Fluke 9640A Series, Instruction Manual

Page: 125 / 324

SCPI and IEEE Bus Descriptions
Initialization and Resetting
4B
4B-13
Initialization and Resetting
Reset Strategy
There are three levels of initialization:
•
Bus initialization
•
Message exchange initialization
•
Device initialization
Bus Initialization
This is the first level of initialization. The controller program should start with this, which
initializes the IEEE-interfaces of all connected instruments. It puts the complete system
into remote enable (REN-line active) and the controller sends the interface clear (IFC)
command. The command or the command sequence for this initialization is controller
and language dependent. Refer to the user manual of the system controller in use.
Message Exchange Initialization
Device clear is the second level of initialization. It initializes the bus message exchange,
but does not affect the device functions.
Device clear can be signaled either with DCL to all instruments or SDC (Selective
device-clear) only to the addressed instruments. The instrument action on receiving DCL
and SDC is identical, they will do the following:
•
Clear the input buffer.
•
Clear the output queue.
•
Reset the parser.
•
Clear any pending commands
The device-clear commands will not do the following:
•
Change the instrument settings or stored data in the instrument.
•
Interrupt or affect any device operation in progress.
•
Change the status byte register other than clearing the MAV bit as a result of clearing
the output queue.
Many older IEEE-instruments that are not IEEE-488.2 compatible returned to the power-
on default settings when receiving a device-clear command. IEEE-488.2 does not allow
this.
When to use a Device-clear Command
The command is useful to escape from erroneous conditions without having to alter the
current settings of the instrument. The instrument will then discard pending commands
and will clear responses from the output queue. For example, suppose you are using the
Instrument in an automated test equipment system where the controller program returns
to its main loop on any error condition in the system or the tested unit. To ensure that no
unread query response remains in the output queue and that no unparsed message is in the
input buffer, it is wise to use device-clear. (Such remaining responses and commands
could influence later commands and queries.)