B&K Precision 4040B - Manual - Manual (Page 50)

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4.3 Message Exchange Protocol
The device decodes messages using the Message
Exchange Protocol similar to the one defined in IEEE
488.2. The following functions implemented in the MEP
must be considered:
4.3.1 The Input Buffer
The device has a 128-byte long cyclic input buffer.
Decoding of remote messages is begun as soon as the
input buffer is not empty, that is, as soon as the controller
has sent at least one byte to the device. Should the input
buffer be filled up by the controller faster than the device
can remove the bytes and decode them, the bus
handshake (CTS/RTS) is used to pause data transfer until
room has been made for more bytes in the buffer. This
prevents the controller from overrunning the device with
data.
4.3.2 The Output Queue
The device has a 100-byte long output queue in which it
stores response messages for the controller to read. If at
the time a response message is formatted the queue
contains previously formatted response messages, such
that there is not enough places in the queue for the new
message, the device will hold off putting the message in
the queue until there is place for it.