52 Remote Programming
The following figure illustrates the SCPI message structure:
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.
ABOR<NL>
VOLT 20<NL>
Headers
Headers (which are sometimes known as "keywords") are instructions recognized by the programming interface. Headers
may be either in the long form or the short form.
Long Form The header is completely spelled out, such as VOLTAGE STATUS DELAY.
Short Form The header has only the first three or four letters, such as VOLT STAT DEL.
Query Indicator
Following a header with a question mark turns it into a query (VOLT? VOLT:PROT?). If a query contains a parameter,
place the query indicator at the end of the last header (VOLT:PROT? MAX).
Message Unit Separator
When two or more message units are combined into a compound message, separate the units with a semicolon
(STATus:OPERation?;QUEStionable?).
Root Specifier
When it precedes the first header of a message unit, the colon becomes a "root specifier". This indicates that the command
path is at the root or top node of the command tree.
Message Terminator
A terminator informs SCPI that it has reached the end of a message. Three permitted messages terminators are:
■ Newline (<NL>), which is ASCII decimal 10 or hex 0A.
■ End or identify (<END>)
■ Both of the above (<NL><END>)
In the examples of this manual, there is an assumed message terminator at the end of each message. If the terminator needs
to be shown, it is indicated as <NL> regardless of the actual terminator character.