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State Purpose
IDLE Wait for messages
READ Read and execute mes
-
sages
QUERY Store responses to be
sent
SEND Send responses
RE
-
SPONSE
Complete sending re
-
sponses
DONE Finished sending re
-
sponses
DEADLOCK The device cannot buffer
more data
Action, Reason
Untermin-
ated,
The controller attempts to
read the device without
first having sent a com-
plete query message
Interrupted, The device is interrupted
by a new program mes-
sage before it finishes
sending a response mes-
sage
Protocol Requirements
In addition to the above functional ele
-
ments, which process the data, the mes
-
sage exchange protocol has the following
characteristics:
–
The controller must end a program mes
-
sage containing a query with a message
terminator before reading the response
from the device (address the device as
talker). If the controller breaks this rule,
the device will report a query error
(unterminated action).
–
The controller must read the response to a
query in a previously (terminated) program
message before sending a new program
message. When the controller violates this
rule, the device will report a query error
(interrupted action).
–
The instrument sends only one response
message for each query message. If the
query message resulted in more than one
answer, all answers will be sent in one re
-
sponse message.
n
Order of Execution
Deferred Commands
Execution control collects commands un
-
til the end of the message, or until it finds
a query or other special command that
forces execution. It then checks that the
setting resulting from the commands is a
valid one: No range limits are exceeded,
no coupled parameters are in conflict, etc.
If this is the case, the commands are exe-
cuted in the sequence they have been re-
ceived; otherwise, an execution error is
generated, and the commands are dis-
carded.
This deferred execution guarantees the
following:
–
All valid commands received before a
query are executed before the query is exe
-
cuted.
–
All queries are executed in the order they
are received.
–
The order of execution of commands is never
reversed.
n
Sequential and Overlapped
Commands
There are two classes of commands: se
-
quential and overlapped commands. All
commands in the CNT-8X counters are
sequential, that is one command finishes
before the next command executes.
Introduction to SCPI
How does SCPI Work in the Instrument? 3-5