IBM 2265 Manual page 40

When an SOH or EOT is received, the 2845 transfers
the contents of the printer buffer to the Printer.
If
the
Printer is in the process of printing out an operator-initia-
ted print operation (PRINT key depressed on the 2265
keyboard), the 2845 holds the computer-generated Printer
message until the key-initiated print operation is completed
before transferring the computer-generated message to the
Printer. (A print operation initiated by depressing the 2265
PRINT key transfers displayed data stored in the display
buffer to.the Printer for printing.) If a parity error is de-
tected when the text is being transferred from the printer
buffer to the Printer, the incorrect character is printed. A
Check symbol (,) is printed at the end of the message to
indicate that an error has occurred.
Referring back to the initial addressing sequence, the
2845 will respond with NAK if the Printer is not ready,
and Printer Request will be set. (See "Printer Request
Condition" below.) The 2845 will respond with EOT if
the 2845 finds the printer buffer busy; i.e., still trans-
ferring text to the Printer as a result of a previous Write
Printer command. A busy condition also sets Printer
Request.
If
both Not Ready and Busy conditions are
detected, the 2845 responds with NAK and Printer Re-
quest is set. No response to the. addressing sequence indi-
cates that the sequence was not successfully decoded or a
VRC error was detected by the 2845.
Other 2845 responses to the Printer message are shown
in Figure 12. A NAK indicates that the 2845 detected a
VRC or LRC error in the message transmission from the
I/O channel. The program should retry the text transmis-
sion. (See "Error Recovery Procedures".)
If the Printer text exceeds 240 characters, the 2845
responds to the message transmission with EOT, indicating
a printer-buffer overrun. The 2845 erases the text entered
in the printer buffer and inhibits the printing operation.
The message should be broken into segments of 240 char-
acters or less and transmitted separately.
PROGRAMMING NOTES:
1. If a VRC or LRC error is detected during printer text transmis-
sion from the I/O channel, upon conclusion of the transmission
the 2845 will inhibit the start of the print operation and issue· a
NAK response to the I/O channel. The program should retry
the text transmission starting with STX.
2. Should the Printer text transmission exceed 240 characters
(buffer overrun), upon conclusion of the transmission, the 2845
will inhibit the start of Printer operation and issue an EOT
response to the I/O channel to indicate that a printer-buffer
overrun has occurred. The program must segment the Printer
text into two or more shorter messages.
3. An EOM code should be transmitted as the last character of
each complete message to ensure separation of messages. The
EOM code causes an EOM symbol to be printed and the print
mechanism to be advanced to the lust print position of the
next line. The EOM symbol serves as a visual indication that
the message has been completed.
40
4. An LF code should be used as the lust character of a message
when additional separation between messages is desired. The
LF code causes the print mechanism to be advanced to the lust
print position of the next line. However, unlike the EOM code,
the LF code does not cause a character to be printed.
5. A message to the Printer that contains more characters (includ-
ing spaces) than can be accommodated on one print line must
be segmented into two or more lines by inserting LF codes at
the appropriate points within the message. Otherwise, all
characters that exceed the capacity of a single print line will
overprint in the last print position.
6. If two successive Write Printer commands are issued, the second
command will not be accepted until
th~
Printer has completed
printing the lust message. For this reason, the Write Printer
command should not be chained.
7. Data chaining of successive Printer messages will result in ex-
cessive line turnaround time. If two or more messages are to
be sent to the Printer, they should be combined and sent as a
single message (not to exceed 240 characters.)
Printer Request Condition
The Printer Request condition will be set in the 2845 if
the I/O channel attempts to select the attached Printer
with a Specific Poll to Printer command or a Write Printer
command while the Printer is busy or not ready. Later,
upon receipt of a General Poll command, because the
Printer Request condition is already set, the 2845
will
test
the Printer ready and busy statuses. If found still busy or
not ready, General Poll is continued by automatic polling of
the Display Station as described under "General Poll".
The Printer Request condition remains unchanged. How-
ever, if the Printer is found to be ready and not busy, the
2845 responds to the I/O channel by sending the follOWing
sequence: STX, the Printer's address, ETX, and LRC
character. This printer-available sequence indicates to the
I/O channel that the Printer is ready and free to receive a
message.
The I/O channel may respond to the printer-available
sequence with a Printer message. If so, no addressing se-
quence is necessary; the operation continues as a Write
Printer operation. However, if the I/O channel does not
respond with a message for the Printer and instead sends
another addressing sequence to the 2845, the new address-
ing sequence terminates Printer
Requ~st.
Therefore, a new
General Poll will not find Printer Request set and will
not elicit the printer-available sequence from the 2845.
In this case, the Write Printer command must be issued to
send a message to the Printer.
Simultaneous Keyboard and Printer Operations
2265 Display Station data and 1053 Printer data are. stored
in separate portions of the 2845 buffer storage and are es-
sentially independent of each other. Therefore, transfer
of data from the 2845 to the 1053 Printer as the result of
a Write Printer command does not interfere with keyboard
entry, display of data, nor any 2845 command other than