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Initialization and
Resetting
Reset Strategy
There are three levels of initialization:
–
Bus initialization
–
Message exchange initialization
–
Device initialization
n
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 lan-
guage dependent. Refer to the user man-
ual of the system controller in use.
n
Message Exchange Initialization
Device clear is the second level of initial
-
ization. It initializes the bus message ex
-
change, 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 instru
-
ments. The instrument action on receiv
-
ing 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 compati
-
ble returned to the power-on de
-
fault 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 instru-
ment. The instrument will then discard
pending commands and will clear re-
sponses from the output queue. For ex-
ample; suppose you are using the Counter
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 re
-
sponses and commands could influence
later commands and queries.)
n
Device Initialization
The third level of initialization is on the
device level. This means that it concerns
only the addressed instruments.
Introduction to SCPI
Initialization and Resetting 3-19