Keithley 6430, Instruction Manual

Page: 297 / 558

Status Structure 14-19
Queues
The SourceMeter uses two queues, which are fi rst-in, fi rst-out (FIFO) registers:
•
Output Queue
— Used to hold reading and response messages.
• Error Queue — Used to hold error and status messages. (See Appendix B.)
The SourceMeter status model (Figure 14-1) shows how the two queues are structured with
the other registers.
Output queue
The output queue holds data that pertains to the normal operation of the instrument. 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 Out-
put 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 SourceMeter 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 de fi nes 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 Table 14-9. When you read a single message in the Error Queue, the “ old-
est ” message is read and then removed from the queue. If the queue becomes full, the message
“ 350, ‘ Queue Over fl ow ’” will occupy the last memory location. On power-up, the Error Queue
is empty. When empty, the message “ 0, No Error ” is placed in the queue.
Messages in the Error Queue are preceded by a code number. Negative (-) numbers are used
for SCPI-de fi ned messages, and positive (+) numbers are used for Keithley-de fi ned messages.
The messages are listed in Appendix B. As shown in Table 14-7, there are commands to read
the entire message (code and message) or the code only.