Introduction to Programming - 6
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Using Queries
Observe the following precautions with queries:
♦ Set up the proper number of variables for the returned data.
♦ Read back all the results of a query before sending another command to the dc source. Otherwise a
Query Interrupted error will occur and the unreturned data will be lost.
Types of SCPI Messages
There are two types of SCPI messages, program and response.
♦ A program message consists of one or more properly formatted SCPI commands sent from the
controller to the dc source. The message, which may be sent at any time, requests the dc source to
perform some action.
♦ A response message consists of data in a specific SCPI format sent from the dc source to the
controller. The dc source sends the message only when commanded by a program message "query."
The following figure illustrates SCPI message structure:
Data
Ke
y
words
Ke
y
word Separator
Messa
g
e Unit Separators
Messa
g
e Unit
Quer
y
Indicator
Message Terminator
Root Specifier
VOLT <NL>
: LEV 20 PR OT 21
;;:
CURR?
Figure 6-2. Command Message Structure
The Message Unit
The simplest SCPI command is a single message unit consisting of a command header (or keyword)
followed by a message terminator. The message unit may include a parameter after the header. The
parameter can be numeric or a string.
ABORt<NL>
VOLTage 20<NL>
Headers
Headers, also referred to as keywords, are instructions recognized by the dc source. Headers may be
either in the long form or the short form. In the long form, the header is completely spelled out, such as
VOLTAGE, STATUS, and DELAY. In the short form, the header has only the first three or four letters,
such as VOLT, STAT, and DEL.