IBM 2265 Manual page 33

However, if no parity errors are detected, a normal I/O
channel response will be STX, text to be printed, ETX,
and the LRC character. Operation from this point is
identical to that of the Write Printer command.
No response by the 2845 to the I/O channel addressing
sequence indicates that the 2845 did not decode the
sequence successfully or detected a VRC error in the se-
quence. The 2701 will time out and the program should
retry.
General Poll (Figure 8)
The General Poll command is used to test the 2265 Display
Station for the presence of a manually entered message
pending transfer to the I/O channel, and, when the Printer
Adapter Feature is installed, also causes the 2845 to test
for a Printer Request condition. Printer Request will be
set if a previous attempt was made (under the Specific
Poll to Printer or the Write Printer command) to select
the Printer while it was busy or not ready. (See "Printer
Request Condition".)
If the 2845 is equipped with the Printer Adapter Fea-
ture., the 2845 first tests for Printer Request condition. If
Printer Request is set and the Printer is ready and not busy,
the 2845 responds with the following "printer available"
sequence: STX, Printer Address, ETX, and the LRC char-
acter. Receipt of this sequence indicates to the I/O chan-
nel that the Printer is now ready to print a message. The
I/O channel may respond by sending a message to the
Printer (STX, text, ETX, LRC character) or it may ter-
minate the operation with STX/EOT, SOH, or EOT.
If the I/O channel sends a message to the Printer at this
time, the operation continues in the same manner as a
Write Printer operation. On the other hand, if the I/O
channel does not respond with a printer message, Printer
Request is ended and a later General Poll to the 2845 will
not elicit a printer-available response from the 2845.
A message to the Printer at a later time will have to be sent
using the Write Printer command. Note that if the opera-
tion is terminated at this time, the poll should be reinitia-
ted with another complete addressing sequence to assure
receipt of a message pending from the Display Station.
However, if the I/O channel sends a message to the Printer,
the operation continues identically to a Write Printer oper-
ation.
If Printer Request is not set, or if it is set but the Printer
is still busy or not ready, the Display Station is automati-
cally polled following the initial addressing sequence. Sim-
ilarly, if the 2845 is not equipped with the Printer Adapter
Feature, the General Poll command causes the 2845 to
poll the Display Station. Operation from this point is
identical to that of the Specific Poll to Display Station
command.
The EOT response to a General Poll addressing sequence
indicates that the 2265 has no . message to send. When the
2845 is equipped with the Printer Adapter Feature, the
EOT also indicates that either Printer Request was not
set or that the Printer is still busy or not ready.
No response to the addressing sequence indicates that
the 2845 did not successfully decode the addressing se-
quence or detected a VRC error in the sequence. The
2701 will time out and the program should retry.
All responses that can occur to a General Poll com-
mand are shown in Figure
8~
Each is summarized under
"Summary of Sequences and Responses".
Read Addressed Full OS Buffer (Figure 9)
The Read Addressed Full DS Buffer command is used to
transfer all characters stored in the display buffer of the
selected 2845 to the I/O channel in the following sequence:
STX
Address of the selected 2265 Display Station
Text (all characters displayed from character position 1
to character position 960, including spaces, except for
the nondestructive cursor)
ETX
LRC character
Data is transferred starting at the upper left corner of the
screen and ending at the lower right corner.
Included among the characters transferred are the codes
for the Check symbol, NL symbol, SOM symbol, and,
when the Destructive Cursor Feature is installed, the
destructive cursor (EOM symbol). A nondestructive cur-
sor is not transferred. If a parity error is detected in the
message by the 2701, the NAK response from the I/O
channel causes the 2845 to retransmit the message.
The normal I/O channel response to receipt of the trans-
mission sequence is STX/EOT, indicating to the 2845 that
the message sequence was received correctly. The STX/
EOT response received by the 2845 causes the nondestruc-
tive cursor to be positioned in the first character position
(upper left corner) of the screen, restores the keyboard,
and terminates the operation. (See programming note
below for operation with the destructive cursor.)
If an internal parity error is detected by the 2845 be-
fore transmission of the 2845 text is completed, the 2845
transmits the CAN control character immediately follow-
ing the text and prior to the ETX character provided the
operator has not intervened by depressing the RESET key.
The CAN control character indicates to the I/O channel
that an internal parity error was detected by the 2845 in
the preceding message. If the operator discovers the error
(indicated by one or more Check characters on the 2265
screen), simply depressing the RESET will prevent the
CAN character from being transmitted should a Read
Addressed Full DS Buffer command be issued. Keyboard
correction of the erroneous characters is optional. Charac-
ters found in error are assigned correct parity by the 2845
before being transmitted to the I/O channel. Subsequent
Principles of Operation 33