PLC vs PC based control


Thread Starter


I am a student currently doing my project.
My project is mainly concerned in automatic feeding control system.

But I am kinda confused in choosing whether to use PLCs or Data Acquisation card(I/O card).

Frankly speaking, I am not familiar with PLC and DAQ card. What are the differences(advantages or disadvantages) between those two systems?

Could anyone please help me or give me some advices?




As a student, you have some freedoms and some restraints that aren't typically addressed on this list. On the first hand, you have no longterm expectations about reliability and spare parts--- once you turn in this project, it's over. On the other hand, industrial projects typically have funding appropriate to the task at hand--- and most students have little funding.

You can successfully demonstrate automatic control using PLCs, PCs, custom electronic circuits, microcontroller boards, and lots of other alternatives; your biggest constraint is what is exactly available to you. They'll all work!

I think you should talk to your instructor about what is available to you for doing this project. Since lots of things will work, the best choice for you is probably the best thing recommended by your instructor.

Hope this helps!

Larry Lawver Rexel / Central Florida

James Ingraham

Let's start with your background. Are you a computer guy? Can you program in VB or VC++? Do you have a copy of VB lying around? If the answer to all of those is "yes", than PC based control with DAQ is a good choice. If you have direct experience with control software (e.g. LabView) use it.

Do you have an electrical background? Can you read electrical prints? Have you never worked with PC-based languages like Visual Basic before? Can't swap out a motherboard? PLC is your choice.

Either way, you have a hell of an uphill climb in front of you.

James Ingraham
Sage Automation, Inc.

california bob

No, No, No. That simply is not true and is not a fair statement at all.

Read - IEC 61131-3 - it doesn't care whether a 'PC' or a 'PLC' is involved. It is what it is - a fantastically well-thought out control platform. Oh, and it includes ladder! You're not stuck in obscure .NET (or earlier) land; but you can be if you want to. It is your CHOICE.

IEC 61131-3, IEC 61131-3 - you're heading the wrong way intellectually and career wise if you do not even give it consideration.

Robert B. Trask, PE
Los Angeles, CA USA
[email protected]

Michael Griffin

Do you have a few good suggestions of where to find complete, working IEC 61131-3 development and hardware (or run-time systems) at a price suitable for typical student projects (e.g. zero)? If so, the rest of us would really like to know.

There is a difference between education and training. I believe the project is to learn to design an automatic feeding system, not to learn a specific software package.

The student who wrote the original question had a fairly specific request. What were the pros and cons of using a PLC versus a DAQ card?

- A PLC is more expensive than a DAQ card (I assume you actually mean a digital I/O card). However, if the digital I/O (DIO) card requires external solid state relays to interface to the control voltages being used, then the DIO card plus extra devices may in fact be more expensive than a small PLC. You should make a list of what types and how much I/O the projected design will need, and then compare this to what sort of hardware you will have available for this project.

- The DIO card also requires a dedicated computer plus OS plus some sort of application software (the software you are going to use to write the control program with). The PLC requires a computer to be used to program it, plus special proprietary programming software and special programming adaptor. The special PLC programming software and adaptor may cost several times more than the PLC itself. Some small PLCs are programmed using a hand held box rather than a computer, but these are suitable only for fairly small programs.

- If you are familiar with programming PLCs, you can write the control program much quicker than if you are using a general purpose programming language on a PC. If you are not familiar with programming PLCs, then you would have to learn that first, which would likely result in a much greater overall effort spent on the software portion of the project than if you had used a programming language you are already familiar with.

- Most PLC programming languages (e.g. ladder) inherently function in a manner which is "multi-tasking". That is, each "rung" or "logic network" is processed independently of the others, which allows you to easily control various independent devices simultaneously. This can also be accomplished with conventional computer programming languages, but with more effort.

- PLCs are typically used in common industrial machines where the people applying and maintaining them require little or no actual computer expertise, but can instead concentrate on the machine itself and the logic to control it. Most industrial machinery falls into this catagory.

- DIO (and DAQ) cards are typically used where a computer has to be part of the system design anyway due to the need for complex algorithms, high speed data acquisition, high speed data processing, large amounts of data storage or retrieval, a complex user interface, or various other factors not easily provided by a PLC. In industrial applications these characteristics are often found in production or laboratory test systems.


Michael Griffin
London, Ont. Canada

Curt Wuollet

And if you need something cheaper than either. I have free PCB artwork that will work with a Measurement Computing DIO48 card (about $100 US) and give you 24 optoisolated 24V DC inputs and 24 optoisolated .5 A DC outputs for a PC. Cost works out to between $3 and $4 a point having the board fabbed at a typical proto house. I can provide programming examples for Linux (which further lowers your cost) up to and including a primitive PLC. This was designed for the lowest possible cost for Industry Standard IO levels and eliminates the somewhat costly SSRs other PC interfaces use. It has been running in the real world for several years now. If you can't find it at, let me know and I can email it to you.



tribhuwan joshi

According to me you should use the PLC's because it is easy to use and it's programming is also easy.In it u have to use only the logic gates and timers and counters which is i think easy for you.
By it also you come to know about the ladder programming and the use of STL lang
In it u have PID control also.


california bob

Download TwinCAT PLC. It is a full 1131 package, not a demo, and runs for 30 days. After that, just reload it again. The assumption of Beckhoff is that one would never ship a machine like that.

There is no distinction between a 'run-time' and 'development'. It's all one thing. It'll turn any NT4.1SP2+, 2000, XP machine into a PLC.

Robert Trask, PE
Los Angeles, CA USA
[email protected]
An important consideration is how far you have to go with it. Do you just have to show a program and some simulation logic, or do you have a bench scale unit that you have to wire up to I/O?

The freebie PC control software mentioned elsewhere is a good (cheap) option if you just need something to program. To make it actually work, I think you'll be better off with an Ebay PLC. Dirt cheap, and your effort to get it working will be much less than with PC I/O, as the parts just plug in and work (assuming basic I/O modules).

Most PLC brands have copy protected programming software, but some are more lenient than others with 'evaluation periods'. Before you buy anything, make sure you have a way to program it.

Michael Griffin

Your web site states "A non-commercial version of OATs is available for universities and private educational usage." Is this version fully funcitonal, or are there technical limitations to it as compared to the commercial version?


Michael Griffin
London, Ont. Canada

Friedrich Haase

Moin Mr. Griffin,
moin all,

A downloaded project has an execution time limit of 30 min. Enough for most experimental plants as long as they don't need long durations of heating or cooling. And the C/C++ interface is currently not available for the
non-commercial version. This might need to change to allow building high performance function blocks to access I/O boards.

The programming environment itself is fully functional.

Please note, that OATs only relies on the IEC 61131. Many other vendors have e.g. ladder diagrams which still keep their historical versions intact. I have successfully used OATs to build Siemens S7-400 projects in Structured Text (mostly a cut and paste job.) I would not expect this for ladder diagram for Siemens, AB or else. Sometimes OATs has more features as the usual PLC implementations, best visible in SFC. This also could reduce portability.

From an educational viewpoint this might be favourable. But OATs is not a cheap replacement for a PLC/vendor specific programming platform.

Hope this helps.

Friedrich Haase