National Instruments DAQ X Series - Page 273

Glossary
© National Instruments G-7 X Series User Manual
encoder A device that converts linear or rotary displacement into digital or pulse
signals. The most popular type of encoder is the optical encoder, which uses
a rotating disk with alternating opaque areas, a light source, and a
photodetector.
external trigger A voltage pulse from an external source that causes a DAQ operation to
begin.
F
FIFO First-In-First-Out memory buffer—A data buffering technique that
functions like a shift register where the oldest values (first in) come out
first. Many DAQ products and instruments use FIFOs to buffer digital data
from an A/D converter, or to buffer the data before or after bus
transmission.
The first data stored is the first data sent to the acceptor. FIFOs are often
used on DAQ devices to temporarily store incoming or outgoing data until
that data can be retrieved or output. For example, an analog input FIFO
stores the results of A/D conversions until the data can be retrieved into
system memory, a process that often requires programming the DMA
controller. This process can take several milliseconds in some cases. During
this time, data accumulates in the FIFO for future retrieval. With a larger
FIFO, longer latencies can be tolerated. In the case of analog output, a FIFO
permits faster update rates, because the waveform data can be stored on the
FIFO ahead of time. This again reduces the effect of latencies associated
with getting the data from system memory to the DAQ device.
filter A physical device or digital algorithm that selectively removes noise from
a signal, or emphasizes certain frequency ranges and de-emphasizes others.
Electronic filters include lowpass, band-pass, and highpass types. Digital
filters can operate on numeric data to perform equivalent operations on
digitized analog data or to enhance video images.
floating The condition where a common mode voltage exists, or may exist, between
earth ground and the instrument or circui
t of interest. Neither the high, nor
the low side of
a circuit is at earth potential.
floating signal sou rces Signal sources with voltage signals that are not connected to an absolute
reference of system ground. Also called non-referenced signal sources.
Some common examples of floating signal sources are batteries,
transformers, and thermocouples.
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