Fluke PM6681R Outdoor Timer User Manual


 
How does SCPI
Work in the
Instrument?
The functions inside an instrument that
control the operation provide SCPI com
-
patibility. Figure 3-3 shows a simplified
logical model of the message flow inside
a SCPI instrument.
When the controller sends a message to a
SCPI instrument, roughly the following
happens:
The GPIB controller addresses the instru
-
ment as listener.
The GPIB interface function places the
message in the Input Buffer.
The Parser fetches the message from the
Input Buffer, parses (decodes) the message,
and checks for the correct syntax. The in-
strument reports incorrect syntax by send-
ing command errors via the status system
to the controller. Moreover, the parser will
detect if the controller requires a response.
This is the case when the input message is
a query (command with a “?” appended).
The Parser will transfer the executable
messages to the Execution Control block
in token form (internal codes). The Exe
-
cution Control block will gather the infor
-
mation required for a device action and
will initiate the requested task at the ap
-
propriate time. The instrument reports ex
-
ecution errors via the status system over
the GPIB and places them in the Error
Queue.
When the controller addresses the instru
-
ment as talker, the instrument takes data
from the Output Queue and sends it over
the GPIB to the controller.
Message Exchange Control
protocol
Another important function is the Mes
-
sage Exchange Control, defined by
IEEE 488.2. The Message Exchange
Control protocol specifies the interactions
between the several functional elements
that exist between the GPIB functions
and the device-specific functions, see
Figure 3-3 .
The Message Exchange Control protocol
specifies how the instrument and control
-
ler should exchange messages. For exam
-
ple, it specifies exactly how an
instrument shall handle program and re
-
sponse messages that it receives from and
returns to a controller.
This protocol introduces the idea of com-
mands and queries; queries are program
messages that require the device to send a
response. When the controller does not
read this response, the device will gener-
ate a Query Error. On the other hand,
commands will not cause the device to
generate a response. When the controller
tries to read a response anyway, the de-
vice then generates a Query Error.
The Message Exchange Control protocol
also deals with the order of execution of
program messages. It defines how to re
-
spond if Command Errors, Query Errors,
Execution Errors, and Device-Specific er
-
rors occur. The protocol demands that the
instrument report any violation of the
IEEE-488.2 rules to the controller, even
when it is the controller that violates
these rules.
The IEEE 488.2 standard defines a set of
operational states and actions to imple
-
ment the message exchange protocol.
These are shown in the following table:
3-4 How does SCPI Work in the Instrument?
Introduction to SCPI