Select Sequence Format - HP 262SA Reference Manual

Data Communications
SELECTING. ttSelecting" occurs when the computer directs a specific terminal or group of
terminals to accept a data transmission.
For example, the sequences illustrated in figure 7-13 select device
ttD"
in group
tt
a".
ASYNCHRONOUS
~
PAD
e e
D D
~
PAD
~--..--
Group ID for select Device ID
SYNCHRONOUS
~~~ ~ PAD ~~~
e e
D D ~ PAD
'-...-" ~
Group ID for select Device ID
*Optional, but if present, should be FF (hex).
Figure 7-13. Select Sequence Format
Note that both the group ID and device ID characters are transmitted twice to reduce line
errors during poll and select sequences. (These transmissions do not use Block Check Charac-
ters.) 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 responds with an ACKO. After receiving the first block
of data the terminal responds with an ACK1.
GROUP SELECT. A ttgroup select" sequence is used to send a single block ofdata to all of the
terminals in a group. The terminals do not send any response to a group select. (Since there is
no response there is no guarantee that the terminals have received the text.) The text
transmission is appended directly to the end of the group select sequence.
The group select sequence 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
tt
c "
you could use the sequences illustrated in figure 7-14.
ASYNCHRONOUS
Group ID
for select Group select characters
* -"--"-
~
PAD
c c
~ ~
\
TEXT
~
Bee PAD
'v/
Included in block check
SYNCHRONOUS
Group ID for select Group select characters
*
~-"-
~~~ ~
PAD
~~~
c c
~ ~
\
TEXT
~
Bee PAD
\ I
Included in block check
*Optional, but if present, should be FF (hex).
Figure 7-14. Group Select Sequences
7-27