EXFO FTB-2 - Manual (Page 257)

Using Your Unit and Modules in an Automated Test Environment
FTB-2 and FTB-2 Pro 247
Writing Remote Control Code
When you write code, you must follow these rules on message reception
and transmission:

The controller must have sent a complete message to the instrument
(including the message terminator) before retrieving a response.

The controller must retrieve all the responses from previous queries
(including the response terminator) before sending a new message to
an instrument.

The controller must not try to retrieve a response from an instrument if
the corresponding query has not been previously sent to the
instrument.

You must pay special attention to queries that return an indefinite ASCII
response. To avoid any confusion, the IEEE 488.2 standard requires that
this data type be immediately followed by a response termination
character. For this reason, when working with compound queries, you
must ensure that a query sending an indefinite ASCII response is the
last query of the series.

Be careful when sending program messages containing multiple
queries that return large amounts of data. Since the controller can only
retrieve data when the instrument has finished processing the queries,
it could result in problems ranging from a saturation of the output
queue to the complete blocking of the whole system.