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For example, in the execution of the Specific Poll to
Display Station command (Figure 6), the addressing se-
quence is initiated by the I/O channel. The 2845 responds
with a transmission sequence (STX, 2265 address, text,
ETX, and LRC). Upon receipt of the transmission se-
quence, the I/O channel responds with ACK, indicating
correct receipt of the transmission sequence. The 2845
responds to ACK with EOT and the poll operation is con-
cluded. As determined by the program, the I/O channel,
instead of responding to the transmitted sequence with
ACK, could respond with one of the following: NAK,
SOH, EOT, STX/EOT, and STX, Text, ETX, LRC.
The function and operation of each command are
described in the following paragraphs. Included are de-
scriptions of sequences and responses that may occur (as
shown in Figures 6 through 13) and descriptions that are
necessary to explain each operation. All sequences and
responses are summarized under "Summary of Sequences
and Responses". Recommended procedures for recovery
from error conditions are described under "Error Recovery
Procedures" .
Note that a poll command is designated as Specific or
General by the device address code that is used in conjunc-
tion with the Poll command. Use of a Display Station's or
Printer's own address code results in a Specific Poll of only
that device. Use of a device address of ali 1-bits (hexadeci-
mal representation
=
FF) results in a General Poll of the
Display Station and, if the 2845 is equipped with the
Printer Adapter Feature, also of the Printer attached to
the Display Control.
Specific Poll to 2265 Display Station (Figure 6)
The Specific Poll command addressed to a 2265 Display
Station enables the 2845 to determine if the ENTER key
has been depressed. If the key has not been depressed,
indicating the 2265 has no message ready to be transmitted,
the 2845 responds to the addressing sequence with EOT.
EOT terminates the transmission. If the ENTER-key has
been depressed, the 2845 transmits STX, the address of the
2265, and all data (text) displayed between the SOM
symbol and the EOM symbol. The exception is any data
between an NL symbol and the end of the display line
containing the symbol. The text is followed by the ETX
and LRC characters. If no SOM symbol is being displayed,
the 2845 transmits STX, the address of the 2265, ETX,
and LRC, but no text. Upon successful receipt of the
transmission sequence, the I/O channel may respond with
STX/EOT. This response causes the SOM symbol to be
I
erased from the 2265 screen, the ENTER PENDING indi-
cator to be turned off, and the keyboard to be restored. It
also terminates the Specific Poll operation; no further
response is required from the 2845.
As an alternate response, the I/O channel may respond
with ACK (instead of STX/EOT) to notify the 2845 that
the display message was received correctly. The ACK
response causes the SOM symbol to be erased from the
12265 screen, the keyboard ENTER PENDING light to be
30
extinguished, and the keyboard to be restored. The 2845
responds to ACK with EOT to conclude the operation.
Another I/O channel response to receipt of the Display
Station message must be considered. As indicated in
Figure 6, the I/O channel may have a message to send to
the polled Display Station. Therefore, after receiving the
message from the 2265, the I/O channel may reply by
sending a message to the 2265. Since the 2845 and 2265
are already selected, no further addressing sequence is
necessary. Essentially, the Specific Poll to Display Station
is converted to a Write Addressed DS. The SOM symbol is
I
erased, the ENTER PENDING indicator is turned off, and
the keyboard is restored. After the message has been re-
ceived and acknowledged by the 2845, the I/O channel can
send another message to the 2265 or terminate the opera-
tion with EOT.
A NAK response by the I/O channel indicates that a
parity error was detected by the 2701. This response
causes the 2845 to automatically retransmit the message
beginning with STX. An EOT or SOH response from the
I/O channel to a 2845 message ending in ETX/LRC indi-
cates a negative response to the transmission and terminates
the operation. A negative response does not result in the
clearing of the SOM symbol nor the unlocking of the key-
board. The Enter operation is still pending.
The CAN character is transmitted foilowing the iast char-
acter of text and prior to ETX only if an internal parity error
is detected by the 2845 after the ENTER key is depressed
and before the message text is completely transmitted. Cor-
rect parity is assigned to the erroneous characters by the
2845 before they are transmitted to the I/O channel. The
characters with corrected parity are identified as those im-
mediately preceding any transmitted Check characters.
Specific Poll to 1053 Printer (Figure 7)
The Specific Poll sequence, when directed to a 1053 Printer,
causes the 2845 Display Control to test the readiness and
busy status of the Printer and to respond accordingly.
Three possible 2845 responses to the Specific Poll se-
quence provide the I/O channel with the busy and ready
status of the Printer:
1. NAK-This response from the 2845 indicates that the
Printer is not ready; i.e., does not have power on or is
out of paper. The 2845, upon responding with NAK,
also sets Printer Request in the 2845 (described under
"Printer Request Condition").
2. EOT - This response from the 2845 indicates that the
Printer is busy. The 2845, upon responding with EOT,
also sets Printer Request in the 2845 (described under
"Printer Request Condition").
3. STX, Printer Address, ETX, LRC Character-This
response from the 2845 indicates to the I/O channel
that the Printer is ready and free to print a message.
Subsequent action is determined by the program.
If the 2701 detects a parity error in this sequence, a
NAK response will cause a retransmission from the 2845.
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