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Chapter 4
Theory of Operation
NI PCI-1200 User Manual
Controlled Acquisition Mode
The NI PCI-1200 uses two counters, counter A0 and counter A1, to execute
DAQ operations in controlled acquisition mode. Counter A0 counts sample
intervals, while counter A1 counts samples. In a controlled acquisition
mode DAQ operation, the device performs a specified number of
conversions, and then the hardware shuts off the conversions. Counter A0
generates the conversion pulses, and counter A1 gates off counter A0 after
the programmed count has expired. The number of conversions in a single
controlled acquisition mode DAQ operation is limited to a 16-bit count
(65,535 conversions).
Freerun Acquisition Mode
The NI PCI-1200 uses one counter, counter A0, to execute DAQ operations
in freerun acquisition mode. Counter A0 continuously generates the
conversion pulses as long as GATEA0 is held at a high logic level. The
software keeps track of the number of conversions that have occurred and
turns off counter A0 either after the required number of conversions has
been obtained or after some other user-defined criteria have been met.
The number of conversions in a single free-run acquisition mode DAQ
operation is unlimited.
Interval Scanning Acquisition Mode
The NI PCI-1200 uses two counters for interval scanning data acquisition.
Counter B1 is used to time the scan interval. Counter A0 times the sample
interval. In interval scanning AI operations, scan sequences are executed at
regular, specified intervals. The amount of time that elapses between
consecutive scans within the sequence is the sample interval. The amount
of time that elapses between consecutive scan sequences is the scan
interval. LabVIEW, LabWindows/CVI, other application software, and
NI-DAQ support only multichannel interval scanning.
Because interval scanning allows you to specify how frequently scan
sequences are executed, it is useful for applications in which you need to
sample data at regular but relatively infrequent intervals. For example,
to sample channel 1, wait 12 µs, then sample channel 0; and if you want
to repeat this process every 65 ms, then you should define the operation as
follows:
•
Start channel:
•
Sample interval:
•
Scan interval:
ch1 (which gives a scan sequence of "ch1, ch0")
12 µs
65 ms
4-8
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