EXFO FTB-500 - Manual (Page 354)

Data Types
346 FTB-500
Applicable Data Types for Output —IEEE 488.2

This data type is used by a device to return blocks of 8-bit binary
information with a fixed and predetermined length.
The actual length of the retrieved data 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 know the total length.
For example, if you receive this response (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 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.
Examples: #14,
#3004
where “” stands for data byte
#
digit>
<8-bit data
byte>