National Instruments AT-MIO-16X - Manual (Page 78)

Chapter 3 Theory of Operation
AT-MIO-16X User Manual 3-12 © National Instruments Corporation
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.
A SCANCLK signal is generated from the sample-interval counter.
This signal pulses once at the beginning of each A/D conversion and is
supplied at the I/O connector. During multiple-channel scanning, the
configuration memory location pointer is incremented repeatedly,
thereby sequencing through the memory and automatically selecting
new channel settings during data acquisition. The signal used to
increment the configuration memory location pointer is generated from
the SCANCLK signal. Incrementing can be identical to SCANCLK,
sequencing the configuration memory location pointer once after every
A/D conversion, or it can also be generated by dividing SCANCLK by
Counter 1 of the Am9513A Counter/Timer. With this method, the
location pointer can be incremented once every N A/D conversions so
that N conversions can be performed on a single-channel configuration
selection before switching to the next configuration memory selection.