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Chapter 4 Theory of Operation
© National Instruments Corporation 4-9 PCI-1200 User Manual
repeat this process every 65 ms, then you should define the operation as
follows:
• Start channel: ch1 (which gives a scan sequence of “ch1, ch0”)
• Sample interval: 12 µs
• Scan interval: 65 ms
The first channel will not be sampled until one sample interval from the
scan interval pulse. Since the A/D conversion time is 10 µs, your sample
interval must be at least this value to ensure proper operation.
Single-Channel Data Acquisition
The PCI-1200 executes a single-channel analog input operation by
performing an A/D conversion on a specified analog input channel every
sample interval. The sample interval is the amount of time that elapses
between successive A/D conversions. The sample interval is controlled
either externally by EXTCONV* or internally by counter A0 of the timing
circuitry.
To specify a single-channel analog input operation, select an analog input
channel and a gain setting for that channel.
Multichannel Scanned Data Acquisition
The PCI-1200 executes a multichannel DAQ operation by repeatedly
scanning a sequence of analog input channels (the same gain is applied to
each channel in the sequence). The channels are scanned in decreasing
consecutive order; the highest-numbered channel is the start channel, and
channel 0 is the last channel in the sequence.
During each scan sequence, the PCI-1200 scans the start channel (the
highest-numbered channel) first, then the next highest-numbered channel,
and so on until it scans channel 0. The PCI-1200 repeats these scan
sequences until the DAQ operation is terminated.
For example, if channel 3 is specified as the start channel, then the scan
sequence is as follows:
ch3, ch2, ch1, ch0, ch3, ch2, ch1, ch0, ch3, ch2, …
To specify the scan sequence for a multichannel scanned analog input
operation, select the start channel for the scan sequence.