Glossary

data transfer

A technique for moving digital data from one system to another.

 

Options for data transfer are DMA, interrupt, and programmed I/O. For

 

programmed I/O transfers, the CPU in the PC reads data from the DAQ

 

device whenever the CPU receives a software signal to acquire a single data

 

point. Interrupt-based data transfers occur when the DAQ device sends an

 

interrupt to the CPU, telling the CPU to read the acquired data from the

 

DAQ device. DMA transfers use a DMA controller, instead of the CPU, to

 

move acquired data from the device into computer memory. Even though

 

high-speed data transfers can occur with interrupt and programmed I/O

 

transfers, they require the use of the CPU to transfer data. DMA transfers

 

are able to acquire data at high speeds and keep the CPU free for

 

performing other tasks at the same time.

dB

Decibel—The unit for expressing a logarithmic measure of the ratio of

 

two signal levels: dB = 20log10 V1/V2, for signals in volts.

DC

Direct current—although the term speaks of current, many different types

 

of DC measurements are made, including DC Voltage, DC current, and DC

 

power.

device

A plug-in data acquisition product, card, or pad that can contain multiple

 

channels and conversion devices. Plug-in products, PCMCIA cards, and

 

devices such as the DAQPad-1200, which connects to your computer

 

parallel port, are all examples of DAQ devices. SCXI modules are distinct

 

from devices, with the exception of the SCXI-1200, which is a hybrid.

DIFF

Differential mode—An analog input mode consisting of two terminals,

 

both of which are isolated from computer ground, whose difference is

 

measured.

differential input

An input circuit that actively responds to the difference between two

 

terminals, rather than the difference between one terminal and ground.

 

Often associated with balanced input circuitry, but also may be used with

 

an unbalanced source.

digital I/O

The capability of an instrument to generate and acquire digital signals.

 

Static digital I/O refers to signals where the values are set and held, or

 

rarely change. Dynamic digital I/O refers to digital systems where the

 

signals are continuously changing, often at multi-MHz clock rates.

digital signal

A representation of information by a set of discrete values according to a

 

prescribed law. These values are represented by numbers.