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Data Communications
Group Polling. In order to reduce the time and pro-
gramming required to poll each terminal on a communica-
tion line you can perform a group poll. This will allow all
of the terminals in a group (terminals having the same
group ID) with data ready to send, to respond
to
a single
poll sequence. When the last terminal in the group with
data to transfer is through sending it will send an EOT to
indicate that the group has finished.
Selection. ((Selection" occurs when the computer directs
a specific terminal or group of terminals to accept a data
transmission. The character sequences used in selection
are the same as those used in polling. The only difference
is that the lower case group ID characters are used. This is
to tell the terminals that a selection is being sent instead
of a poll. For example, to address the same device as in the
polling example, the sequence would be as follows:
The group poll sequence is similar to the normal poll
sequence. The .. character (042 octal) is used in place of
the device ID characters. For example, to poll all of the
terminals in group A you can use the following sequence:
ASYNCHRONOUS
Group Poll Characters
I \
EPA A
II
II
E P
o
A
H A
T D
Q
D
ASYNCHRONOUS
E P
e e
D D E P
0
A
H A
T D
Q
D
SYNCHRONOUS
555
E P
555
e e
D D E P
VVV
0
A VVV
H A
NNN T D NNH
Q
D
• Optional PAD character
SYNCHRONOUS
Group Select. A ((group select" sequence can be used to
send data to all of the terminals in a group. The terminals
will not send any response to group select. (Since there is
no response there is no guarantee that the terminals will
receive the text.) The text transmission is appended di-
rectly to the end of the group select sequence. The group
select is the same as a device select sequence except that
the device ID character is replaced with a tilde (-) (octal
176). For example, to send data to all of the terminals in
group C the following sequences would be used:
Note that both the group ID and device ID characters are
transmitted twice to eliminate line errors during Poll and
Select sequences. (These transmissions do not use Block
Check characters.) The two group ID characters must be
the same and the two device ID characters must be the
same for a terminal to accept a poll or select sequence.
Then, if the group and device IDs are the same as the
terminal's, the terminal will respond with an ACKO. After
receiving the first block of data the terminal will respond
with an ACKl.
(Terminals of group A have no data to trans-
mit.)
+
EOT
I
AA'"
(Group POLL. group A)
I
555 E P·SSS A A
II
..
E P
VVV 0 A VVV
H A
HHH T D HHH
Q
D
-
- =
BB'
ASYNCHRONOUS
5 TEXT E B P
T
TeA
X
xeD
\
I
Included in block check
5 TEXT E B P
T
TeA
X
xeD
Lower case for select Group select characters
I
\ I \
E Pee
o
A
T D
Lower case for select Group select characters
I
\ I \
555 E P 555
c c
VVV 0 A VVV
HHH T D HHH
SYNCHRONOUS
(Terminal group B device A: ETB indicates
more text blocks ooming from terminal.)
(Group B terminals have no more data to
transmit.)
(ETX indicates last tex1 block from terminal.)
(Terminal group B. device G.)
t
EOT
I
t
STX Text ETX (BCC PAD)
I
t
STX B G Text ETX (BCC PAD)
I
...
f
ACKO
I
t
ACK1
I
(Group POLL. group B)
•
STX B A Text ETB (BCC PAD)
I
t
ACK1
I
5-40
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