advantages/disadvantages of PLC vs. PC control


Thread Starter


I'm a Controls student at Fanshawe College in London, Ont. Canada. As a requirement to complete a technical report class, I am searching for information regarding PLCs/PCs. Originally, I chose to look into the "best" manufacturers of PLCs on the market today (naive I know). However, upon my preliminary research I discovered (much to everyone's surprise I bet!) that this topic is far too broad to ever be possible to produce in a 5000 word report. The sheer number of PLC manufacturers and diversity of their products are completely overwhelming to deal with my proposed scope. So, my plan is to keep researching as much as I can in the next while and hope that I can come across a more
refined angle to tackle this project... or look into the PLC vs. PC debate. I noticed from the message board that there is quite a heated interest in this issue and started thinking to myself that this might be a better topic for evaluating. I realize that this topic is also
very widespread, but I am hoping that maybe by studying the general advantages/disadvantages of each platform I can compare the two systems reasonably well. I think it might be better to compare PLC vs. PC control, than AB vs. ABB vs. Modicon vs. Omron vs. Siemens etc etc etc. If anyone has any information regarding said topics, personal experience, possible new topic ideas, or any input in general, it would be greatly appreciated.

Michael Griffin

Whether you are looking at comparing two different PLCs, or PLCs to PC soft-logic systems, "better" is not meaningful except in the context of an application. DIfferent applications have different needs, and I don't think you are in a position to judge what is "best" for each.

If you are looking for a subject to write about, perhaps you might consider showing how several different types of industrial PC systems could be put together entirely from open source software. You could define several arbitrary applications and show what software is available for each, and how it could be configured. I am sure that there are several list members who could direct you to some suitable sources of information on this subject.

The reason why I suggest open source software is that if you decide to pursue the subject to the point of setting up a system, the price (free) fits your budget (which is none). This is a point which needs to be kept in mind with student projects.


Michael Griffin
London, Ont. Canada
Try this approach, take one manufacturer, such as AB that offers both products, a PLC (Logic5000)and a soft PLC (Logix5800) that runs on a PC, and compare suitability for a particular application. AB has a number of white papers
available online at "":

James Ingraham

Not too long ago I switched my company from one PC-based control solution to a different one. To make the decision, I implemented a skeleton of one our more common software problems in each of the options that I had. This goes along with what the others are saying about comparing based on the application.

I say, DO SOMETHING. Pick a few platforms. I might suggest Allen-Bradley ControlLogix, Entivity's Think & Do, and CTC's QuickStep. This
gives you 3 different languages on 3 very different platforms. Then try a few sample applications. Possibilities: (1) A conveyor is
running at constant speed; when a product enters the system, randomly assign it a lane; when the product reaches it's lane, divert it (i.e.
trigger an output). (2) Using PID with an analog output and a thermocouple, maintain a constant temperature on a heater bar. (3) Calculate
all the primes up to 32767. (Don't laugh! Sometimes mathematical manipulation comes in handy. This is a canned, well-understood example
that tests the limits of a controller.) You might like calculating digits of Pi better. (4) Use an indexing conveyor to move multiple parts through an assembly process.

These are all "thought experiments" that can be programmed into a controller without ever having to have the equipment; you just simulate it. Even if it doesn't really work, it gives you an idea of what you're up against trying to really get it to work.

It's a thought, anyway. I had fun just thinking about it, so thanks!

Sage Automation, Inc.
[email protected]

Roderick K. Duet

The management of a PLC is better suited to the engineering staff of a plant than a PC based control system. I've worked in the field now for seven years, from conception to startup of process control systems, even the packaged HMIs are sometimes over the head of most plant personel. I as part of these control solutions would rather have the plant personel able to take care of program additions or changes than get phone calls. Most plant personel are afraid of PCs, in my experience, or are affraid to screw with the investment made in the original