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Introduction to Programming - 2
17
Introduction to SCPI
SCPI (Standard Commands for Programmable Instruments) is a programming language for controlling
instrument functions over the GPIB. SCPI is layered on top of the hardware-portion of IEEE 488.2. The
same SCPI commands and parameters control the same functions in different classes of instruments. For
example, you would use the same DISPlay command to control the ac source display and the display of a
SCPI-compatible multimeter.
Conventions Used in This Guide
Angle brackets < > Items within angle brackets are parameter abbreviations. For example,
<NR1> indicates a specific form of numerical data.
Vertical bar | Vertical bars separate alternative parameters. For example, NORM | TEXT
indicates that either "TEXT" or "NORM" can be used as a parameter.
Square Brackets [ ] Items within square brackets are optional. The representation
[SOURce:]LIST means that SOURce: may be omitted.
Braces { } Braces indicate parameters that may be repeated zero or more times. It is
used especially for showing arrays. The notation <A>{<,B>} shows that
parameter "A" must be entered, while parameter "B" may be omitted or
may be entered one or more times.
Computer font Computer font is used to show program lines in text. TRIGger:DELay .5
shows a program line.
Types of SCPI Commands
SCPI has two types of commands, common and subsystem.
u Common commands generally are not related to specific operation but to controlling overall ac
source functions, such as reset, status, and synchronization. All common commands consist of a
three-letter mnemonic preceded by an asterisk:*RST*IDN?*SRE 8
u Subsystem commands perform specific ac source functions. They are organized into an inverted
tree structure with the "root" at the top. Some are single commands while others are grouped
within specific subsystems.
Refer to appendix A for the ac source SCPI tree structure.
Types of SCPI Messages
There are two types of SCPI messages, program and response.
u A program message consists of one or more properly formatted SCPI commands sent from the
controller to the ac source. The message, which may be sent at any time, requests the ac source
to perform some action.
u A response message consists of data in a specific SCPI format sent from the ac source to the
controller. The ac source sends the message only when commanded by a program message
called a "query."