5.2
sePI
PRIMER
5-2 Remote Operation
SCPI (Standard Commands for Programmable Instruments) traces its lineage to IEEE
488.1 and IEEE 488.2. Although the IEEE 488.2 standard addressed sorne instrument
measurements,
it
principally dealt with common commands and syntax
or
data formats.
Please refer to the IEEE 488.2 and SCPIreference manuals for more information.
The IEEE 488.1 relates to the physical connection between the instrument and its remote
control unit; how the
data
is transmitted between the two;
and
the method used to
determine master and slave.
The IEEE 488.2 encompassed and built upon IEEE 488.1
byadding
syntax and data
requirements for the communication path.
In
addition, it also defined the commands
which were to be common to any and aIl units, and the query format for data retrieval
from the remote instrument.
With
SC:PT,
Il
vlltif\fy
of
moônillf instl1Jments have a universallanguage. even though
they have different functions and manufacturers. SCPI added
the
fine detail
of
instrument setup by establishing a hierarchy
of
standard command formats and subsystem
routing, reducing multiple ways to control similar functions. An exarnple
of
vertical
consistency (same instrument type) would be multimeters from different manufacturers
implementing the command
ta
measure a value
of
OC voltage in the sarne mannerusing
the
Measure sub-system.
An
example
of
horizontal consistency (different instrument
types) would be different instruments using the sarne command to trigger a function
using the
Trigger subsystem. Refer to Figure 5-1. Queries
of
the instrument by the
controller result in weIl-defined status response and measurement data. By building on
the IEEE 488.2, all
of
the earlier commands that
it
had
defined have become a part
of
sePI
and, to the limit that an instrument can be operated by them, they are valid.
T~
VERTICAL
Il
0000
11
COOSI
I
~I
,
...
<E--------
HORIZONTAL
--------
CONSISTENCY
Figure 5-1. Command Consistency
The user need not
be
concerned about the interface commands, the common commands
or
the syntax and datastructures, they are IEEE 488.1 and IEEE 488.2 definitions and
have
not changed; they are encompassed within SCPI.
Not
all instruments use
an
commands, but aIl instruments use the same command format.
Using the language rules and the hierarchical nature
of
the
command
structure, new
commands, pararneters, and subsystems can be developed from the existing primitive
elements and commands as new instruments are introduced.
The hierarchy
of
subsystem commands in SCPI is called a Command Tree(sometimes
also called
aCommand Flow
Chart).
The SCPI Command Tree is up-side-down, the
Root
is
at the top
with
branches extending downward,
ending
with
the
parameter
required for the branch function. Refer to Figure 5-2.
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