Using Queries; Types Of Scpi Messages; The Message Unit; Headers - Keysight Technologies N3301A Programming Manual

Using Queries

Observe the following precautions with queries:
♦ Set up the proper number of variables for the returned data. For example, if you are reading back
a measurement array, you must dimension the array according to the number of measurements
that you have placed in the measurement buffer.
♦ Read back all the results of a query before sending another command to the electronic load.
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 electronic load. The message, which may be sent at any time, requests the
electronic load to perform some action.
♦ A response message consists of data in a specific SCPI format sent from the electronic load to
the controller. The electronic load sends the message only when commanded by a program
message called a "query."
The following figure illustrates SCPI message structure:

Headers

Header Separator

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 electronic load. 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.
Data Message Unit
VOLT:LEV 20 ;
Message Unit Separators
Figure 2-2. Command Message Structure
Query Indicator
TLEV 30 ; : CURR?
Message Terminator
Root Specifier
Introduction to Programming - 2
<NL>
19