Advantech PCI-1710UL Manual de usuario descargar pdf (Pagina 6)

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Data Acquisition Boards

Glossary

Accuracy

Accuracy is the deviation of a measurement from a known standard. Accuracy is normally

speciﬁed in percent.

ADC - Analog-to-Digital Converter

ADC is used to convert DC voltage from transducers into digital words (data). The voltage

representing a temperature, pressure, ﬂow, pH, or speed and must converted to a digital

word before it can be passed to an intelligent device like a computer.

Ampliﬁer

Ampliﬁers are used to boost the analog level (voltage) of the signal.

AO - Analog Output

The D/A converter performs the opposite function of an A/D converter. It interprets

commands from the computer and outputs the proper DC voltage or current. The output

stays at this output level until the computer tells the D/A converter to output a new value.

Auto Calibration

The built-in auto-calibration circuitry corrects gain and offset errors in analog input and

analog output channels thereby eliminating the need for external equipment and user

adjustments.

Automatic Channel/Gain/SD*/BU* Scanning

Advantech’s DAQ cards with this function feature an automatic channel/gain/SD/BU

scanning circuit. This circuit controls multiplexer switching during sampling in a way

that is more efﬁcient than software implementation. Onboard SRAM stores different gain,

SD and BU values for each channel. This combination lets users perform multi-channel

high-speed sampling with different gain, SD and BU values for each channel.

*Note: SD: Single-Ended/Differential; BU: Bipolar/Unipolar

BoardID Switch

BoardID DIP switch helps deﬁnes each card’s unique identity when multiple identical PCI

cards have been installed in the same computer. The BoardID switch is very useful when

you build your system with multiple identical PCI cards. With the correct BoardID switch

settings, you can easily identify and access each card during hardware conﬁguration and

software programming.

C/T - Counter/Timer

The counter card can be used to sense the presence or absence of a voltage, much like

digital input card. The counter is used to count the number of electronic pulses (totalize),

the duration of the pulse (pulse width), or the rate of pulses (frequency) coming out of

an external device.

Channel-freeze

The channel-freeze function can be enabled either in dry contact or wet contact mode

(selected by the onboard jumper). When the channel-freeze function is enabled, the last

status of each digital output channel will be safely kept for emergency use. Moreover,

you can enable this function through software as it is useful in software simulation and

testing program.

Common Mode Noise

Common mode noise most often results when the ground potential between the measuring

instrument and the device being measured are different. The difference in grounds results

in a ground loop, a current ﬂowing through ground and the low lead. Once this current

appears in the low lead wire it will cause a voltage because the wire has some resistance.

The longer the lead, the more lead resistance and greater the voltage error.

TIP : To reduce common mode noise, use a guarded voltmeter. Tie the guard to the low

side of the device being measured. This will shunt any ground loop currents away from

the high and low measurement wires.

DI - Digital Input

A digital input card is used to determine whether an external device is on or off by sensing

the presence or absence of a voltage. The DI can only report ON/OFF status and not the

value of the voltage on each channel (sometimes called a bit). The bit is considered to

be ON if the voltage exceeds a certain value.

Digital cards usually feature 8, 16, or 32 channels. They can monitor a number of devices

at the same time. For example, a digital card can be attached to a single operator panel

to sense the position of switches on that panel.

Digital Filter

The digital ﬁlter function is used to eliminate glitches on input data and reduce the

number of changes to examine and process. The ﬁlter blocks pulses that are shorter than

the speciﬁed timing interval and passes pulses that are twice as long as the speciﬁed

interval. Intermediate-length pulses that are longer than half of the interval, but less than

the interval, may or may not pass the ﬁlter depending on your settings.

DMA - Direct Memory Access

A method of transferring data from or to memory at a high rate without involving the

CPU. DMA is the hardware/software technique that allows the highest speed transfer of

data, to or from random memory (RAM) . Given the potentially more expensive hardware,

DMA can provide the means to read or write data at precise times without restricting

the microprocessor' tasks. For example, one system under DMA control can read or

write any combination of analog, digital or counter/timer data to or from RAM at rates

up to 360KB/second. This is accomplished without taking time from the other tasks of

the microprocessor. The amount of time required to respond to a DMA request is much

smaller than the one required to service an interrupt. This makes the goal of foreground/

background operation, at high speed, possible.

DO - Digital Output

The digital output card interprets a command from the computer and outputs a high or

low voltage on each of its channels (bits). It is commonly used to turn on/off small lights

or to send digital words to machinery.

FSR - Full Scale Range

FSR stands for the difference between the largest positive and negative voltages which a

AI channel can measure or a AO channel can output.

Gain - Magnitude Ratio

For a linear system or element, the ratio of the magnitude (amplitude) of a steady-state

sinusoidal output relative to the causal input; the length of a phasor form the origin to a

point of the transfer locus in a complex plane.

GPIB - General Purpose Interface/Instrument Bus

A standard for IEEE-488 communication interface.

Interrupt Data Transfer

Interrupts provide a mean of tightly controlling the timing of events, while allowing the

processing of more than one task. Multitasking systems are also known as “foreground/

background” systems. One way of putting data acquisition in the background is to relegate

it to an interrupt routine. The clock or external timing signal, rather than being polled

continuously, is used to generate an interrupt to the computer. Whenever the interrupt

occurs, the computer suspends current activity, and executes an “interrupt service routine”.

The interrupt service routine in this case might be a short program which acquires one

frame of data, and stores it in memory. The computer can perform other operations in

the foreground while collecting data in the background. Whenever a clock tick or external

interrupt occurs, the computer will automatically stop the foreground processing, acquire

the data, and then resume where it left off.

The reaction speed of the interrupt system is much higher than that of a polling loop.

Speed, for a PC, is about 10 kHz ~ 30 kHz in the interrupt mode.

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