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Chapter 3 Theory of Operation
© National Instruments Corporation 3-5 Lab-NB User Manual
multichannel (scanned) data acquisition. Scanned data acquisition uses a counter to
automatically switch between analog input channels during data acquisition.
DAQ timing consists of signals that initiate a DAQ operation, initiate individual A/D
conversions, gate the DAQ operation, and generate scanning clocks. Sources for these signals
are supplied mainly by timers on the Lab-NB board. One of the two 8253 integrated circuits is
reserved for this purpose.
An A/D conversion can be initiated by a high-to-low transition on the counter A0 output
(OUT A0) of the 8253(A) Counter/Timer chip on the Lab-NB or by a high-to-low transition on
EXTCONV* input. During data acquisition, the onboard sample-interval counter–counter 0 of
8253(A)–is used to generate pulses that initiate A/D conversions.
The sample-interval timer is a 16-bit down counter that uses the 1-MHz clock onboard to
generate sample intervals from 2 µsec to 65,535 µsec (see Timing I/O Circuitry later in this
chapter). Alternatively, it can use the output from counter B0 (OUTB0) of the 8253(B)
Counter/Timer chip on the Lab-NB. Each time the sample-interval timer reaches 0, it generates
a pulse and reloads with the programmed sample-interval count. This operation continues until
the counter is reprogrammed.
As stated in Chapter 4, Register-Level Programming, only counter A0 is required for DAQ
operations in freerun acquisition mode. The software must keep track of the number of
conversions that has occurred and turn off counter A0 after the required number of conversions
has been obtained. In controlled acquisition mode, two counters (counters A0 and A1) are
required for a DAQ operation. Counter A0 generates the conversion pulses, and counter A1
gates off counter A0 after the programmed count has expired.
Single-Channel Data Acquisition
During single-channel data acquisition, the channel select and gain bits in the A/D Configuration
Register select the gain and analog input channel before data acquisition is initiated. These gain
and multiplexer settings remain constant during the entire DAQ process; therefore, all A/D
conversion data is read from a single channel.
Multichannel (Scanned) Data Acquisition
Multichannel data acquisition is performed by enabling scanning during data acquisition.
Multichannel scanning is controlled by a scan counter.
For scanning operations, the scan counter decrements from the highest numbered channel
(specified by the user) through channel 0 and then repeats the sequence. Thus, any number of
channels from 2 to 8 can be scanned. Notice that the same gain setting is used for all channels in
the scan sequence.