03 Interfacesmm

7/28/2019 03 Interfacesmm

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Ch. 3

Computer Interfacing

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Data Acquisition (DAQ)

Measuring a real-world signal and bringing

that information into the computer.

LabVIEW can command plug-in DAQ

boards to acquire (A/D conversion) or

generate (D/A conversion) analog anddigital signals.


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Data Acquisition (DAQ)

A DAQ system

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DAQ boards


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Transducers

Converting one physical phenomenon toan electrical signal.

E.g. thermocoupleconverts temperature

to a voltage.

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Sensors

For data acquisition, you need to know aboutyoursensors: Nature of the produced signals. E.g. voltage and

frequency response. We need to determine what kindof signal conditioning, analog-to-digital converter(ADC), or other hardware are needed.

You need to know how to calibrate the sensorbecause the calibration may be nonlinear or it mayhave problems with repeatability, etc.

What happens if you turn off your data acquisitionequipment while the sensor still has power applied?Will there be damage to any components?


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Data acquisition

A general sensor model

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Actuators

Converting a signal into a physical phenomenon.

Required when you need to control something

such as temperature, pressure or position.

You may often see there are feedback control

loops associated with the actuators. This isbecause most actuators produce responses in

the physical system that may be nonlinear and

sometimes unpredictable.


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Virtual Instrumentation Software

Architecture (VISA)

A standard I/O Application ProgrammingInterface (API).

Independent of the hardware protocol or

bus.

VISA can control GPIB or serial

instruments.

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VISA

Instrument I/O control >> VISA palette.


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VISA

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VISA


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General Purpose Interface Bus

(GPIB)

Complies with IEEE-488 standard. It is a digital, 24-conductor parallel bus.

It is the means by which computers and

instruments transfer data.

Many instruments and computers can be

connected to the same GPIB bus.

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GPIB


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General Purpose Interface Bus

(GPIB)

A typical GPIB system

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GPIB

Connects a maximum of 15 devices.

Consists of 16 signal lines, 7 signal ground

return, 1 main ground return.

16 signal lines8 data lines, 3 handshake

lines, and 5 interface management lines.


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GPIB

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GPIB

Listenera device capable of receiving dataover the interface when addressed. E.g. printers,display devices.

Talkera device capable of transmitting dataover the interface when addressed. E.g.

frequency counters, digital voltmeters. Controllera device which controls the flow of

data information on the bus and can issuespecial commands for bus management. E.g. acomputer with an appropriate I/O card.


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GPIBData Lines

DIO1 through DIO8 are used for datatransfer in the form of a sequence of 8-bitcharacters (bytes).

Information transferred includes interfacecommands, addresses, and devicedependent data.

ASCII 7-bit code is generally used, with

the 8th bit as a parity check. DIO1 is LSB.

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GPIBHandshake Lines

3 handshake lines (NRFD, NDAC, DAV) control

the transfer of message bytes among the

devices and ensure data transfer integrity.

Technique is characterized by:

Asynchronous data transfer. More than one device can accept data at the same

time.

Every byte transferred undergoes the handshake

(except for parallel poll response).


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GPIBHandshake Lines

NRFD (Not Ready For Data)used toindicate the condition of readiness of

device to accept data.

NDAC (Not Data Accepted)used to

indicate the acceptance of data by device.

DAV (Data Valid)used to indicate the

condition of the info. on the DIO lines.

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GPIBHandshake Lines


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GPIBInterface Management

Lines

The remaining 5 lines comprise the control bus. ATN (ATtentioN) functionthe controller alone

drives the ATN line and all devices must monitorATN at all time. When ON, ATN places the interface in the

COMMAND MODE where all devices accept data onthe Data Lines and interpret it as COMMANDS orADDRESSES.

When OFF, it places the interface in the DATA MODE

where the active talker sources device dependentDATA to all active listeners.

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GPIBInterface ManagementLines

IFC (InterFace Clear)the line is used by theSystem Controller to halt current operations(communications) on the bus.

REN (Remote ENable)the line is used by theSystem Controller to enable devices to be

subsequently places in the remote programmingmode. When TRUE, all listeners if capable, are placed in

remote.

When FALSE, devices return to local front paneloperation.


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GPIBInterface Management

Lines

SRQ (Service ReQuest)the line is usedto indicate the need for attention, and canact as an interruption of the currentsequence of events.

EOI (End Or Identity): When ATN is TRUE, the EOI line is used by a

controller to execute a parallel poll.

When ATN is FALSE, the lines is used by anactive talker to indicate the last byte of a datamessage.

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GPIB in LabVIEW

Instrument I/O control >> GPIB palette.

All GPIB instruments have an address,which is 0~30.

You must provide the address of the

instrument (in string format) in order tocommunicate between the GPIB VIs.

Example in LabVIEWOpen>>LABVIEW\examples\INSTR\smplgpib.llb, selecting LabVIEWGPIB.vi


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GPIB

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Serial Communication

Another popular means of transmitting

data between computers or a peripheral

device.

It uses a transmitter to send data 1 bit at a

time in a single communication line to areceiver.


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RS-232

RS-232built-in serial port Slower and less reliable than GPIB.

No need for additional hardware.

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RS-232

A typical serial communication system


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Universal Serial Bus (USB)

USBbuild-in serial port. More complex protocol.

Widely-used nowadays.

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USB connected to GPIB


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GPIB

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USB connected to RS-232


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Instrument Drivers

A collection of functions that implementthe commands necessary to perform the

instruments operations.

They receive, parse, and scale the

response strings from instruments into

scaled data.

Instrument I/O >> Instrument Drivers

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Connectivity

Internet Connectivity

http://ni.com/labview/

Share data through the Internet, Free VIs,

LabVIEW Demo

Networking

Separate computers communicate through

LAN or the Internet.


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Connectivity

ActiveX A piece of code can be shared in different

application.

E.g. A Word document can be embedded in

LabVIEW VI.

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Connectivity

Dynamic Link Libraries (DLLs), Code

Interface Node (CINs)

DLL

A library of shared functions

Done at runtime

CIN

A special block diagram structure

Used to link conventional, text-based code to a VI.


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Digital I/O

Porta collection of digital lines that areconfigured in the same direction and can be

used at the same time.

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Digital I/O

Port widthno. of lines in a port.

State1 or 0.

Patterna sequence of digital states, e.g.

1001.

Least-significant bit (LSB)rightmost bit

on the pattern.

Most-significant bit (MSB)leftmost bit on

the pattern.

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