The *OPC? command outputs a “1” after all previous commands are completed (see “Step 8:
Check for Command Synchronization” for more information on the command).
Step 3: Query the Instruments for Errors
All instruments that follow the SCPI definition have an Error Queue. Each error that occurs is stored into
the queue. To read the queue, send the command:
SYSTem:ERRor?
the returned message consists of an error number and message. If no errors are in the queue, the command
returns:
+ 0,“No error”
This command is a very useful command since it can determine if the correct SCPI command has been sent
to an instrument and that the parameters sent with the command are correct. If, for example, an incorrect
command was sent, an error message is stored in the error queue after the command is parsed. Since the
incorrect command may not otherwise indicate an error condition, the only way to find out is to query the
Error Queue. The following shows how to continuously read the Error Queue until no more errors are
stored using IBASIC:
REPEA T ! Start a continuous loop
OUTPUT 80900;"SYST:ERR?" ! Send query command
ENTER 80900;A,A$ ! Enter returned data
PRINT A;A$ ! Print error number and message
UNTIL A= 0 ! Loop until no errors are returned
Step 4: Query All Command Parameter Settings that are Set
If there is an error in the Error Queue, you must find out which command caused the error. A command
with parameters that are incorrect will not be executed, but will generate an error in the queue. Reading the
queue may not tell you which parameter caused the error. To do this, query the command itself to determine
which parameters have been set. When the queried setting is different than what it is intended to be, the
error is then located. For example, the following shows a typical command and the query form of the
command:
:SOUR:CLOSE (@100) command to close a channel
:SOUR:CLOSE? (@100) command to query a channel
SCPI commands have levels similar to a computer path name. A colon separates the different levels. The
spelling in each level must be correct or the instrument generates an error. Note that each command also has
an abbreviated version. However, the command can only be sent as the full command or the abbreviated
version. The abbreviated version are normally shown as upper case letters in the instrument manuals. All
commands can be sent either using upper or lower case letters or a combination thereof.
Some common command errors are as follows:
C-4 Debugging VXI SCPI Programs
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