IBM 2265 Manual page 36

I/O channel response to the CAN control character is de-
termined by the program. The characters with corrected
parity are identified as those immediately preceding the
Check characters.
The I/O channel may respond to the 2845 transmission
with SOH or EOT. Either response terminates the Read
operation but does not restore the 2265 keyboard.
Figure 9 illustrates another I/O channel response that
can be made following the readout of the 2845 buffer. If
the program has a computer-generated message to send to
the Display Station, the message can be sent after the 2845
buffer readout is complete. Thus, the Read Addressed Full
DS Buffer operation is essentially converted to a Write Ad-
dressed DS operation and, since the 2845 and 2265 are
already selected, no addressing sequence is necessary. The
operation may be terminated as described under "Write
Addressed DS".
No response to the addressing sequence by the 2845 in-
dicates that the sequence was not successfully decoded or
that the 2845 detected a VRC error in the sequence. The
2701 will time out and the program should retry.
PROGRAMMING NOTE: When the 2845, equipped with the
Destructive Cursor Feature, executes a Read Addressed Full DS
Buffer command, the destructive cursor symbol
VJill
remain sta-
tionary on the screen during the command operation. After the
operation is finished (buffer completely read out), depression of a
character key on the 2265 keyboard will cause the entered charac-
ter to be displayed in the first character position and a second
destructive cursor symbol to be displayed in the second character
position of the screen. Because operation with two destructive
cursor symbols is not defmed, an Erase/Write Addressed DS com-
mand" should be issued to the selected 2265 immediately following
a Read Addressed Full DS Buffer operation. In lieu of the Erase/
I
Write Addressed DS command, the keyboard ERASE DPLY (erase
display) key can be used or the original EOM symbol can be over-
written by the operator.
Write Addressed OS (Figure 10)
The Write Addressed DS command is used to transfer data
from the I/O channel to a selected 2265 for presentation
on the Display Station screen. Upon receipt of the Write
Addressed DS addressing sequence, the 2845 Display
Control responds with ACK. No response from the 2845 to
the addressing sequence indicates that the sequence was
not successfully decoded or that the 2845 defected a VRC
error in the sequence. The 2701 will time out and the pro-
gram should retry.
The ACK response indicates that the 2845 and the ad-
dressed 2265 are selected and are ready to receive a mes-
sage. Note that if a keyboard key is depressed at any time
after the Write Addressed DS command is received, the
keyboard locks and the character is not entered. (The
IERASE EOL, ERASE EOS, ERASE DPLY, TEST, and
RESET keys do not lock the keyboard; neither do they
perform their normal functions.)
36
PROGRAMMING NOTE: Ifthe ENTER key was depressed before
the Write Addressed DS command is transmitted, upon decoding
of the command, the ENTER indicator is extinguished, the key-
board is restored, and the Write operation continues.
Upon receiving the ACK response, the normal response
is the following transmission sequence containing the
message to be written on the screen of the selected 2265:
STX, Text (message to be written), ETX, and LRC charac-
ter.
The incoming USASCII characters are parity-checked
and placed in the display buffer of the addressed 2845 at
the location indicated by the cursor. Upon entering the
display buffer, each character is displayed on the 2265
screen. The cursor automatically advances to the next
character position as each character is placed in the display
buffer and displayed on the screen. If a Line Feed (LF)
code is received, an NL symbol is displayed, and the cursor
is stepped to the first position of the next line without al-
tering the contents of the remaining character positions on
that line. The cursor proceeds to step normally with each
additional character received. At the termination of text
transmission, the cursor is displayed in the character posi-
tion immediately following the last transmitted character.
Upon receipt of ETX and the LRC character, the 2845
responds with ACK if no parity errors were detected. The
ACK response indicates to the I/O channel that the mes-
sage was successfully received. The I/O
ch~nnel
may con-
tinue the operation by sending another message or it may
terminate the operation by responding with SOH or EOT.
If the operation is ended with SOH or EOT, the Display
Station keyboard remains locked as a positive indication
to the operator that an I/O operation has taken place.
The screen should be visually checked by the operator for
a computer-generated message before proceeding to enter
data from the keyboard. The keyboard may be unlocked
by depressing the RESET key.
PROGRAMMING NOTES:
1.
Transfer of data from the I/O channel is under I/O channel
byte count control. If a message length exceeds the total num-
ber of character positions on the screen, screen wraparound
occurs. That is, the excess message data replaces previously
written data, beginning at the fust character position.
2. If a parity error is detected by the 2845 during the transmission
sequence, a Check symbol is displayed in place of the character
in error.
3. When operating in conversation mode, it may be desirable to
unlock the keyboard following the Write Addressed DS com-
mand. If so, the program should respond with STX/EOT to
the second ACK response from the 2845. See Figure 10.
("Conversation mode", in this sense, is defined as a communica-
tion between the computer and a display station in which each
entry from the computer elicits a response from the display
station, and vice versa.)