Technical data

16 Programmer’s Guide
2 Programming Fundamentals
Program and Response Messages
To understand how your instrument and controller communicate using
SCPI, you must understand the concepts of program and response
messages.
Program Messages
Program messages are the formatted data sent from the controller to the
instrument. Conversely, response messages are formatted data sent from
the instrument to the controller. Program messages contain one or more
commands, and response messages contain one or more responses.
Response Messages
The controller may send commands at any time, but the instrument
sends responses only when query commands is received. All query
mnemonics end with a question mark. Queries return either measured
values or internal instrument settings.
Forgiving Listening and Precise Talking
SCPI uses the concept of forgiving listening and precise talking outlined
in IEEE 488.2.
Forgiving listening means that instruments are very flexible in accepting
various command and parameter formats. For example, the spectrum
analyzer accepts either
:FREQuency:CENTer:STEP:AUTO ON or
:FREQuency:CENTer:STEP:AUTO 1
Precise talking means that the response format for a particular query is
always the same. For example, if you query RF output state when it is
on (using
:FREQuency:CENTer:STEP:AUTO?), the response
is always 1, regardless of if you previously sent
:FREQuency:CENTer:STEP:AUTO ON
or
:FREQuency:CENTer:STEP:AUTO 1.