Stanford Research Systems RGA100, Operating Manual And Programming Reference

Page: 297 / 362

12 Appendix D
SRS Residual Gas Analyzer
Note that this block diagram is very similar to the Simple Table block diagram. Again, only six VIs are
used. All the VIs in gray are taken from the operations layer, while the only one that is not is the general
SRSRGA Error handler VI. Just as in the communications layer example, you can see that an error
cluster is propagated through the six VIs. However, rather than propagating the SRSRGA VISA session
explicitly, the session is propagated by referring to its reference number (or refnum). By double-clicking
on the sub-VIs that appear to the right of the Open VI, you will see that each VI takes the refnum as input,
then passes it to SRSRGAo Get Session. This VI accesses the serial communications session referred to
by the SRSRGA refnum, and sends commands or receives data from that session. By using the
operations level, you don’’t have to worry about dealing with the VISA session directly; the VIs in this
layer will handle that for you.
In this VI, an array of all the points in the scan is created beforehand and each point initialized as ““not a
number””. This is different from the previous example of Simple Table scanning, where the points are
simply plotted as they are read in. In Simple Analog (and Histogram) the initialized array changes in the
ordinate as new values are read in, but the abscissa (the mass range) always stays the same. Also, rather
than using a multiple number of milliseconds in the while loop, a fixed delay of 250 milliseconds is used.
Very tight while loops can put unnecessary stress on a multi-threaded application or operating system.
This slight delay helps guard against problems stemming from having no delay in the execution of the
while loop.
After taking your input from the specifications cluster and opening an SRS RGA session, the Start Analog
VI sets the RGA parameters you request, performs a calibration, calculates the number of points that will
be coming from the RGA, and triggers a scan. Within the while loop, the Measure Pressures VI formats
the binary data into an array of pressure readings. This array is plotted in the graph that is visible on the
front panel. The condition for the while loop is simply that the user has not stopped the scan and that no
errors are present. If the while loop ends, the mass filtering is turned off, as is the electron multiplier and
the filament. Any errors are reported at this time.