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6 - Introduction to Programming
68
SCPI Command Completion
SCPI commands sent to the dc source 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 8. Some practical considerations for using these
commands are as follows:
*WAI
This prevents the dc source 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 subsystem must be in the Idle state for the status OPC bit to be true. As far as
triggers are concerned, OPC is false whenever the trigger subsystem is in the Initiated
state.
Using Device Clear
You can send a device clear at any time abort a SCPI command that may be hanging up the GPIB
interface. The status registers, the error queue, and all configuration states are left unchanged when a
device clear message is received. Device clear performs the following actions:
♦ The input and output buffers of the dc source are cleared.
♦ The dc source is prepared to accept a new command string.
The following statement shows how to send a device clear over the GPIB interface using Agilent BASIC:
CLEAR 705 IEEE-488 Device Clear
The following statement shows how to send a device clear over the GPIB interface using the GPIB
command library for C or QuickBASIC:
IOCLEAR (705)