PC based Control .vs. PLC


Thread Starter

Don Baechtel

Are there any features or properties that a PLC has that PC based controllers do not?
Specific reasons that are affecting your purchasing decisions are very appreciated.
What could we do that would change your mind ?

Herman De Schepper

(Originally posted Wed, 26 Aug 1998)
I reply on the mail sent by Don Baechtel.
There are reasons why the PLC is still used and popular :
-The reboot time is minor compared to a PC (this can be important in some industries)
-The cycle time of the PLC can be made deterministic ; you know the sequence in which the program is executed.
-Reliability: a PLC that runs doesn't crash, you can't guarantee this with a PC
-The PLC hardware is robust + you don't have to remove a frame to exchange cards (in case of a defect). With some PLCs you even don't have to cut off the tension (voltage/power)
-The technical maintenance personel is more familiar with the PLC; this counts as well for programming as for hardware problems.
-Security reasons

In general, I think that the PC still has to prove its reliability in many industries. As long the real-time processing is more important than the information processing, the PLC is in favor.

Herman De Schepper
Technical coordinator PLC
Process Systems
Egemin N.V. Belgium

R A Peterson

(Originally posted Wed 08/26/1998)
1. convince me that NT will crash as rarely as a PLC5. I have yet to have a PLC5 crash on me. I've had dead ones, I can accept that. I can't accept random crashes where after cycling power "everything is fine". I have had numerous blue screens of death in my relatively small amount of NT work.

2. convince me that PC hardware (such as harddrives) is as reliable as a PLC5. I want something I can prom, flash prom, eeprom, or put into battery backed ram. no harddrives, no isa or pci bus. I'd accept a pcmcia slot, because its commonly used in the industrial control world and has had few problems (at least in my experience).

3. BTW, I think ASAP is looking at windows CE for certain platforms. My windows CE handheld computer has yet to crash! I think that the real PC control is probably not NT but CE longer term. NT is just too BIG. CE is about the right size so it can actually be adequately tested and proven. It also has the advantage of minimal outside interfrence in the OS. For better or worse, MS has a lot more control of CE then NT in this respect (for such mundane things as drivers).

4. make online programming available (I think ASAP is the only one that does this). Its a big nuisance to have to shutdown a machine to make a simple change. I'm amazed that any enduser would accept this for any decent sized machine.

5. Convince hardware makers to make a std line of hardware, with common features that are more or less interchangeable. The only thing going for the open s/w is that it runs on what amounts to an open hardware platform (the PC), which is a really poor choice for industrial control, regardless of how ruggedized it gets. Maybe VME is the long term solution (just maybe) in this respect, but a lot of PLC makers have made VME products over the years that just haven't taken off (other then the obvious 90-70 stuff that is based on VME).

6. Stop making the promise of saving money on either hardware or software.

There maybe long term advantages, but all the stories of money savings is so much bunk as far as I can tell (based solely on my experience with people who have made the switch or who were forced to make the switch for a machine or two). Maybe longer term there is some advantage, but I don't see it.

(I used PLC5 as an example - insert 984 if you want, or 90-70, etc.)

Marc Sinclair

(Originally posted Wed 08/26/1998)
I seem to remember that we went through this some time ago but, You ask
”What could we do that would change your mind ?”

Put the PC in a small DIN rail mountable enclosure, (10cm *10cm *25cm would do)with cage clamp connectors for all IO.
Include io onboard, maybe 24 in and 16 out and two built in RS232 / 485/ PTP, PROFIBUS ports and allow me to add on (or replace) more IO easily with no tools.
Make available a range of analogue IO at reasonable cost.
Make it easy to program and supply the programming software for free which
allows me and the customer to view and modify the program.
Make it so that I can protect the software if needed and update via small
EEPROM modules for less than £25
Make sure that the unit is available worldwide within a day so that any
breakdown or damage can be rectified quickly
Make available a range of HMI units from a simple 2 line LCD and function keys through to full colour screens with no extra programming software required.
An ASI master card / Module for less than £175 Worldwide technical assistance.
Make it cost less than £400
I get all this already (and more!!) from a plc so you’d have to do better than this!

Robert Phillips

(Originally posted Wed 08/26/1998)
My preference is PLC due to fact that they are designed and tested to hold up under 24 hour operation. As far as components go, the brand I recommend used plug in relays for outputs. These relays have a mechanical life of over 50,000,000 operations.
For the most part, you cannot get detailed specs on the pc components.

Robert Phillips
City of Wichita Falls, Tx

Davis Gentry

(Originally posted Wed 08/26/1998)
>As long the real-time processing is more important
than the information processing, the PLC is in favor.<

While I agree with some of the points made in this email, the one above is open for debate. Many highly demanding real-time processes cannot be run on PLCs due to their slow speeds and limited programming languages. High speed complex machine vision processing, precision motion control, complex machine control (semiconductor manufacturing is one I’ve had experience with), etc are examples. In some instances there can be PLCs or PLC like devices (GE FANUC CNC controllers for example) which can do some or all of the tasks, often a computer (PC or other - some VME/VXI based minis have some PLC like characteristics)can do the job better, faster, and cheaper.

Davis Gentry
Controls Engineer
Research and Development
Carpenter Company

Darwin Frerking

(Originally posted Wed, 26 Aug 1998)
I’ve been implementing PC-based and PLC-based solutions for 15 years now. There’s a place for both.
I’ve used PC-based, programmed in BASIC, Pascal, C, or VB for applications that required complex motion control, calculations, communications, and also a graphical user interface.
I have used PLCs in simpler stand-alone applications and also as I/O for PC-based systems. It is hard to beat the integrated control and I/O of a PLC.
If the PLC manufacturers would offer general purpose PC-based CPUs and intelligent motion control modules in their micro-PLCs, they would have a real winner.
You don’t need to change my mind; only offer PC-based CPU and I/O hardware packages for the price of a micro-PLC, i.e. 300-500 dollars.

Darwin Frerking, Control Engineering Manager
FAS Technologies - 10480 Markison Road - Dallas, TX 75238

David Lawton Mars

(Originally posted Wed, 26 Aug 1998)
A quick response to Hermann’s note:
<< The technical maintenance personel is more familiar with the PLC; this
counts as well for programming as for hardware problems. >>
I don’t buy this train of thought ! If there are true benefits to be had from technology, industry will buy it (eventually!). Fieldbus is a good example of this !
<< Reliability: a PLC that runs doesn’t crash, you can’t guarantee this
with a PC >>
You’ve just about hit the nail on the head ! In many industries (FMCG included, which is where I work), downtime means serious money. Robustness is a KEY consideration when choosing a platform to control the “real plant”. Okay, PC based SCADA etc for data collection, SPC etc etc., but actually running the plant? I think this is still some way off.
(Originally posted Wed, 26 Aug 1998)
Please allow me to add a couple items to those already mentioned in previous replies to your request:

(1) We need a migration path. We have a HUGE installed base of PLC’s. Let us say that we accept that PC control is the wave of the future. Just exactly how do we get from where we are today to PC based control? Specifically, what are the steps to be taken for a GLOBAL corporation to make this migration?

Want to know what PC control vendors come in and tell us? They say: “Give us a machine to do a prototype on.” Man! That is not a migration path! We don’t just go throwing any old solution on even ONE of our machines.
Want to know the other thing vendors do? They come in and show us all kinds of neat new technology. Again, this is great stuff, but what is the migration path?
Also, what’s the migration path for our maintenance people? We have a HUGE investment in PLC training for our maintenance folks. What is to become of that?
Vendors need to work closely with global corporations to develop realistic long range migration plans and systems if they expect to see corporations make such a migration.

(2) Programming standards are an issue. Basically, from what I’ve seen, the SFC’s generated by PC based software are used to generate ladder diagrams that implement the logic of the SFC. The problem which arises is that the ladder logic so generated does not conform to our company’s current (worldwide) ladder programming standards. How can we make it conform?

For example, let’s say that we use (in A-B PLC’s) B3 bits for our “fault summary bits”. How can we ensure that, if we draw an SFC with your software, the ladder generated from the SFC will use the same B3 bits for fault summaries that we would use? Or, to say it another way, how can we ensure this without effectively writing a ladder program ourselves. If what it boils down to is more work for the programmer to implement a certain format of ladder diagram through PC based control than it would take to implement the same ladder directly in the PLC, then PC based control won’t go anywhere soon.

(3) All this being said, I do think that PC based control has its place.
The big selling point of it, in my mind, is memory. We are tapping the limits of what can be done with even the largest PLC’s. This is where PC based solutions seem to make the most sense. Give us a migration path and we’ll use PC based control in such applications.

Randall Reed

(Originally posted Wed, 26 Aug 1998)
The last “real” reason i heard for plc was from an omron rep. he was very happy that day as a customer just cut a 50k order for plc. the reason was 100k of mechanical damage to a pharamaceutical line. (pc based).


(Originally posted Wed, 26 Aug 1998)
PLC is the replacement of the relay controller.It’s memory is low compared to PC.It does not has hard disk which can content a lot of memory.It does not support data acquisition,dos and windows.The PLC software is downloaded to the ROM in the PLC.The PLC software is depend on the manufacturer like OMRON, AB,Siemen and others.Now there is the company try to promote PLC Open which not depend on the particular brand.PLC is widely used in the automation industry.Now Siemen try to promote micro PLC which the price is very low.
MARDI Malaysia
(Originally posted Thu 08/27/1998)
What about price? In my system design, price is a major factor and then the amount of I/O.

Mike Esposito

(Originally posted Thu, 27 Aug 1998)
...get your head out of the sand...
-what u described would be fine for a coke machine or car wash
-but more than 2 axes of servo motion and a real hmi (recipes... communicate w/ IS systems)
-your cute little shoe box won’t come close
-don’t take me wrong...i love plc’s...however u overstate your case

1. the software is never free...in fact quite the opposite... constantly needing to upgrade $$$ as new generation plc’s and processors come out..., and portability of the software does not exist—koyo to AB and Siemens...software costs $$$
-also i’ve never seen any ab slc500 shareware??

2. plc hmi (anything over 2 line display) also requires some separate ($$$) software
-another unique programming language which is tied to that model hardware
-just had terrible experience w/ Siemens interfaces and which rev. of software i was using
-even worse were the compatibility problems w/ my Siemens eeprom programmer
-it liked one cpu but not the hmi ???
-changed its firmware...it liked the hmi but not .... u guessed it ...the cpu

3. by the way...pc’s are avail in rack mount...various types std, din rail box, vme....many others

Again i’m not saying PC’s are better than PLC’s just different....
-also PLC’s CAN”T do motion and they do a s*cky job w/ HMI
-however they are GRRRReat w/ I/O and ease of use, durability is super.
(Originally posted Thu, 27 Aug 1998)
Industrial PLC’s are far more robust in their processor operation and are not prone to the need to reboot as PC based controllers are. They are also usually installed in a control system, for the most part, as the sole control for that system with a PC acting only as the HMI interface with that system. This allows the user to run other applications on the PC without the fear of losing control of the system in the event of a PC failure or an application failure. Each control system application has it’s pros and cons as to whether it should be operated with a PLC control or PC based control. Personally, if I am only performing data acquisition functions on a non critical system, then PC based control is acceptable. If I am performing building control for a critical computer room support application I would prefer the PLC controller.

Steve Preslin

(Originally posted Thu, 27 Aug 1998)
The PC may be the way for a small company with a DIY engineer. I can’t imagine running an Electrogalvanizing line on PC based stuff. If your PC takes a dump or starts having mysterious problems, who you gonna call? AB and others have years of experience and generally can get you through a problem with a phone call. Parts are also easy to come by.
Thanks, I’ll stick to PLCs
Try to tell a production manager how much money you saved by using “Joe Blows Turbo PC” for your control when its down and you can’t get parts or tech support.
Steve Preslin
Metro Metals

Frank Powell

(Originally posted Thu, 27 Aug 1998)
I use PLCs because they are being used in a mobile application and are under extreme vibration and shock conditions. I didn’t want to deal with hard drives and all of the connections in a PC. (I could have used flash RAM) I already had somewhat limited experience in ladder logic and none what so ever with C or C++.
I also had a requirement of using 12VDC power for the system which was quite limiting.
I will be interested in seeing Real Time Windows CE (flame shields UP!!) and all of its possibilities. Especially since it has built in flash ram, VGA for displays, and communications (serial and email).
Frank Powell
Mechanical Engineer
California Pavement Maintenance
9390 Elder Creek Road
Sacramento, CA 95829

Andy Piereder

(Originally posted Thu, 27 Aug 1998)
I would like to add that many automation applications simply do not require even all the features and power of current PLC models. The low price of many PLCs often means that many PLCs are ‘oversized’ for the applications they are implemented in.
In view of this, PC control in its traditional sense would seem to be destined to remain a specialized, high-end application. On the other hand, new offerings that blend OI functionality with controllers and CE based OSs offer the potential of real cost and space savings as well as programming and communications flexibility.
As has been historically the case, new technologies don’t replace old ones, but simply broaden the palette of solutions.

Andy P.

Timothy P. Gabrys

(Originally posted Fri 08/28/1998)
Price can vary between PLC’s systems as well as PC based. Some of the Micro or brick PLC’s are inexpensive ($150.00-$300.00). Larger PLC systems can run as high as $8000.00 for the processor alone. Price can vary based on I/O requirements as well as processing capabilities. You typically have to use the micro PLC manufacturers proprietary software to program these systems.
PC hardware can also be inexpensive especially if you look at some of the embedded microprocessors on the market. These systems typically have no moving parts (flash disk and RAM) and have shock and temperature specifications far superior to the traditional PLC. These embedded PC’s are used in the transportation (on the vehicle) as well as gas and pipeline industries where shock and temperature swings can be severe. Prices of these microprocessors can range from ($300.00 - $1000.00). DOS is used as an operating system in a majority of these type applications. We have customers that still use 386 processors for their applications because of the low cost. The interesting point here is that in some cases the 386 processors out perform their PLC counterparts in program execution speed. Using embedded processors also open up your options when choosing software. Their are packages available out there that are operating system independent and offer you the choice of programming language. These packages come in many flavors (Object Oriented, IEC1131.....).
For more info on embedded PC’s check out (http://www.octa.com)
or contact RTA at (414) 453-5100

Darwin Frerking

(Originally posted Fri, 28 Aug 1998)
Marc is talking about his concept of a PC-based system that would meet his needs; not about PLCs. He is pretty close.
For example, what is really needed is for someone like PLC Direct to get Koyo to develop several PC-based CPU modules for their Direct 205 PLC. The high end would be Pentium based with 64/128 MB of memory. Then get MEI to develop DSP-based motion modules for the same PLC. And offer it all at reasonable prices: $300-800 for the CPUs and $200-300 per motion axis.
They would have a winner and we would have the processing power and hardened I/O for the majority of our applications.

Darwin Frerking, Control Engineering Manager
FAS Technologies - 10480 Markison Road - Dallas, TX 75238
(Originally posted Mon, 31 Aug 1998)
>>There are reasons why the PLC is still used and popular: - The reboot time is minor compared to a PC (this can be important in some industries)<<

Depends on the BIOS ... most good IPCs have a BIOS option for fast boot.

>>- The cycle time of the PLC can be made deterministic; you know the
sequence in which the program is executed.<<

Use an IEC1131-3 environment and a RTOS like QNX and your PC based solution will be more predictable than the PLC solution.

>>- Reliability: a PLC that runs doesn't crash, you can't guarantee this with a PC<<

Interesting ... a PLC that runs doesn't crash?? Why??

>>- The PLC hardware is robust + you don't have to remove a frame to
exchange cards (in case of a defect). With some PLCs you even don't have to cut off the tension.<<

With our PC and fieldbus based solution you can exchange the IO modules at run time! (as long as a clean bus topology is used .. e.g. like PROFIBUS or CAN ... )

>>- The technical maintenance personel is more familiar with the PLC; this counts as well for programming as for hardware problems.<<

With IEC1131-3 and the usage of fieldbusses. I don't see any differences ...

>>- Security reasons<<

Well chosen embedded PC hardware has the same quality as the PLC hardware.

>>In general, I think that the PC still has to prove its reliability in many industries. As long the real-time processing is more important than the information processing, the PLC is in favor.<<

A PC solution under QNX gives you a better real-time behavior and the possibilty to use the information processing and communication features ...

Armin Steinhoff