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 1 (DAV) 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. The first bit is set to 0 between calculation and updating of measured data, and is set to 1 when data updating is complete. The update period is the timing in which value switches from 0 to 1. 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 1 of the status register changes from 0 to 1) bit 1 of the extended event register is set to 1. Send the "STATUS:FILTER2 RISE" communications command, and set FILTER2 (the filter corresponding to bit 1 of the status register) so that when bit 1 of the status register "RISES" from 0 to 1, bit 1 of the extended event register is set. Bit 1 is the status register's data update bit, but the filter uses bit 2. Note that the filter's bits are numbered starting from 1 instead of 0.
This setting makes it so that bit 1 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 1 is set to 1, this does not indicate at what time the update occurred. Send a dummy "STATUS:EESR?" command, and clear the contents of the extended event register. Then issue repeated "STATUS:EESR?" commands, confirm the changing of the returned value's bit 1 to 1. Then if you send the NUMERIC:NORMAL:VALUE?" command, the data is not read out until it is updated.
For information on the extended event register, status register, and transition register please refer to the PZ4000 Power Analyzer Communications Interface Manual IM 253710-11E, Page 5-4.