EXFO FTB-500 Guía Del Usario página 380

Data Types
Applicable Data Types for Output —IEEE 488.2
<DEFINITE LENGTH ARBITRARY BLOCK RESPONSE DATA>

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:


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<DAB><DAB><DAB><DAB>,
#3004<DAB><DAB><DAB><DAB>
where "<DAB>" stands for data byte
372
<non-zero
digit>
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.
<digit>
<8-bit data
byte>
FTB-500