Agilent Technologies 34420A User Manual page 186

Chapter 4 Remote Interface Reference
An Introduction to the SCPI Language
Using " ? " Commands The bus controller may send commands at any
time, but a instrument may only send responses when specifically
SCPI
instructed to do so. Only query commands (commands that end with
a "?") will instruct the instrument to send a response message. Queries
return either measured values or internal instrument settings.
C a u t i o n If you send two query commands without reading the response from the
first, and then attempt to read the second response, you may receive some
data from the first response followed by the complete second response. To
avoid this, do not send a query command without reading the response.
When you cannot avoid this situation, send a device clear before sending
the second query command.
Using " * " Commands Commands starting with a " * " are called
common commands. They are required to perform the identical function
for all instruments that are compliant with the interface
IEEE-488.2
standard. The " * " commands are used to control reset, self-test, and
status operations in the meter.
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