B&K Precision 4078 - Manual - Manual (Page 42)

4.5 Message Exchange Protocol
The device decodes messages using the Message Exchange Protocol (MEP) defined in IEEE 488.2. The following
functions implemented in the MEP must be considered:
4.5.1 The Input Buffer
The device has a 256-byte long cyclic input buffer. Decoding of remote messages begins 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 is not
completed until room has been made for more bytes in the buffer. This prevents a fast controller from overrunning
the device with data.
If the user has sent part of a Program Message, but not the Program Message Terminator, and he wishes to abort the
message decoding and execution, the Device Clear command may be sent, or front panel operation resumed (in
REMS only).
4.5.2 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 are
not enough places in the queue for the new message, the device will put off putting the message in the queue until
there is place for it.
The Status Byte MAV bit, when set, indicates that part or all of a response message is ready to be read.
4.5.3 Response Messages
The device sends a Response Message in response to a valid query. All queries return a single Response Message
Unit. In only one case is the Response Message generated when the response is read (as opposed to when the
response is parsed), and this is when querying Arbitrary Waveform data. All other queries generate the Response
Message when they are parsed.
4.5.4 Coupled Commands
Coupled Commands are either commands whose execution validity depends on the value of other parameters, or
commands whose execution changes the value of another parameter. The execution of commands designated as
being coupled is deferred until all other commands in the same Program Message have been executed. The coupled
commands are then grouped together according to their functionality, and executed as a group. These groups of
coupled commands are defined in the MODEL 4075 and 4078:
a) The commands to set the amplitude, the offset, and to switch the output on. The output being switched on is
included here in order to prevent possible damage to the equipment being driven as a result of the amplitude and
offset not being executed as intended by the user, due to an execution error.
b) The commands to set the function, frequency, the point rate, the wavelength, and the waveform start address. In
ARB function, setting the frequency or the point rate causes the other to change, keeping the wavelength constant (if
it, too, is not specified in the same program message). If the wavelength is specified as well, the frequency or point
rate must change in accordance with the new value. The validity of the start address is a function of the wavelength.
Please refer to the individual commands for more detail.
The maximum frequency is also dependent upon the waveform, so that changing the waveform may render the