Fluke 9640A Series, Инструкция по эксплуатации

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9640A
Instruction Manual
4B-6
The IEEE 488.2 standard defines a set of operational states and actions to implement the
message exchange protocol. See Table 4B-1 and Table 4B-2.
Table 4B-1. States for Message Exchange Protocol
State Purpose
IDLE Wait for messages
READ Read and execute messages
QUERY Store responses to be sent
SEND Send responses
RESPONSE Complete sending responses
DONE Finished sending responses
DEADLOCK The device cannot buffer more data
Table 4B-2. Actions for Message Exchange Protocol
Action Reason
Unterminated The controller attempts to read the device without first having sent a complete
query message.
Interrupted The device is interrupted by a new program message before it finishes sending
a response message.
Protocol Requirements
In addition to the above functional elements, which process the data, the message
exchange protocol has the following characteristics:
•
The controller must end a program message 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
response message.
Order of Execution - Deferred Commands
Execution control collects commands until 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 executed in the
sequence they have been received; otherwise, an execution error is generated, and the
commands are discarded.
This deferred execution guarantees the following:
•
All valid commands received before a query are executed before the query is
executed.
•
All queries are executed in the order they are received.
•
The order of execution of commands is never reversed.