EXFO FTB-500 (Window XP) - Manual (Page 369)

Data Types
FTB-500 361
Applicable Data Types for Input—IEEE 488.2

<ARBITRARY BLOCK PROGRAM DATA>
This data type is used to send blocks of arbitrary 8-bit information
when you need to work with large amounts of data.
The actual length of the data that you send has the following structure:

The first byte contains the # character.

The byte that immediately follows contains the number of
subsequent bytes that you have to check to obtain the total length.
Note: If you use a zero as the first digit (#0), it has to be followed by a
<PROGRAM MESSAGE TERMINATOR > so that the device will detect the
end of the <ARBITRARY BLOCK PROGRAM DATA>. This will also force
immediate termination of the message.
For example, if you send the following data (here, values are expressed
in decimal instead of binary for easier readability):
# 2 1 3 7 6 8 9 2 ...
The byte that immediately follows the # contains 2, which means that
you would have to read the two following bytes to know the length (in
bytes) of the retrieved data. The bytes indicate 1 and 3. The length will
then be 13 bytes. The actual response will begin at byte number 5, in
this case.
#
<non-zero
digit>
<digit>
<8-bit data
byte>
0
<8-bit data
byte>
<PROGRAM MESSAGE
TERMINATOR>