11-20 Status Structure
Model 6220/6221 Reference Manual
Queues
The Model 622x uses two queues that are first-in, first-out (FIFO) registers:
•
Output queue — Used to hold reading and response messages.
•
Error queue — Used to hold error and status messages.
The Model 622x status model (
) shows how the two queues are struc-
tured with the other registers.
Output queue
The output queue holds data that pertains to the normal operation of the instru-
ment. For example, when a query command is sent, the response message is
placed in the output queue.
When data is placed in the output queue, the message available (MAV) bit in the
status byte register sets. A data message is cleared from the output queue when it
is read. The output queue is considered cleared when it is empty. An empty output
queue clears the MAV bit in the status byte register.
A message is read from the output queue by addressing the Model 622x to talk
after the appropriate query is sent.
Error queue
The error queue holds error and status messages. When an error or status event
occurs, a message that defines the error/status is placed in the error queue.
When a message is placed in the error queue, the error available (EAV) bit in the
status byte register is set. An error/status message is cleared from the error
queue when it is read. The error queue is considered cleared when it is empty. An
empty error queue clears the EAV bit in the status byte register.
The error queue holds up to 10 error/status messages. The commands to read
the error queue are listed in
. When you read a single message in the
error queue, the “oldest” message is read and then removed from the queue. If the
queue becomes full, the message “350, ‘queue overflow’” will occupy the last
memory location. On power-up, the error queue is empty. When empty, the mes-
sage “0, No Error” is placed in the queue.
Messages in the error queue are preceded by a code number. Negative (-) num-
bers are used for SCPI defined messages and positive (+) numbers are used for
Keithley defined messages. The messages are listed in
.
On power-up, all error messages are enabled and will go into the error queue as
they occur. Status messages are not enabled and will not go into the queue. As
listed in
, there are commands to enable and/or disable messages. For
these commands, the <list> parameter is used to specify which messages to
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