
sion sequence from the 2845 ending in ETX/LRC indi-
cates a negative response to the transmission sequence and
terminates the operation. It does not restore the keyboard
of the selected 2265.
Start of Text/End of Transmission (STX/EOT)
The STX/EOT sequence may be used as a positive response
in substitution for ACK, following a Specific Poll to Dis-
play Station, General Poll, Write Addressed DS, or Read
Addressed Full DS Buffer operation. This sequence con-
cludes the operation and does not require a reply from the
2845. When used following a Specific Poll to Display Sta-
tion or General Poll, it also causes the SOM symbol to be
erased from the screen, the ENTER light to be extin-
guished, and the keyboard to be restored.
Line Feed (LF)
The LF character is.a formatting character which is recog-
nized by the 2845 only when it is transmitted as part of
the message text. When the text is addressed to a Display
Station, the LF character causes the cursor to be placed in
the first character position of the next line. When included
in text addressed to a Printer, the LF character causes
the Printer to perform a carriage return-line feed operation.
PROGRAMMING NOTE: Transmission of an SOH or EOT com-
munication control character within the body of a message aborts
the operation and places the 2845 in control mode. Therefore,
they should not be transmitted as part of text. When the ACK,
NAK, or STX communication control characters are transmitted
within the body of a message, the character location occupied by
anyone of these three control characters within the body of text
is displayed as a space. Transmission of an ETX communication
control character within the body of text will cause a line turn-
around. The LRC accumulation will not compare equally and the
2845 will respond with NAK.