H-2 Measurement Queries
:FETCh?
What it does
This command will simply return the latest available reading from an instrument.
Limitations
If the instrument does not have a reading available (indicated by dashes in the display),
sending this command will cause a –230, “Data corrupt or stale” error. This query will not cause
the box to trigger a reading, nor will it “wait” for a result if a reading is in progress. It is possible
to get the same reading over and over using this query. It will continue to give the same result
until one of two things has happened:
•A new reading has been triggered
• The old reading has been invalidated by changing ranges, or by changing function.
Where appropriate
Since this query does not trigger a reading, and can give duplicate results, there are not many
cases where this command should be used. The “:DATA:FRESh?” query (see page H-4) is often
a better choice. If this query is used, the following conditions should be met:
•A reading has been triggered, either by free running (:INIT:CONT ON and
:TRIG:SOUR IMM), by some event such as a bus trigger (*TRG), or by an external
trigger (:TRIG:SOUR EXT).
• It is confirmed that the reading is completed, either by the setting of the RAV bit in the
status model, or by allowing sufficient time to pass for the reading to complete.
:READ?
What it does
This command performs three actions. It will reset the trigger model to the idle layer
(equivalent to the :ABORt command), take the trigger model out of idle (equivalent to the :INIT
command), and return a reading (equivalent to a “FETCh?” query). This command will always
return a new reading, since aborting the trigger model will invalidate any old readings and
trigger a new one. This query will “wait” for a new reading to become available before the
instrument sends a result back.
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