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Nathan

Your Opinion On Pc Based Plcs

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I'm curious about the viability of PC based PLCs in general. The major drawback that I'm aware of is that PLCs will let you edit the program while it's running. Are there PC based controllers that don't require the traditional "compiling" that will interrupt the process? I'd like to hear opinions on different brands. ---- Nathan Boeger Integrator, Microsoft Certified Systems Engineer Inductive Automation "Specializing in SQL and web based HMI systems"

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In my opinion, the major downside of "soft" PLC's is that they run on Microsoft Windows, and this OS is not stable enough. Who wants to be in the middle of some critical process operation and get the dreaded "blue screen of death". Yes, it's improved over the years, but it's still not ready for prime time. On the other hand, a unix based system might work, it would depend on the platform and the hardware used.

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Another downside of the PC based control is the lifespan of the hardware compared to a PLC. Yes, you can find the used parts on Ebay, but you will have a hard time finding new components for a 20 year old PC system

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with performance levels of todays plcs, i see little need for soft plcs. in my opinion, soft plcs may have place but only in cases where for example: - downtime is not critical (it is not the same thing to shut down 15min. line with 5000 workers and small research lab with 2 techicians who play minesweeper most of the time anyway). - extra memory is important (loads of recipies, cam tables etc.) - extra speed is important (so far even fastest plcs have to deal with scan times of 1ms magnitude, while with PC this should not be the problem, at least if using RTOS) - loads of complicated calculation is required. - high speed sampling / data collection - space is constraint so there is no room for both PC and PLC and PLC alone is not suitable for job.

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We have a PC controlled machine that we replaced with a Controllogix. Aside from it having problems Bill Gates Rolex (hour glass) getting stuck on it atleast 10 times a day and requiring a reboot, the operators played games all day instead of working. I don't know much about Unix so this might not be a problem with it.

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I've used a couple of PC Based Control systems with Beckhoff PC (Win2000) and TwinCAT. The first one was comissioned in 2002 and still working fine. I didn't see anyone playing games on the PC, but this might be because there in no keyboard or mouse, we just have a touch screen with a VB6 app running as a front end. I think the future is PC Based Control and only when you work with traditional PLC and soft PLC you can see the difference. I'm working now on a project with RSLogix5000, and there is a big difference, believe me... There are also other software that runs under Linux or QNX (like CoDeSys) so there is hope for those that are affraid of Bill Gates. -CoS

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I think I need a definition of a "traditional PLC". Thats a pretty broad term.

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In my opinion, a traditional PLC is a PLC that uses the '90 technologies (hardware and software). I would include in my list SLC with RSLogix500 software (without tag-based programming capability), Mitsubishi PLC with GX Developer 8.12 (an extremely limited DOS software running under a Windows OS and also I found it to be difficult to use for special IO configuration), and there might be some other PLCs, but I didn't use them so I cannot put on my list. There are also some advanced PLC, like ControlLogix with RSLogix5000, and top of the list Modicon with Concept 2.5. The major limitation that I see on a PLC (without an OPC server or some other interfaces) is that you cannot have some basic features, like: data logging, data storing, web page, retentive data, partial download, reusable code (excluding Concept), OOP (excluding Concept), and so on. - CoS

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There is no right answer to this question. Just figured I would bring a few points up. Shouldn't you compare a 90s SLC (actually 1989) to a 90s PC based software? Data logging, data storing, web page, and retentive data are on most of your new PLCs Partial download and reusable code are on some

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I certainly know that you can do data logging to compact flash, data storing (compact flash), retentive data, Online editing, reusable code, user defined function block progamming, on a basic CJ1MCPU11 around $AUS500. Would you regard user defined function blocks as OOP (Object Oriented Programming) ?

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Changing a program on the fly while a PLC is running is also not possible for a lot of PLC’s. I use softplcs from Sigmatek. It is running on a realtime OS it is called ‘ON TIME’ I think. The development software is object oriented. The code is ‘structured text’, IL, ladder or C (Debugging in C is almost impossible). Other softplc that I know of are Beckhoff (already mentioned) and Bachman. Bachman softplcs are often used in windmills the OS for this softplc is VXworks the development is Codesys. In general I think plcs are more reliable than softplcs and softplcs are a bit cheaper. Henry

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Dear Nathan: I must say that after 20+ years in programming PLC's and Industrial COntrols, I fall strongly in the PLC camp for control. Most PC based systems either run on the Windows OS {a large neagative} or if they have their own kernel {Entivity VLC Steeplechase} are severly limited in what online editing you can do. BobLFoot

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I'm not saying that there are no PLC that are capable of doing some of the functions that I've mentioned in my thread (a good example is Modicon with Concept), but must of them are limited in this regard, and I'm talking about big players in the field (Allen Bradley, Mitsubishi). DFBs are still a dream for most of the PLCs... - CoS Edited by cstoica

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IMHO FBs would only be the equivalent to functional programming in the computer world. They CAN be used to do object oriented programming (as any language can - object orientation merely is a programming philosophy, so you could theoretically program object orientated in assembly), but that doesn't really make them any more object-oriented than C, a classical functional language, which indeed has been used for object oriented development (Gnome/GTK as an example). Arguments for my position: please read the definition of object-oriented programming at http://en.wikipedia.org/wiki/Object_orientation . PLC programming in general is far from that, and partly, that's actually a good thing (normally it would be unnecessary abstraction - making the system more complex than necessary). To have an understanding of the principles is a very good way to get a possibility to be able to write better programs - but part of understanding principles is also understanding when it is okay to break them. That object orientation would add complexity in PLCs actually is a bit self-contradictory, as one of the objects of introducing object oriented programming was to reduce programming complexity. (!). The explanation I think it's unnecessary complexity for PLC programming is that PLC projects very seldom are huge - projects not being understandable by one single person would be more or less unheard of. Compare that to your general computer code monkey scenario: loads of cubicles for a single application, where many different parts of an application are put together by different programmers. PLCs rarely have more than a MB of memory - computers already have several gigabytes. The added performance comes with a downside - more complexity - which has been fighted with OO techniques. OO reduces complexity - but isn't really necessary to use a language with specific OO language constructs to be able to use OO as design philosophy when developing a program.

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I've also used Beckhoff TwinCAT quite a bit over the years as PC based and as PLC based. The best feature for those that are unsure about PC based is that you can use it as a PC based control and if you decide the reliability is not there, then switch over to and non-PC based CX1000 and keep the exact same program and IO hardware. Beckhoff even allows you to download TwinCAT on a 30 day trial that will function fully as PC based control. I tend to favor Beckhoff's real time ethernet IO. It works the same with PC based and with a CX1000. If you write a VB front end for the PC based control, the front end will also work with the CX by only changing one address. For anyone interested, I have developed an HMI template based on Visual Basic Express 2005 for the Beckhoff TwinCAT system. Visual Basic Express is a free download from www.microsoft.com. The template is written to be able to have the application installed on any PC on the same network as the TwinCAT controller and not require any setup (e.g AMS Router entries) or separate driver software loaded. I have written methods that allow a lot of things to be done with only one line of code for each variable/tag. The best part..... it is a totally free HMI (no hidden costs).

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Brian (esys) I received your email and attempted to respond, but it came back as a failed delivery. If you can send me another email address, I can send you some things.

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BeckHoff TwinCat sounds interesting, how does it do with online editing when running.

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TwinCAT handles online editing quite well. To perform an online edit you would logout, make the changes, then log back in. It will then prompt if you would like to make the online changes. If you wish to keep the changes, you then create a boot project from the online menu. This basically writes the changed program into flash memory or disk so that it is used the next time the controller boots. You also have the option to download source code to the controller. This allows anyone without the original program file to upload with all documentation and go online.

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It sounds like we define Online Editing differently. For me from the PLC perspective if I have a running machine making parts and on a given rung one contact drives one coil I can change that rung to an or of two contacts driving an oring of two output coils. And i can change this while the process is running and without rebooting the PLC. Does your rebooting your TwinCat mean it must stop running and making parts?

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After so called online change TwinCAT does not need rebooting. In order to make online change you have to leave online view (Beckhoff call it logout). After editing the program you have to login back. Then changed program (in fact not all but only so called delta code) is loaded to the TwinCAT PLC and takes effect immediately. However if the boot project isn't created the change will be discarded during next TwinCAT restart (as the old boot project will be loaded).

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HI I know it's a bit late in this discussion but I thought I would throw my tuppence in ref PC PLC's. We use a version here in all our control systems for exactly the opposite reasons others have mentioned. We design and build underwater robots and the product used is from a company called Softplc www.softplc.com which we have over 12 years experience with. It does allow live editing, the only thing you have to stop the PLC for is to save the edited code. It is fully compatible and has all the features of Allen Bradley PLC5 including a range of protocols for comms including DF1. It runs on any Dos, is compact (obviously) allows data logging, storage of setups in data files, Full web server capability with an embedded TCP/IP stack, retentive data (Kind of, you need to deliberately save the datafile) and reusable and extendible code. ie If there are any major routines used regularly, (like for us, a decent analogue filter for sensors), we write it in C code and build it into Softplc as a new instruction. It's as rugged as hell which is just as well for us and we don’t have a problem with obsolescence BECAUSE it’s based on PC architecture. We still support systems built over a decade ago by using newer and faster boards. (All our stuff is PC104). We occasionally get bitten on the bum by some old code that expects a slow machine (That’ll be Moore and his sodding laws). As time progresses, the electronics get smaller and convergence pushes more functionality onto the same board which means we occasionally have to write a new driver but again it’s no big deal. The biggest reason for us to use Softplc is that the hardware is tiny compared to the equivalent PLC. (all our electronics has to go inside pressure housings.) Another biggie is that I can write an application in a couple of days for a new robot and then use a SCADA package to create some cracking control screens. I can’t do that with C code. The code can also be understood by others. (ever tried to read someone elses C code….ugh) The only problem with SoftPlc’s in general is that the I/O cards from most manufacturers don’t incorporate proper isolation so we had to create a set of PC104 boards to do that for us. With underwater kit, keeping water out of connectors is impossible so the isolation cards prevent the expensive brains of the system from blowing up. We considered moving to PCMCIA cards for the product but although the electronics is there, the connectors on the cards are shoot so we are sticking with PC104 for the next few years. The reason I came to this site was to find information on an obsolete PLC used in a Russian Crane. You may not like PC architecture, not many do but at least it will still be around in 20 years time even if it is a bit smaller.

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DOS based - interesting. What kind of power supplies, etc do you use with it. I like the idea of these (PC based PLCs, not specifically Softplc), but like BobLFoot, I'm placed firmly in the PLC camp for critical controls based on the actual, real world implementations that I've had experience with on both sides. Keep the learnin's flowin'... (in response to Ken's early post) - I agree with your comment - I would hardly consider a PLC emulator running on top of Windows to even fit into this category. Using that for industrial control would be begging for chaos. Feel free to prove me wrong if there's a solid product based on this idea for that purpose. I doubt it. Edited by Nathan

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Sounds like the Softplc works nice in your application. Once a system is assembeled how often do you edit and change it once running. It doesn't sound like it would work in a running factory. You can't shut the line and plc down to save edits during the production week.

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Hi, From preference i prefer real PLC's on grounds of reliability but 15 years ago I worked for a company that had many rack mounted PC controlled automatic test jigs running microsoft quick basic - never any reliability problems - and easy to code - execution time was fast enough too!

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Quick Basic. OMG! At least a simple program should be easy to test.

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