SME Structure and Syntax of the Messages
1038.6002.02 3.5 E-13
3.4.2 Device Messages (Commands and Device Responses)
Device messages are transferred on the data lines of the IEC bus, the "ATN" control line not being
active. ASCII code is used. The device messages are largely identical for the two interfaces (IEC bus
and RS232) .
A distinction is made according to the direction in which they are sent on the IEC bus:
– Commands are messages the controller sends to the instrument. They operate the device
functions and request information.
The commands are subdivided according to two criteria:
1. According to the effect they have on the instrument:
Setting commands cause instrument settings such as reset of the
instrument or setting the output level to 1 volt.
Queries cause data to be provided for output on the IEC-bus,
e.g. for identification of the device or polling the active
input.
2. According to their definition in standard IEEE 488.2:
Common Commands are exactly defined as to their function and notation in
standard IEEE 488.2. They refer to functions such as
management of the standardized status registers,
reset and selftest.
Device-specific commands refer to functions depending on the features of the
instrument such as frequency setting. A majority of
these commands has also been standardized by the
SCPI committee (cf. Section 3.5.1).
– Device responses are messages the instrument sends to the controller after a query. They can
contain measurement results, instrument settings and information on the
instrument status (cf. Section 3.5.4).
Structure and syntax of the device messages are described in Section 3.5. The commands are listed
and explained in detail in Section 3.6.
3.5 Structure and Syntax of the Device Messages
3.5.1 SCPI Introduction
SCPI (Standard Commands for Programmable Instruments) describes a standard command set for
programming instruments, irrespective of the type of instrument or manufacturer. The goal of the SCPI
consortium is to standardize the device-specific commands to a large extent. For this purpose, a model
was developed which defines the same functions inside a device or for different devices. Command
systems were generated which are assigned to these functions. Thus it is possible to address the same
functions with identical commands. The command systems are of a hierarchical structure. Fig. 3-1
illustrates this tree structure using a section of command system SOURce, which operates the signal
sources of the devices. The other examples concerning syntax and structure of the commands are
derived from this command system.
SCPI is based on standard IEEE 488.2, i.e. it uses the same syntactic basic elements as well as the
common commands defined in this standard. Part of the syntax of the device responses is defined with
greater restrictions than in standard IEEE 488.2 (see Section 3.5.4, Responses to Queries).
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