Networking with LabVIEW

Author: Marco Parra, Jr.

Course: Computer Networks (CEN4516)

Professor: Janusz Zalewski

Florida Gulf Coast University

Fort Myers, Florida

December 2, 2005

1. Introduction

LabVIEW is a programming language which has been created mainly for data

acquisition (or DAQ in LabVIEW terminology). As such LabVIEW has been used as an

interface for hardware tools to interact with software tools. By acting as a software

interface, LabVIEW is able to get signals from a variety of hardware and give

instructions to the hardware to be controlled so it behaves in a desired manner.

According to Bruce Mihura, author of the book LabVIEW for Data Acquisition

and developer for LabVIEW at National Instruments (NI), states the one of the simplest

ways to create a DAQ Virtual Instrument (way programs are created in LabVIEW or VI

for short) is by using NI’s Measurement & Automation Explorer (or MAX). MAX is a

tool that works along any DAQ device that generates an analog signal and is capable of

automatically recognizing any device attached to it [1]. Also, by using MAX it is possible

to analyze the data acquired and produce graphs and produce readings through the

LabVIEW software.

Knowing that LabVIEW was created to collect data from hardware connected to a

computer card, the next question rises. How does data acquisition happen? Before we go

into the workings of data collection, let us borrow some terms from Bruce Mihura that

will help us understand how this process is done.

Channel: An electrical connection to a signal. This might be in the form of an

analog input.

Sample: The conversion of an electrical signal into a digital value.

Scan: The collection of one or more samples, across one or multiple channels.

Amplifiers and Multiplexers: Devices that take an electrical signal and scale it

accordingly for further processing. However if only one channel is to be used, an

amplifier can do the job for reading that channel, but if many signals need to be

read at the same time a multiplexer can be used so there is only one amplifier

reading various signals.

All definitions are taken from LabVIEW for Data Acquisition by Bruce Mihura.

So what will happen when data is collected is that an electrical signal representing the

data to be collected and analyzed goes through a channel provided by the DAQ device,

then an amplifier or multiplexer will take that signal an scale it so it can be understood by

following devices that process this signal. Upon this, an ADC (or analog to digital

converter get de processed signal from the amplifier and assign it a digital value which

can be understood by LabVIEW [1]. In addition, this process can be done backwards to

generate an analog output to be understood by certain DAQ devices. Here a digital signal

is generated upon a command given by LabVIEW, this signal is converted into an analog

one by a DAC (not an ADC-look at the letters) scaled by an amplifier and sent to the

DAQ device.

In addition to analog signals, there are digital signals which may be easier to deal

with. Because this type of signal only has two values, like 0 and 5 Volts, and since it is

already in a digital format, which a computer can understand, there is no need for an

ADC to interpret this signal.

So now that we understand how data is collected through a series of signals, from

an instrument to a computer. How can LabVIEW understand these signals and produce

data which is meaningful to us or a given problem? Well, to oversimplify matters and just

spend a little more time on data acquisition rather than networking, to have LabVIEW

understand the input of any device coming into the computer the first step is to use MAX

to recognize any device you want to plug in. Then, you have to assign a number to the

device you are working on in LabVIEW so the software can recognize it and from here

everything else is just programming and smooth sailing, hopefully.

To start programming any data collection application, LabVIEW provides a

structure panel with many tools which are already developed and ready to be

implemented in a program so LabVIEW can read the desired input. This variety of tools

comes from the data acquisition panel which looks like this:

From this panel we can see the building blocks available in LabVIEW to start

building an application which makes use of DAQ devices. From left to right and top to

bottom, the displayed functions in this panel are: Analog Input, Analog Output, Digital

IO, Counter, Calibration and Configuration, Signal Conditioning, and the DAQ Channel

Name Constant which allows LabVIEW to incorporate DAQ names into any VI.

As on the following example, provided by NI’s LabVIEW tutorials [2], this flow

diagram was created with the intent of reading a continuous analog input.

[2].

As illustrated on the VI above, there are an input limits, device number, and

channel number, which serve as parameters for the function “AI 1-SCAN” which returns

a reading from the DAQ device’s input board directly and finally the VI creates a graph

of from the input received in a Cartesian (x, y) coordinate plane.

Finally, I’ll make some statements about other data collection software and

technologies. First there is SciTech from SCI technologies, which is a SCADA system

used in various industries and relies on ladder logic to control a variety of variable

frequency drives and systems relay systems. This system can actually be coupled with a

real time operating system in order to collect data and issue desired commands ion order

to control the drives which in order may control, a pump a switch, and many other

systems. Unfortunately, writing about these technologies in great extent would be the

topic of another paper.

2. Problem Definition and Solution

On today’s world it is rare for a computer to complete a variety of projects all by

itself. With the rise of the internet and communication among computers, integration of

computer resources and sharing of data is a must now. Protocols like TCP/IP and UDP,

make it possible to use standard processes which multiple platforms can use to

communicate with each other, share resources, and execute commands to perform desired

operations. Take this for example: an operator on an assembly line needs to make a subtle

change to one of the parameters in which machinery works to assemble a product.

Therefore by the use of networking and LabVIEW or comparable software, a parameter

can be changed on a machine all the way across the plant; machinery can be turned on

and off wirelessly by using radio telemetry units or data can be collected from miles

away.

Unfortunately, LabVIEW was not created with networking in mind. Therefore, it

could not be considered as one of the leading technologies in this field. However this

does not stop this technology from offering routine that take advantage of these protocols

in order to produce an extended usability and allow for certain DAQ devices in a certain

computer to be used by another one. Some of the protocols used by LabVIEW are

illustrated in the communications palette.

[2]

By glancing at this palette, it is easy to see the LabVIEW makes use of ActiveX

controls, Data Sockets (enabling socket programming), HiQ (which according to the

book don’t have any relevance to networking and I don’t know what they are for),

TCP/IP controls (which allow for a reliable transmission of data), and UDP (a lightweight

protocol that is used because its performance) [3].

[2]

In LabVIEW, many network protocols work in the same manner. They all start

by opening a connection for the desired protocol with the symbol. Afterwards

reading ad writing through such connection can be done by the read symbol and

write symbol which allow for the transfer of data. Then, of course, you have to

create a listener by which VIs can receive connections and a waiter so VIs can

standby until connections are established and data is received and finally at the end

of the VI execution, you have to close the connection created .

3. Description of Technology

So, after all this reading about LabVIEW, what exactly is it? The

Laboratory Virtual Instrument Engineering Workbench or LabVIEW is a powerful tool

designed to work with instrumentation and develop analysis software. Originally created

by National Instruments in Texas, LabVIEW is the brainchild of James Truchard, Jeffrey

Kodosky, and William Nowlin which by working on sonar applications for the Navy they

were also looking for a way to connect their equipment to the computers used at the time.

Faced with this challenge, they created an interface bus and created prototype of what

later would become a “virtual instrument” or LabVIEW project [4].

Unlike C/C++, JAVA, PERL, FORTRAN, and many other programming

languages which are mainly text based, LabVIEW uses graphical elements that represent

commands, functions, and other programming actions. In addition, by putting these

graphical elements together and connecting them with a series of wires, users are able to

develop virtual instruments by using a terminology similar to that used by scientists and

engineers. Therefore, by making use of its graphical nature, LabVIEW is a programming

language comprised of two parts-the front panel and the block diagram.

The front panel is mainly constructed out of “real things” that people recognize in

everyday life, instruments like gages, switches, graphs, sliders and other instruments

serve as the building blocks for an interface with the user.

[2]

As seen in the VI above, taken form the demos present in LabVIEW [2], it is

made clear why LabVIEW is called a graphical language. In the demo, which simulates

the vibration of a given object, it is visible that all the inputs to the VI are given through

this interface which is entirely graphical. The “acquisition rate” is placed in the form of a

slider, the “set velocity” is placed in form of a dial and a digital control, and the outputs

are given in the form of a gage and a couple waveform charts.

On the other hand, the block diagram is where the actual programming takes

place. As its name states it, this part of the diagram is composed of different blocks that

serve different functions and are connected by wires so functions can interact with each

other.

[2]

By observing the block diagram above, it is possible to see many of the blocks

that make the code for a VI, and the connections they make so parameters and variables

can be passed from one block to the next one. In addition, by means of this diagram, we

can also see that abstract concepts like loops and if statements are represented in

LabVIEW by mean of an arrow going in a circle encapsulating most of the code shown in

the diagram and if statements shown by a frame with a true on the top encapsulating

some more code.

4. Programming Example/Application

The example shown for this report was obtained from the LabVIEW tutorials, and

it is designed to get the system time from one host and send it to the other [2].

Starting with the client, we can see that the front panel is just made out of an input

to determine the port of connection and another one to determine where to connect and a

button to stop the program.

[2]

However, in the block diagram we can see that this VI starts by taking the input

provided by the “machine” and “port” fields. Afterwards, the VI tries to open a

connection and goes into a while loop in which this client VI set out to read any input

provided by the server with and displays it on the “date-time” text field. Upon pressing

the “STOP” button, the VI will exit the loop, notify the server that the client is stopping

by writing in formation into the TCP connection and finally it closes the connection.

Like the client side for the program, the front panel for the server is very simple, it

only shows the port number for connection and the number of connections established.

[2]

However, the code for the server is not as simple as that for the client. For starters, the

first step this VI takes is to recognize the input provided by the port field and then sets up

a connection listener and a waiter in order to put the program on standby until a

connection is accomplished. Upon this step, the VI enters a loop in which it gets the

system time through the symbol and writes it to a string which in turn is connected

to the TCP writer in case there is a connection. Following this, there is an “if” statement

to take care of what to do in the case there are no connections or if all clients quit-which

is to close TCP connections and not to send anything. Finally, upon pressing the stop

button the VI will exit the “while” loop close all TCP connections to existing clients and

exit.

5. Conclusion

After viewing the capabilities that LabVIEW brings, it is visible to see that this

technology brings benefits that no other provides. By using graphical elements, the task

of programming is made more intuitive so more attention can be shifted towards a

solution to a given problem rather than on coding. In addition, by being able to provide

an actual module that gives LabVIEW networking capabilities, tasks of data acquisition

and measurements can be made from miles away all with intuitive programming. Also,

by being able to interface with other programming languages, LabVIEW would have the

capability of using the networking capabilities of these other languages like C++.

6. References

[1] B. Mihura, LabVIEW for Data Acquisition, Upper Saddle River, New Jersey, 2001.

[2] National Instruments Corporation, LabVIEW 6i software and tutorials, 2000.

[3] National Instruments Corporation, Using LabVIEW with TCP/IP and UDP, 2000.

[4] R. H. Bishop, Learning With LabVIEW 6i, Upper Saddle River, New Jersey, 2001.