Error Recovery Procedures; Nak Response To Addressing Sequence; Nak Response To Text; Eot Response To Addressing Sequence - IBM 2265 Manual

ERROR RECOVERY PROCEDURES
Certain error recovery procedures are recommended when
a particular response is received from the 2845 Display
Control. The responses to the addressing sequence and/or
text of a message are described below together with the
recommended error recovery procedure.
NAK Response to Addressing Sequence
The NAK response is a valid response to the Write Printer
and Specific Poll to Printer addressing sequences. In either
case, NAK indicates that the Printer is not ready. The pro-
gram should notify the operator via a Write Addressed DS
command to make the Printer ready. The Write Printer or
Specific Poll to Printer can then be sent periodically until
the printer available sequence is received in response. At
this time the text of the Printer message may be sent.
The NAK response to other than the above addressing
sequences is invalid. If NAK is received in response to other
addressing sequences, the program should respond with
EaT and retry the addressing sequence three times. If the
three attempts are unsuccessful, the operator should be
notified.
NAK Response to Text
NAK is a valid response from the 2845 to any text trans-
mission from the I/O channel. It indicates that a VRC or
LRC error has been detected by the 2845. All NAK re-
sponses to the transmission of text should be logged by
the program.
If the text was sent to a Display Station as a result of a
Specific Poll, Read Addressed Full DS Buffer, Write Ad-
dressed DS, or Erase/Write Addressed DS command, the
recommended recovery procedure is to issue the Erase/
Write Addressed DS command to the selected Display Sta-
tion. Depending upon the application:
1. The text should be retransmitted.
2. The text should be retransmitted with notification to
the operator.
3 . .A
recovery procedure via the application program
should reconstitute the entire display plus the additional
message text.
4. The application program should generate a recovery dis-
play summarizing the transaction in process.
If the text transmission was addressed to the Display
Station as a result of a Write DS Line Address command,
the message should be retransmitted starting with STX.
The 2845 will address the display line, and, if the previous
transmission was correctly positioned to start on the speci-
fied line, the retransmission will overlay and replace the
erroneous text.
If the text transmission was addressed to the Printer as
a result of a Write Printer, a Specific Poll, or a General Poll
command, a Write Printer command should be reinitiated
and, following a positive 2845 response (ACK), the text
retransmitted.
EOT Response to Addressing Sequence
The EaT response is a valid response to the Write Printer,
Specific Poll, and General Poll commands. If EaT is re-
ceived in response to commands other than these three
the program should reattempt the command up to
thre~
times. If the three attempts fail, the operator should be
notified.
EOT Response to Text
The EaT response to text addressed to a Display Station
is not a valid response. If this response is received, the
program should reinitiate the addressing sequence and,
follOWing a positive 2845 response (ACK), retransmit the
text. Should a third attempt fail, the operator should be
notified.
The EaT response to text addressed to the Printer indi-
cates that the capacity of the printer buffer has been ex-
ceeded (buffer overrun). The program must segment the
Printer text into two or more shorter messages and trans-
mit them individually.
CAN Response
The CAN control character transmitted as the last charac-
ter of text (immediately preceding ETX) indicates that the
transmitted text may contain characters with 2845-cor-
rected parity, and that no operator intervention occurred
prior to buffer readout. The characters are identified as
those immediately preceding any transmitted Check char-
acters. The Check characters may be corrected and elim-
inated by program overwrite or by keyboard overwrite.
If keyboard-overwrite correction is required, the operator
should be so notified.
No Response to Addressing Sequence or Text
No response from the 2845 indicates one of the follOWing:
1. An invalid 2845 address (second byte of an addressing
sequence).
2. An invalid 2265, 1053 Printer, or general address.
(Third byte of an addressing sequence.)
3. An invalid command (fourth byte of an addressing
sequence).
4. A malfunction in the 2845 Display Control.
5.
An
incorrect sequence of characters (either addressing
or text) transmitted to the 2845.
.6. Transmission error in a control or addressing character
(for example, garbled by line noise).
7. A malfunction in the communication facilities.
8. The 2845 in Test status.
9. The 2845 in a power-off condition.
When no 2845 response is received to a command ad-
dressing sequence or to text transmitted by the 2701, the
2701 will time out. The program should log the condition
and reattempt the command or text transmission up to
three times. If the three attempts fail, the operator should
be notified.
Principles of Operation 45