EXFO FTB-1v2 Guide De L'utilisateur page 402

Data Types
Applicable Data Types for Output —IEEE 488.2
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.
390
Value is
Under range 2143289345.000000
Over range 2143289346.000000
Invalid 2143289347.000000
Inactive 2143289348.000000
Value is
Under range 9221120237577961472 7FF8000020000000
Over range 9221120238114832384 7FF8000040000000
Invalid 9221120238651703296 7FF8000060000000
Inactive 9221120239188574208 7FF8000080000000
ASCII 4 bytes
ASCII 8 bytes
PACKED 4 bytes
7FC00001
7FC00002
7FC00003
7FC00004
PACKED 8 bytes
FTB-1v2 and FTB-1v2 Pro