SCPI Introduction
Copyright © Itech Electronic Co., Ltd. 21
Message exchange protocol
Two rules summarize the message exchange protocol:
Rule 1. You must always tell the Model 2000 what to send to the computer.
The following two steps must always be performed to send information from the
instrument other computer:
1. Send the appropriate query command(s) in a program message.
2. Address the Model 2000 to talk.
Rule 2. The complete response message must be received by the
computer before another program message can be sent to the Model 2000.
2.6 SCPI Command Completion
SCPI commands sent to the electronic load are processed either sequentially
or in parallel. Sequential commands finish execution before a subsequent
command begins. Parallel commands allow other commands to begin
executing while the parallel command is still executing. Commands that affect
trigger actions are among the parallel commands.
The *WAI, *OPC, and *OPC? common commands provide different ways of
indicating when all transmitted commands, including any parallel ones, have
completed their operations. The syntax and
parameters for these commands are
described in chapter 4. Some practical considerations for using these commands are
as follows:
*WAI-This prevents the electronic load from processing subsequent commands
until all pending operations are completed.
*OPC?-This places a 1 in the Output Queue when all pending operations have
completed. Because it requires your program to read the returned value before
executing the next program statement, *OPC? can be used to cause the
controller to wait for commands to complete before proceeding with its
program.
*OPC-This sets the OPC status bit when all pending operations have
completed. Since your program can read this status bit on an interrupt basis,
*OPC allows subsequent commands to be executed.
NOTE: The trigger system must be in the Idle state in order for the status OPC
bit to be true. Therefore, as far as triggers are concerned, OPC is false
whenever the trigger system is in the Initiated state.
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