Introduction to Programming - 6

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

Keywords

Message Unit

Query Indicator

VOLT : LEV 20 ; PROT 21 ; : CURR? <NL>

Keyword Separator

Message Unit Separators

Message Terminator

Root Specifier

),*￿￿￿￿*$/

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.

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Agilent Technologies 66111A, D, 66311B, 66309B manual Types of Scpi Messages, Using Queries, Message Unit, Headers

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