How do I Retrieve Data From the Instrument Only When it is Updated Using IEEE488.2 Commands?

You can use the "STATUS:ESSR?" command to access the extended event register and determine whether the data was updated. You can judge the data update status by referencing bit 0 (UPD) of this register. However to do so, you must use the status register and transition filter in addition to the extended event register.

The status register (also called the condition register) is a 16-bit register providing information about the instrument. Bit 0 of this register is set to 1 during measurement and is set to 0 when data updating is complete. The update period is the timing in which value switches from 1 to 0. Changes in each bit of the transition filter and status register are detected and the results are stored in the extended event register.

The transition filter is set so that when data is updated (bit 0 of the status register changes from 1 to 0) bit 0 of the extended event register is set to 1. Send the "STATUS:FILTER1 FALL" communications command, and set FILTER1 (the filter corresponding to bit 0 of the status register) so that when bit 0 of the status register "FALLS" from 1 to 0, bit 0 of the extended event register is set. Bit 0 of the bottom-most bit is the status register's data update bit, but the filter uses bit 1.
Note that the filter's bits are numbered starting from 1 instead of 0.

This setting makes it so that bit 0 of the value returned by the "STATUS:EESR?" command is set to 1 every time the measured value is updated. "STATUS:EESR?" is what is reset, and only when the value of this register is read out.
Therefore, even if you suddenly read the extended event register and determine that bit 0 is set to 1, this does not indicate at what time the update occurred. Therefore, send a dummy "STATUS:EESR?" command, and clear the contents of the extended event register. Then issue repeated "STATUS:EESR?" commands, and confirm the changing of the returned value's bit 0 to 1. Then, data is not read out until updated

For information on the extended event register, status register, and transition register, please refer to:
 

• WT1600 Digital Power Analyzer Communications Interface User's Manual
  Page 5-4
• WT1010 Digital Power Meter User's Manual
  Page App2-47
• WT1030/1030M Digital Power Meter User's Manual
  Page App2-51
• WT2010 Digital Power Meter User's Manual
  Page App2-58
• WT2030 Digital Power Meter User's Manual
  Page App2-59
• WT110E/WT130 Digital Power Meter User's Manual
  Page App2-38
• WT200 Digital Power Meter User's Manual
  Page 14-44
• WT210/230M Digital Power Meter User's Manual
  Page 14-47