Stanford Research Systems RGA100, Operating Manual And Programming Reference

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Programming the RGA Head 6-19
SRS Residual Gas Analyzer
x
A Total Pressure measurement is performed at the end of each scan and transmitted
out to the host computer. In the event where the CDEM is turned on, the total
pressure returned from the head is 0.00. (Please see AP and TP Commands).
Setting up a histogram scan
The following list of commands sets a scanning range of 1 to 50 amu and the slowest
scan rate, and, after checking the number of currents to be measured per scan (not
including total pressure), it triggers a single scan under those conditions. During the scan,
50 scan currents and one total pressure current are transmitted to the computer, and each
current is 4 bytes long.
MI1 Initial mass=1 amu.
MF50 Final mass=50 amu
NF0
Slowest scan rate selected.
HP? Histogram Points query. The number 50 is echoed. Add one for
total pressure.
HS1 Histogram Scan trigger: 1 scan is collected and transmitted.
Histogram Scan Programming Tips
x
It is good programming practice to follow each command that sets a parameter with a
query of the parameter setting. For example, the first command in the list should be
followed by a MI?, and a number 1 should be echoed by the RGA.
x
Notice that MI and MF are shared by analog and histogram scans.
x
Any command sent to the RGA during scanning will immediately halt the scanning
action and clear the RGA’’s transmit buffer. Remember to also clear the computer’’s
receive buffer to reset the communications. The new command responsible for
stopping the scan will be executed!
x
It is good practice to perform an analog scan before triggering a large set of
histograms to assure the correct peak tuning (i.e. correct peak locations and widths)
of the quadrupole mass filter. Perform a complete Peak Tuning Procedure as
described in the RGA Tuning chapter of this manual if significant shifts in the peak
locations are observed.
x
The RGA has the ability to store a complete scan in its output buffer. The scan
remains in memory until all the data is transmitted out to the host computer over
RS232. As a result of the high acquisition rate of the RGA there might be a delay
between the time at which the data is collected and the time at which a complete
spectrum is displayed by the host computer. The time lag between data acquisition
and display depends on a large number of factors including the scan rate (NF setting)
of the RGA, the host computer’’s processing speed, and the amount of handshaking
activity over the RS232 line. As computers get faster, they will be able to catch up
with the RGA data acquisition speed and this problem will no longer need any
consideration.
x
Before a new scan starts, the RGA checks its internal memory to make sure that no
data from any previous scan is pending to be transmitted. If data is still pending, the
RGA must first finish transmitting the stored data before the new scan can start. This