ORTEC DSPEC Pro, Hardware User Manual

Page: 105 / 108

794380D / 0914 APPENDIX B. LIST MODE IN THE DSPEC PRO
95
The ADC Word contains a raw 14-bit ADC value and a 16-bit real time stamp. The Hardware
Time Word contains the same 16-bit real time stamp. In both cases this value represents the time
that the word was created as represented by an internal clock that increments every 200 ns. The
200 ns clock rolls over from 49999 to 0, which gives it a period of 10 ms. Every time the 200 ns
clock rolls over a RT Word and LT Word are created. The 30-bit real time stamp on the RT word
corresponds to the number of times the 200 ns clock rolls over. The 10 ms real time clock and
the 10 ms live time clocks are cleared when the CLEAR command is sent to the spectrometer.
Otherwise they increment every 10 ms while data is collecting. Using the RT Word as a time
standard and the 200 ns time stamps from the ADC word you can determine the time that any
ADC event occurred, ±200 ns, relative to the beginning of the acquisition.
LT Words are created by the spectrometer at the same time as are RT Words. The 30-bit live
time stamp represents the ADC live seconds since the beginning of the acquisition (or since the
last CLEAR command). By calculating the relative change in live time between any two LT
Words in a list mode stream you can calculate the live time for a given block of data, ±10 ms.
The ADC CRM word contains the value of the spectrometer’s count-rate meter and is generated
at the same time as the RT and LT words (i.e., it always appears next to these words in the list
mode data stream). The count-rate meter is incremented each time the ADC fast channel detects
a pulse. Since many pulses may be rejected before one is converted into an ADC event, this
number is not the simple sum of the ADC events over the past 10 ms; instead it represents the
true input count rate, as well as it can be measured by the instrument. Note that this value is
zeroed after an ADC CRM word is created, therefore it represents only 10 ms of ADC input
counts.
The Ext Counter words contain the value of the external input pulse counters and are generated
at the same time as the RT and LT words. The external pulse counters count the positive pulses
at the external input of the spectrometer. Note that the external inputs must stay high for at least
40 ns for the pulse to be counted. These counters are zeroed after an Ext Counter word is
created, therefore, it represents only 10 ms of external pulses.
Optionally, the external counters can be configured to be a simple level sensitive input. In this
case the counter value is either 0 or 1 depending on the level of the input at the time the counter
word is generated.