National Instruments AT-MIO-16D, User Manual

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Theory of Operation Chapter 3
AT-MIO-16D User Manual 3-8 © National Instruments Corporation
time interval called the scan interval to each cycle through the mux-gain memory. The scan interval
is basically the time between starts for each cycle through the mux-gain memory.
Data acquisition timing consists of signals that initiate a data acquisition operation, initiate
individual A/D conversions, gate the data acquisition operation, and generate scanning clocks. The
sources for these signals can be supplied by timers on the AT-MIO-16D board, by signals
connected to the AT-MIO-16D I/O connector, or by signals from other AT Series boards connected
to the RTSI bus.
Single Conversions
You can initiate single A/D conversions by applying an active low pulse to the EXTCONV* input
on the I/O connector or by writing to the Start Convert Register on the AT-MIO-16D board.
During data acquisition, the onboard sample-interval counter (Counter 3 of the Am9513A
Counter/Timer) generates pulses that initiate A/D conversions. External control of the sample
interval is possible by applying a stream of pulses at the EXTCONV* input. In this case, you have
complete external control over the sample interval and the number of A/D conversions performed.
Sample-Interval Timer
The sample-interval timer is a 16-bit down counter that can be used with the five internal timebases
of the Am9513A to generate sample intervals from 2
m
sec to 6 sec (see the Timing I/O Circuitry
section later in this chapter). The sample-interval timer can also use any of the external clock
inputs to the Am9513A as a timebase. During data acquisition, the sample interval counts down at
the rate given by the internal timebase or external clock. Each time the sample-interval timer
reaches zero, it generates a pulse and reloads with the programmed sample-interval count. This
operation continues until data acquisition halts.
Sample Counter
The onboard sample counter can control data acquisition. Load this counter with the number of
samples to be taken during a data acquisition operation. The sample counter can be 16-bit for
counts up to 65,535 or 32-bit for counts up to (2
32
- 1). If a 16-bit counter is needed, Counter 4
of the Am9513A Counter/Timer is used. If more than 16 bits are needed, Counter 4 is
concatenated with Counter 5 of the Am9513A to form a 32-bit counter. The sample counter
decrements its count each time the sample-interval counter generates an A/D conversion pulse, and
the sample counter stops the data acquisition process when it counts down to zero.
You can trigger the sample counter externally with the STOP TRIG input on the AT-MIO-16D I/O
connector. The counter does not begin counting the A/D conversion pulses until a rising edge
signal occurs on STOP TRIG. With this method, A/D conversion samples can be collected both
before and after a hardware trigger is received.
You can initiate the data acquisition process by writing to the Start DAQ Register on the
AT-MIO-16D board or by applying an active low pulse to the START TRIG* input on the
AT-MIO-16D I/O connector. These triggers start the sample-interval and sample counters. The
sample-interval counter then manages the data acquisition process until the sample counter reaches
zero.