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

You can determine whether data was updated by performing a serial poll and referencing the status byte.

Bit 0 of the status byte (D101) changes to 1 when data is updated. When bit 0 changes to 1, bit 6 (D107) also changes to 1.
Also, bit 0 and 6 are reset when a serial poll is performed.
You can judge the data update status by referencing bit 0 and bit 6.

However, even if you suddenly read the status byte and determine that bit 0 and bit 6 are set to 1, this does not indicate at what time the update occurred. Therefore, perform a dummy serial poll, and clear the contents of the extended event register. Then perform repeated serial polls, and confirm the changing of the returned value's bit 0 and bit 6 to 1. Then if you send the "OD" command, the data is not read out until updated.

For details on the status byte, see the following:
 

• WT1010 Digital Power Meter User's Manual
  Page App1-13
• WT1030/1030M Digital Power Meter User's Manual
  Page App1-15
• WT2010 Digital Power Meter User's Manual
  Page App1-17
• WT2030 Digital Power Meter User's Manual
  Page App1-17
• WT110E/WT130 Digital Power Meter User's Manual
  Page 11-3
• WT200 Digital Power Meter User's Manual
  Page 10-4
• WT210/230M Digital Power Meter User's Manual
  Page 10-4