Keysight E3632A - Using *OPC to Signal When Data Is in the Output Buffer

3 Remote Interface Reference 
SCPI Status Registers
116 E3632A User’s Guide
Using *OPC to signal when data is in the output buffer
Generally, it is best to use the “Operation Complete” bit 
(bit 0) in the Standard Event register to signal when a
command sequence is completed. This bit is set in the
register after an *OPC command has been executed. If you
send *OPC after a command which loads a message in the
power supply’s output buffer (query data), you can use the
“Operation Complete” bit to determine when the message is
available. However, if too many messages are generated
before the *OPC command executes (sequentially), the output
buffer will overload and the power supply will stop
processing commands.