EXFO FTB-500 (Window XP) - Manual (Page 376)

Data Types
368 FTB-500
Applicable Data Types for Output —IEEE 488.2
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Special Numeric Values Received on Output
In some cases, an instrument may send values indicating that an
unusual event has occurred. The following tables present the possible
values.
Value is ASCII 4 bytes PACKED 4 bytes
Under range 2143289345.000000 7FC00001
Over range 2143289346.000000 7FC00002
Invalid 2143289347.000000 7FC00003
Inactive 2143289348.000000 7FC00004
Value is ASCII 8 bytes PACKED 8 bytes
Under range 9221120237577961472 7FF8000020000000
Over range 9221120238114832384 7FF8000040000000
Invalid 9221120238651703296 7FF8000060000000
Inactive 9221120239188574208 7FF8000080000000