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Theory of Operation
Chapter 3
AT-MIO-64F-5 User Manual
3-10
© National Instruments Corporation
A signal is generated at the end of the sequence to indicate its completion. An interrupt request
can be generated from this signal if desired. Because the sample counter begins counting
immediately after the application of the trigger, this is a posttrigger sequence. If samples are
necessary before and after the trigger, then a pretrigger sequence is needed. This sequence is
described in the following paragraphs.
Figure 3-6 depicts a pretrigger data acquisition sequence. It is called a pretrigger sequence
because the first trigger initiates the sample-interval timer without enabling the sample counter.
Conversions occur after this initial trigger and are stored in the ADC FIFO for later retrieval in
the same way they are for a posttrigger sequence. After a second trigger is received, the sample
counter begins counting conversions. In this example, there are three pretrigger samples, and
seven posttrigger samples. Only the number of posttrigger samples is programmable.
Trigger*
DAQPROG
DAQCMPLT
DAQCLEAR*
Interrupt
CONVERT
6
Sample CTR
5
4
3
2
1
7
6
Samp CTR Gate
Figure 3-6. Single-Channel Pretrigger Data Acquisition Timing
The pretrigger sequence is programmed in much the same way as a posttrigger sequence. The
sample-interval timer is programmed to generate conversion pulses under a gate signal, and the
sample counter is programmed to count the number of conversions. The only difference between
pretrigger and posttrigger sequences for all data acquisition modes is that the sample counter
waits for a gating signal in the pretrigger mode before beginning the count. For posttrigger
sequences, the sample timer is independent of the gating signal, and for pretrigger sequences, the
sample timer is dependent on the gating signal.
Multiple-Channel Data Acquisition
Multiple-channel data acquisition is performed by enabling scanning during data acquisition.
Multiple-channel scanning is controlled by the configuration memory register.
The configuration memory register consists of 512 words of memory. Each word of memory
contains a multiplexer address for input analog channel selection, a gain setting, a mode setting
(single-ended or differential), and a range setting (unipolar or bipolar). Each word of memory
also contains a bit for synchronizing scanning sequences of different rates, a bit enabling serial
data transmission of channel conversion data over the RTSI bus to the AT-DSP2200 digital
signal processing board, and a bit indicating if the entry is the last in the scan sequence. In
interval scanning, a scan list can consist of any number of scan sequences. Whenever a
configuration memory location is selected, the information bits contained in that memory
location are applied to the analog input circuitry. For scanning operations, a counter steps
through successive locations in the configuration memory at a rate determined by the scan clock.
With the configuration memory, therefore, an arbitrary sequence of channels with separate gain,
mode, and range settings for each channel can be clocked through during a scanning operation.
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