One of the most common applications of SDDS for APS commissioning and operation is logging of measured data values at intervals. A set of generic EPICS monitoring programs sddsmonitor, sddsvmonitor (vector monitoring), and sddswmonitor (waveform monitoring) are used for this. One example is the vacuum pressure in the APS ring, which is logged continuously by sddsvmonitor; this data consists of readings from ion gauges around the ring. Another example is logging of beam-position-monitor readouts in the Positron Accumulator Ring (PAR) and its input and output beam transport lines using the program sddsmonitor.
For use in examples, I'll assume the existence of two files called SR.vac and par.bpm. These are simplified from actual files collected with the programs just mentioned.
SR.vac is a file containing an arbitrary series of data pages, each consisting of a snapshot of the vacuum gauge readings around the ring. There are 40 such readings, one for each sector of the accelerator. Typically, one set of readings is taken every 15 minutes.
par.bpm is a file containing a single page of data with any arbitrary number of rows. The PAR has 16 beam-position-monitors (BPMs), each providing a horizontal (x) and vertical (y) readout. In addition, the beam transport line downstream of PAR (known as the PTB line), contains five BPMs for x and five for y. The data included in the distribution contains only the x values, since these are more interesting:
Time-- A double-precision column giving the elapsed number of seconds since monitoring begain. The values are approximately equispaced.
TimeOfDay-- A double-precision column giving the time of day in hours since midnight.
x-- 16 single-precision readouts of the horizontal beam orbit just prior to beam extraction. quadrant ranges from 1 to 4, as does number.
y-- 16 single-precision readouts of the vertical beam orbit just prior to beam extraction. quadrant ranges from 1 to 4, as does number.
x-- four single-precision readouts of the horizontal beam trajectory as the beam passes through the PTB transfer line. number ranges from 2 to 5.