Measurement Computing Data Acquisition Systems rev.10.4 - Manual (Page 359)

Programmer’s Manual
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Glossary G-1
Glossary
This list of terms is intended to give a brief background on some of the terms used throughout the Daq Programmers Manual.
These definitions should prove a highly informative primer for those unfamiliar with the data acquisition terminology.
8255
Refers to Intel 8255 chip. This chip (or emulation of the chip) is used in Daq products for P2 Digital I/O support.
9513
Refers to the Texas Instruments 9513 chip. This chip is used by DaqBook/100 Series, DaqBook/200 Series, and
ISA-type DaqBoard products for Counter/Timer operations.
A/D
A/D refers to an “analog-to-digital” converter. A/D’s convert a voltage over a specific range to a digitized reading.
The value of the digitized reading depends upon the resolution of the A/D device. Most A/D devices have resolution
of 12 or 16 bits. The range over which the conversion is performed depends upon the gain and polarity selected.
A/D Resolution
With 12-bit A/D’s, the significant values of the converted reading can range from 0 to 4,095 over the specified
voltage range. With 16-bit A/D’s, the significant values of the converted reading can range from 0 to 65,535 over
the specified voltage range.
A/D Data Representation
At times, the presentation of A/D values may differ, depending on the device or the current mode of the device. For
instance, a 12-bit reading may be normalized to a 16-bit ranging from 0 to 65,535, where the 4 least significant bits
are not relevant to the actual value of the reading. Also, a reading may be signed if the range specified is a bipolar
voltage, where the digitized reading will range from –32,768 to +32,767 rather than from 0 to 65,535.
Acquisition
An acquisition is the collection of analog, digital or counter input based upon a common input synchronization
event. The common synchronization event can be an internal time-base generated by an on-board clock, or it can be
an external signal. The start event (also referred to as the ”trigger event”) for an acquisition may take one of several
forms—triggers can be based on input channel values, external events, or programmatically defined events.
Likewise, the end of an acquisition (also referred to as the “stop event”) may be based on similar criteria.
Acquisition Frequency
The rate at which an acquisition takes place, measured in terms of frequency (Hz).
Acquisition Period
The rate at which an acquisition takes place, measured in terms of period (nanoseconds).
Acquisition Rate
The rate (in frequency or period) at which channel scans are to be taken, if using an internal clock source for the
acquisition.
ADC acquisition
See Acquisition.
Analog
A signal of varying voltage or current that communicates data (compare with Digital).
API (Application Program Interface)
The interface program within the Daq system’s driver that includes function calls specific to Daq hardware and can
be used with user-written programs (several languages supported)
Arm
An action that enables a device to detect the trigger/start event specified.