and then run another sequence while processing the last set of results, perhaps to produce
a quick update from a repeating sweep, similar to that provided by sequence 15.
If the LA102 does not receive a sequence within a fixed time (typically 10s) then it will
time out and generate sequence error 10. This prevents the system locking up completely
if the audio path is not connected. After sending an SQ command the computer should
check the error number (using E?) and take corrective action if error 10 occurred
(typically, this will mean prompting the user to plug the audio leads in). The time out
period can be adjusted or disabled using the SP command (see below). It is useful to
disable it where the LA102 is monitoring a programme circuit which perhaps only carries
one sequence per day. The computer can instruct the LA102 to listen indefinitely and
then read the results when a sequence has been received. Alternatively, the SQ command
can be repeated every time the LA102 times out, but this is more clumsy and possibly
error-prone.
The LA102 will abort and accept further commands if a sequence error occurs, and again
this can be detected by reading the error number with the E? command. If a sequence
error is detected it is possible to continue receiving the sequence by repeating the SQ
command immediately (the command will be ignored by the LA101 while it is generating
a sequence).
Normally both units will be connected to the computer and to the equipment under test
and both units will respond to the command simultaneously. Note that they do not rely on
the command for synchronisation, so it does not matter if there is a small delay (perhaps
when testing a 3-head tape machine), as the LA102 will always synchronise on the FSK.
The command may be sent to one unit only by prefixing it with a ‘1’ or a ‘2’ to make the
LA101 send a sequence or to make the LA102 receive one. This would normally be done
when testing lines and links, where only one unit is available at a particular site, or
perhaps when recording or replaying a sequence from a 2-head tape machine which does
not provide off-tape monitoring. In such cases, it is important that the LA102 is ready to
receive a sequence, so if the two units are under computer control the SQ command
should be sent to the LA102 first and then the identical command should be sent to the
LA101. Alternatively one unit may be under manual control, but again the LA102 must
be in sequence mode when it receives the sequence.
Notes for advanced users:
1. If the sequence was run from remote mode the handshake line of the LA102 will
prevent further remote commands interrupting the sequence, so it is permissible to send
an E? or R? command without waiting for the sequence to finish (since it will be held up
by the handshake line). However, sequences which are run from the LA100 front panel
(using the
key) can be interrupted by sending a command (since the handshake line
will not indicate that the unit is busy). In such cases the units should be left to finish the
142
9. Remote Control
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