Fluke PM6681R, PM6685R manual Protocol Requirements, Order of Execution, Deferred Commands

Introduction to SCPI

State	Purpose
IDLE	Wait for messages
READ	Read and execute mes-
	sages
QUERY	Store responses to be
	sent
SEND	Send responses
RE-	Complete sending re-
SPONSE	sponses
DONE	Finished sending re-
	sponses
DEADLOCK	The device cannot buffer
	more data

Action,	Reason
Untermin-	The controller attempts to
ated,	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.

￿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.

￿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.

How does SCPI Work in the Instrument? 3-5