Data Capture Commands - Stanford Research Systems SR865A Operation Manual

134 Programming

Data Capture Commands

Overview
The SR865A can capture data points in an internal capture buffer. This can be up to
1 Mpoints of (X), 512 kpoints of either (X,Y) or (R,θ) or 256 kpoints of (X,Y,R,θ). Data
points are stored as floating point values (4 bytes per data point). The capture buffer is
made of 2 kbyte blocks (512 total data points per block), but specified in terms of its
overall length in kbytes—which must therefore always be an even number of kbytes. The
buffer can be configured between 2 kbytes (512 total points) and 4 Mbytes (1,048,576
total points).
The capture buffer is not retained when the power is turned off.
Data Format
The captured data is stored and read in 32-bit floating point format. Data is stored and
transferred in little endian byte order.
Capture Rate
The capture rate sets how often data points are sampled and added to the capture buffer.
All parameters are sampled simultaneously at the same rate.The maximum capture rate is
determined by the time constant of the SR865A. Shorter time constants allow faster
capture rates, up to a limit of 1.25 MHz. The actual capture rate for periodic sampling can
be set to the maximum allowed rate divided by factors of 2.
Note that if the time constant is modified during a capture, the sampling rate will change
mid-capture. This will likely create confusing results and should be avoided.
For the case of one sample per trigger capture (see below), data is stored when a trigger is
detected. The hardware trigger rate shouldnever exceed the maximum capture rate,
otherwise the SR865A will capture fewer samples than expected.
Capture Time
The entire capture time is the buffer length (number of data points of each parameter)
divided by the capture rate. For example, a 4 MByte buffer holds 1 M total data points. If
X and Y are captured, the buffer holds 512 kpoints of X ,Y pairs (captured together). The
total capture time is thus (512 kpoints)/(1.25 MHz) = 0.419 seconds at the fastest rate.
OneShot or Continuous
When the capture buffer becomes full, data capture can either stop or continue.
The first case is called 'OneShot' (data points are captured for a single buffer length).
When the buffer fills, data capture stops. Capture may also be stopped early with the
CAPTURESTOP command.
The second case is called 'Continuous'. In this case, data capture continues at the end of
the buffer. The buffer will fill and start at the beginning again, overwriting the oldest
data. Only the most recent points will be contained in the buffer. Data is recorded until
explicitly stopped using the
SR865A DSP Lock-in Amplifier
CAPTURESTOP command or by hardware trigger (see
Chapter 4