tbolsh
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Re: Turn Raspberry Pi into a P.L.C (Programable Logic Controller) Advanced

Mon Oct 10, 2011 7:13 pm

In general, I assume that for this project (PLC) we will need to find out how to work with watchdog, how to work with means of communications (I2C, SPI etc) - harmless stuff. If Broadcom care for something to be "not disclosure" it should be rather description of their Graph processor - not the standard stuff I just mentioned.

So, it might be a good idea for them (I mean generally good idea) to make "public" and "NDA" versions of their SoC description to make themselves friendlier to user community and still keep the desired level of security. This way they also will decrease the amount of requests to their customer support.

tbolsh
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Re: Turn Raspberry Pi into a P.L.C (Programable Logic Controller) Advanced

Mon Oct 10, 2011 7:14 pm

P.S. GPIO pins we might need as well.

thepanoguy
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Re: Turn Raspberry Pi into a P.L.C (Programable Logic Controller) Advanced

Tue Oct 11, 2011 6:43 am

It is very obvious from the comments posted in this thread that the people driving the use of the Raspberry-Pi as a PLC training tool have not conducted any professional, electrical and electronic engineering research into the design criteria required for an educational PLC or are prepared to acknowledge the legally required European Union safety compliance to IEC61508 and IEC 61131-3.

A PLC’s functional criterion is an industrial control device used to automate electrically driven machinery. The objective of any real world PLC design and training criterion is to write and test PLC programs compliant to IEC 61508 and IEC 61131-3. Being able to simulate watchdog timer failure in both software and hardware to IEC 61508 and IEC 61131-3 in the classroom educational environment is also paramount to any professional PLC training course.

Therefore; for classroom training purposes, a PLC used to emulate real world functionality requires three levels of watchdog compliance and the provision to connect the PLC to real world electrically independent failsafe simulation circuits using a low voltage power supply, usually 24 volts AC and DC.

First Level – An IEC61508 and IEC 61131-3 complaint machine language operating system containing a software watchdog timer programmed in pure machine language and burnt in a non-volatile ROM. The purpose of the CPU watchdog timer is to monitor all CPU memory functions for checksum errors and watchdog failsafe on any error generated.

Second Level – An IEC61508 and IEC 61131-3 complaint PLC will also contain an external electronic hardware watchdog circuit independent of the First Level operations. The second level watchdog’s purpose is to monitor the PLC input and output (I/O) electronics. In the event that any I/O device malfunctions, the secondary watchdog de-energises all PLC outputs.

Third Level – An IEC61508 and IEC 61131-3 complaint PLC program and PLC associated electrical interlocking circuits. This is the real world functional design.

Fourth Level – The fourth level contains electrical interlocking, failsafe safety wiring not attached to the PLC. The purpose being that in the event of any emergency all electrical equipment attached to the PLC is safely de-energised. Once de-energised; all the electrical equipment must be manually checked for electrical faults and re-energised.

Level 1 PLC programming is always written in machine language. Level 1 and Level 2 electronics functions are tightly integrated into the PLC electronics design. Level 3 is where the PLC is programming and field testing is conducted. Level 3 and Level 4 electrical interlocking wiring electrical wiring is purely the domain of electrical engineering and commissioning professionals.

If IEC61508 and IEC 61131-3 compliance cannot be programmed into the Raspberry-Pi then all you have is a programmable simulator that will not teach a person seeking PLC programming experience any serious PLC programming. Those same PLC simulators are freely available for the Windows Operating System. Therefore; I cannot see any value in investing money in a Raspberry-Pi PLC project when I can carry out the same PLC simulation on a Windows PC.

References:

PLCopen: http://www.plcopen.org/

Safety Integrity Level (SIL)
http://en.wikipedia.org/wiki/S.....rity_Level
ANSI/ISA S84 (Functional safety of safety instrumented systems for the process industry sector)
IEC EN 61508 (Functional safety of electrical/electronic/programmable electronic safety related systems)
IEC 61511 (Safety instrumented systems for the process industry sector)
IEC 62061 (Safety of machinery)
EN 50128 (Railway applications - Software for railway control and protection)
EN 50129 (Railway applications - Safety related electronic systems for signalling
EN 50402 (Fixed gas detection systems)
MISRA, various (Guidelines for safety analysis, modelling, and programming in automotive applications)
Defence Standard 00-56 Issue 2 - accident consequence

The use of a SIL in specific safety standards may apply different number sequences or definitions to those in IEC EN 61508.

IEC 61508 Safety: Safe software solutions according to IEC 61508 (SIL2 up to SIL3)
http://www.kw-software.com/com.....y/2990.jsp

SafeOS: Safe PLC runtime system according to IEC 61508 to SIL3
http://www.kw-software.com/com.....y/3059.jsp

Training Specification Sheets:
http://www.kw-software.com/com.....ad/431.jsp

ICE65108 Safety Conformity:
http://www.tuv-fs.com/HH80661TT.pdf

Quantum Safety PLC Safety Reference Manual:
http://www.tuv-fs.com/33003879.....000_00.pdf

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Gert van Loo
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Re: Turn Raspberry Pi into a P.L.C (Programable Logic Controller) Advanced

Tue Oct 11, 2011 7:30 am

Although I agree that safety is an important issue, I don't think any of the above prevents using the Raspberry Pi for PLC programming education. As with any teaching you have to go through a lot of basic stuff before you can do some serious work. I did three years of electronics before we got around to NEN1010 (The Dutch safety regulations for installing electrical equipment). In fact I don't think the Raspberry Pi is a good tool for PLC, it never was designed for PLC, but it is cheap and it can probably keep you going for a long time. The major goal of the Raspberry Pi is to get people enthusiastic. After that: anybody who wants to have a job in PLC should definitely do a serious follow-up coarse which I assume will teach the safety aspects.

vorrias
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Re: Turn Raspberry Pi into a P.L.C (Programable Logic Controller) Advanced

Tue Oct 11, 2011 11:25 am

thepanoguy I agrree on what you write. It looks that you are a pro in the subject and I am glad to expunge all that info with people like you. I did not realize when I first started the thread how much work is need it and most important that many technical matter is so difficult to obtain. Looks to me that Broadcom will not give us any info and at the end we will wast our time asking this and that in a time that other companies like Atmel have better CPUs to run a PLC on them (and use the R-Pi as an HMI controler).
May be bad idea in the first place becouse even for educational purposes hardware MUST HAVE all the pro safety features.
So I will first wait for the R-Pi to come out and see some I/O boards. Then I came and see if it is worth it to spend my time making R-Pi a PLC.
Meanwhile for controlling projects Arduino remains very high in scale. The new boards with ARM are even better. So having a little free time I want to spend it wisely.
May be I was running very fast and not seen all the problems. But never the less the exiting thing is that are people like you out there having the spirit of creating. This is fantastic

pmario
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Re: Turn Raspberry Pi into a P.L.C (Programable Logic Controller) Advanced

Wed Oct 12, 2011 12:48 am

Hi folks,
I'm following this topic for a wile now. It's quite interesting. I didn't find any links to open source fieldbus software yet. So I'll add some [1]:

With openPOWERLINK [2] you'll have a full software stack for a RT Ethernet communication for Managing Node (master) and for the Controlled Nodes (slaves). Seems to be BSD lizensed.

openCONFIGURATOR [3] is an open-source configuration tool. Seems to be BSD lizensed.

With openSAFETY [4], you'll even have the possibility to create safety functionality, if you are able to do so. Seems to be BSD lizensed as well.
--------
IMO for teaching purpose, it is allready interesting, to let a student implement this type of protokoll to the R-Pi. If they succeed, fine. Learned something, about a state of the art industrial fieldbus.

Watchdogs are common for plc's. But if you have none, buy two R-Pi's and let one of them be the watchdog for the other. Proof, that it works and that it is sufficient. If that's not enough, let both of them watch the other. Maximum hw cost for a watchdog is the price of an additional R-Pi + wiring.

[1] http://www.ethernet-powerlink.......php?id=11
[2] http://sourceforge.net/project.....powerlink/
[3] http://sourceforge.net/project...../openconf/
[4] http://www.ixxat.com/ethernet_.....opensafety

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Burngate
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Re: Turn Raspberry Pi into a P.L.C (Programable Logic Controller) Advanced

Wed Oct 12, 2011 11:08 am

Had a situation years ago, two boxes,each watching for a heartbeat from the other. If one saw the other's heartbeat went missing, it shut off the power to it. Trouble was, start first one up, it saw no heartbeat so it powered off the second before it could get going. Worked on paper, not in real life.

RichC
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Re: Turn Raspberry Pi into a P.L.C (Programable Logic Controller) Advanced

Wed Oct 12, 2011 12:41 pm

Sounds like a nasty real-world problem that students could learn from :)

I've put a few thoughts in a wiki http://rpiplc.wikispaces.com/Hardware (as provided by ErvKosch, thanks).

I think use SPI for the I/O bus because it's faster than I2C, and makes the I/O boards easier than USB. (For USB the I/O boards would need a chip like FTDI232 for USB-to-RS232, but the I/O chips like ADC and DAC use I2C or SPI, so now you need a PLD to convert the signal)

Real I/O board hardware needs electrical isolation to avoid damage from people accidentally connecting voltages and from ground loop current. As a minimum you need 1kV isolation. That means a 10mm gap on the PCB, opto-isolators for the SPI bus, and a tiny DC-DC converter to provide power to the I/O.

Serial ports would allow you to make a DCS between multiple controllers, each with their own I/O. RS422 or RS485 are long reach.

What do you think?

pmario
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Re: Turn Raspberry Pi into a P.L.C (Programable Logic Controller) Advanced

Wed Oct 12, 2011 5:15 pm

Quote from Burngate on October 12, 2011, 12:08
...snip... Worked on paper, not in real life.
Redundant systems have to deal with this situation all the time. It is a real life problem, thats solved allready. But imo quite interesting for students to solve it too.

eg: 3 cpu's controlling each other, two different networks for cpu/cpu communication and I/O transport. synchroniced program execution. Programms written using 2 different programming languages. ( If possible using 2 different programming environments. )

Seems to be possible, with a configuration discussed here. It's overkill even for most real world applications. But it would be fun to deal with it.

Svartalf
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Re: Turn Raspberry Pi into a P.L.C (Programable Logic Controller) Advanced

Wed Oct 12, 2011 6:10 pm

Quote from pmario on October 12, 2011, 18:15
Redundant systems have to deal with this situation all the time. It is a real life problem, thats solved allready. But imo quite interesting for students to solve it too.

Heh... They're something currently best left to collegiate and advanced placement students. But, still VERY interesting. Lots of solutions out there. Some good, some not so much so. I've seen some very bad ones over the years. Simple is better where possible.

In the case of the example...all things being equal, if your heartbeat system has known good links (simple serial, etc...), you would probably do well to listen to your peer's activity on the link on start up. If you're a single master, hot/warm failover system, you would wait 1-5 seconds for activity and bounce the peer if there is none. That, and you never do a reboot against a peer unless they show up on the heartbeat link for the first time (lack of presence means just that and not a failed system...yet...)- and then peel them out of the presence list after having done it when they fail presence. This doesn't cover multi-master and other similar configurations- they may need slightly differing rulesets and gear.

vorrias
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Re: Turn Raspberry Pi into a P.L.C (Programable Logic Controller) Advanced

Wed Oct 12, 2011 8:16 pm

IMO for teaching purpose, it is already interesting, to let a student implement this type of protocol to the R-Pi. If they succeed, fine. Learned something, about a state of the art industrial fieldbus.
My intention was that from the beginning. To try do things with this little board that never did before. It is a challenge to success . But students and everyone involved will learn a lot trying to implement real automation with this. I am learning everyday from you. Playing with I/O design, porting IEC-61131-3 environment, connecting boards with real time Ethernet and the list is too long.
In case of field bus Powerlink my opinion is that EtheCat is taking momentum over Powerlink . Many Inverters and positioning systems have EtherCat already not Powerlink. And EtherCat is becoming cheaper that CAN bus.
I am already using Beremiz with a REAL PLC. It is very good and it is open-source.

pmario
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Re: Turn Raspberry Pi into a P.L.C (Programable Logic Controller) Advanced

Thu Oct 13, 2011 11:24 am

Quote from vorrias on October 12, 2011, 21:16
In case of field bus Powerlink my opinion is that EtheCat is taking momentum over Powerlink . Many Inverters and positioning systems have EtherCat already not Powerlink. And EtherCat is becoming cheaper that CAN bus.
I am already using Beremiz with a REAL PLC. It is very good and it is open-source.

Could you link to some souce download sites, to sneek into the code a bit. I can't find any.

I also found a table [1] (table 1 near the end of the article), that says for EtherCat, a "special EthernetController" is needed. (Since the article is 2006) this may not be true anymore. For EthernetPowerlink is says, that it uses standard Ethernet controller.
=============
IMO, having a CAN bus module, connected to the R-Pi, would be sufficient for many problems allready. Including cheap wiring.

have fun!
mario
[1] http://rtcmagazine.com/article.....iew/100489

vorrias
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Re: Turn Raspberry Pi into a P.L.C (Programable Logic Controller) Advanced

Thu Oct 13, 2011 12:25 pm

EtherCat can be found on http://www.ethercat.org/
It is proposed by Beckhoff (http://www.beckhoff.com/). The only thing is that the development studio (CoDeSys or TwinCat) for supporting EtherCat must have Intel chipset. That does not mean that in the actual controller you need Intel chip set for ethernet. For instance there are Inverters and positioning control in the market with EtheCat slave (one German firm is http://drives.lt-i.com/). As far as informed from Beremiz team Beremiz development tool (www.beremiz.org) will implement EtherCat in the next months..
Today CoDeSys environment (http://www.3s-software.com/) has very high impact in automation. It is costly for vendors but it is paradise for customers/plc programmers.
The free alternative is Beremiz. I am trying Beremiz now. I will be back for more info.
Feel free to ask me anything (I know).

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TonyD
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Re: Turn Raspberry Pi into a P.L.C (Programable Logic Controller) Advanced

Tue Oct 18, 2011 1:03 pm

The PLC programmers at work have a lot of respect for CoDeSys, a much better development tool than most of the others.
Tony

vorrias
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Re: Turn Raspberry Pi into a P.L.C (Programable Logic Controller) Advanced

Tue Oct 18, 2011 6:03 pm

Yes TonyD I agree and I wrote it in my last post.
Today CoDeSys environment (http://www.3s-software.com/) has very high impact in automation. It is costly for vendors but it is paradise for customers/plc programmers.
In fact it is the best that happened to Automation.
Still Beremiz is free, needs some development but it is a working studio right now. Eventually will became better and better. Both share the idea of PLCOPEN which is the best the Automation community has to offer. Standards. write once and use it in many PLC vendors.
So can we do something to bring a similar environment to Raspberry ? May be Yes may be No. That is why I did opene this thread. Do we (students as well) have this possibility ? The will learn a lot in the process. First Automation Standards....Second learn programming..Third enjoyment to create something new and many more.

jean_bruder
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Re: Turn Raspberry Pi into a P.L.C (Programable Logic Controller) Advanced

Mon Oct 24, 2011 9:59 am

Great topic : this will certainly help me to "hack" the Raspi into a device for a "domotic" use purpose ... Following your posts in the future :-)

nickname
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Re: Turn Raspberry Pi into a P.L.C (Programable Logic Controller) Advanced

Wed Nov 09, 2011 2:52 pm

@thepanoguy
The requirements from IEC 61508 you cited apply only to a safety PLC.
Commercially available standard PLCs do not meet IEC 61508 requirements, and do not have a SIL rating.

Anyone who is interested can read about the differences between a standard PLC and a safety PLC in a White Paper from Rockwell Automation: http://samplecode.rockwellauto....._-en-e.pdf

There is absolutely no need for a hobbyist homemade PLC project to meet any of the requirements from IEC 61508. I'll even go so far as to say that anyone who claims that their hobbyist homemade PLC project actually does meet any of the requirements from IEC 61508 is reckless and irresponsible.

thepanoguy
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Re: Turn Raspberry Pi into a P.L.C (Programable Logic Controller) Advanced

Thu Nov 10, 2011 4:39 am

I am a qualified electrician who lives in Perth, Western Australia. I worked on specialised PLCs for 15 years. (Philips traffic light controllers manufactured by Philips Australia) I also worked on commercial and industrial electrical installations.

I have read the equivalent European Union, USA and Canadian electrical standards and compared them to the Australian electrical standards. PLC’s are manufactured compliant to the relevant electrical standards for the country it is manufactured. If the PLC is exported, the PLC is submitted with the relevant electrical certifications to the relevant electrical certification authority in the country where the PLC is installed.

Before that PLC is approved for Australian use, all the relevant international electrical and manufacturing standards are cross referenced to the Australian standards. If the PLC is compliant to the Australian standards, it is given an Australian electrical certification number. The cost of Australian compliance can be up to $50,000 per item.

The European Union, USA and Canada follow similar procedures to Australia before issuing electrical compliance certification for electrical equipment manufactured in Australia.

The Rockwell white paper states that a safety PLC costs 25% to 30% more than the standard counterparts. The article also states that there are considerable cost savings in using a safety PLC over a standard PLC. ie, electrical engineering design costs, electrical compliance certification, switchboard manufacturing costs, electrical installation and commissioning costs, production line costs, etc.

Even on a small PLC installation the 30% cost difference between the cost of standard and safety PLC is easily absorbed by the design, programming and installation costs.

For example a $500 PLC may be used in the heart of a $30,000 PLC electrical control module. The $150 PLC purchase cost savings equates to one hour corporate labour costs. For the cost difference, the design team would pick the safety PLC for one reason; the real world electrical interfacing costs would be significantly more than $150.

Considering that the overall commissioned cost of a small PLC production plant may be over $1,000,000 and that plant may run at $30,000 per hour production output, the $150 PLC cost saving between a standard and safety PLC is irrelevant.

Therefore; when purchasing a PLC for a project, the overall costs of the commissioned project and the production machine failure rate and non-production maintenance have to be considered.

Frankly; I would waste my time using a non safety PLC in a commercial or industrial installation. For home automation I would not waste my time using a PLC and programming in ladder logic when I can purchase commercial pre-programmed equipment and charge the customer for the installations costs.

nickname
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Re: Turn Raspberry Pi into a P.L.C (Programable Logic Controller) Advanced

Fri Nov 11, 2011 3:34 pm

Safety PLCs are a wonderful tool when they are used for their intended purpose. Their intended purpose is to replace Safety Relays in systems. When properly selected and designed into a safety system, you can wire Emergency Stops, Light Curtains and other critical life-safety inputs into a Safety PLC and trust that the Safety PLC will respond to these critical inputs and stop your machinery (remove control power) in a safe manner.

Your Emergency Stops no longer directly remove control power from machinery protected by as Safety PLC - the Safety PLC does.

IEC 61508 defines the technical requirements a PLC has to meet before it can be considered a life-safety device, as described above. IEC 61508 compliant devices are of the "you can bet your life on it" variety.

One can NOT simply claim that their PLC is a life-safety device, compliant with IEC 61508 by meeting the technical requirements. Compliance must be verified by an authorized 3rd party (TUV, FM, ...)

During the compliance testing and review, you can expect to be asked questions about your supply chain, such as what batch of silicon each of your redundant controllers was made from, and you will be required to provide certified documents from your suppliers attesting to the fact that their chips weren't produced in the same batch.

I fully support the idea of hobbyists developing a homebrew PLC and using it to run their model trainset or turn their christmas tree lights on and off or even run the pump in their pool but under no circumstances can anyone claim that they turned the $25 computer they bought off the internet into a life-safety device.

One can NOT post schematics, plans or any other how-to style documentation for a do it your self electronics project that claims in any way shape or form that the final product is a life-safety device.

For the sake of everyone's safety lets make this clear:

Your stop button must be hardwired to remove control power from your system outputs.
Do not wire your stop button to a PLC input.
Do not rely on your homebrew PLC as a life-safety device.

vorrias
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Re: Turn Raspberry Pi into a P.L.C (Programable Logic Controller) Advanced

Sun Nov 13, 2011 2:43 pm

Nice reading all these writen by experienced people. I totally agree with what posted lately by thepanoguy and Blueberry. This is the case. Raspberry PLC if ever comes alive will be a home made PLC ONLY for hobbies. Still will be a very nice project. At the end a lot will learned considering that a lot of programming effort must be done to make the I/O drivers and make the hole "design studio" looks and feels professional. This is a chalenge for programers. But it seams that I/O Rasbperry devices (boards) needed before we further proceed.

chmodman
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Re: Turn Raspberry Pi into a P.L.C (Programable Logic Controller) Advanced

Sat Mar 10, 2012 5:11 pm

I think some are missing the point of the Raspberry Pi as a PLC.  Not all applications require SIL rating or act as a Safety PLC.  For example, lets say I want to control some office lights or an irrigation system.  A simple web interface running on the R-Pi and some Remote I/O (Like a WAGO ethernet coupler), would be a great low cost solution.

Of course nobody in their right mind would use a R-Pi to control a traffic light or an amusement park ride! Lol. But believe me, there are thousands of applications for R-Pi at this price point.  Just look at Arduino to see what's possible, and this thing has a lot more capability.  Let's hope 3S develops a Codesys Target for it soon.

DaveDaGr8
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Re: Turn Raspberry Pi into a P.L.C (Programable Logic Controller) Advanced

Sun Mar 11, 2012 11:20 am

Perhaps we COULD build it as a mock up Safety PLC. A proper one though with managed inputs and outputs. It wouldn't be THAT much extra work ??? ... or would it. Provided it was modular it should be ok.

FYI, i work on GE Turbines. The controllers have 5 Processors, R,S,T a C-Core and a P-Protected core.

All inputs are replicated 3 times, checked and managed. All outputs are sent by one controller and are managed by all 3, so if the controller is NOT sending out the signal one of the other 2 will take over. The logic itself is actually split into 1/4's and run on R,S,T and C.

To put into perspective, these controllers run into the 10's of millions and run a handfull of inputs and outputs, so the average person just isn't going to get to touch them.

HOWEVER, if i wanted to build a controller to manage the engine in my car, this is what i'd look at building.

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Gert van Loo
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Re: Turn Raspberry Pi into a P.L.C (Programable Logic Controller) Advanced

Sun Mar 11, 2012 11:38 am

I once visited a company which did faill-save-computers. These worked with pulses all the time. No pulse = computer dead. Then the had three in parallel for 'majority decision'. The main idea was that if a computer failed the final stage would go to  a 'safe ' condition. e.g. closing the feed valve and opening all pressure relieve valves.

piters
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Re: Turn Raspberry Pi into a P.L.C (Programable Logic Contro

Thu Jul 26, 2012 5:12 pm

Any progress here since the last post?
I believe many of us already got their RPi's and started hacking the GPIOs...
Anybody tried getting CoDeSys or Beremiz onto the RPi?
(with somewhat limited resources, I'd be happy to hear any hint before embarking into the project).

mvdmeij
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Re: Turn Raspberry Pi into a P.L.C (Programable Logic Contro

Sun Aug 26, 2012 11:38 pm

This would be THE perfect project! I just thought about this myself and then I just assumed someone else must have come up with the same idea. I normally deal with Mitsubishi but the prices they ask for here in Australia are so ridiculous, it is just not funny. Charging $500 for a simple HMI that has a manufacturing cost of probably $15 or less.

The thing that I do care about though is real time processing with an allowable jitter of say 10% on a 1ms time base. There must be a guarantee that timers will always be updated on this say 1ms base and that there is no situation that it takes 1ms @ 99.99% of the times but 10ms @ 0.01% of times.

Someone mentioned forgetting the OS all together and directly run the PLC software. I think that is not the best way as the OS has useful features for networking and the graphic interface can be used for a build-in HMI.

For some projects I now use SBC computers with PC/104 interface, running DOS with GO32 extender . Not a bad choice if you make all the rest of the hardware yourself but if you need a quick solution, I grab a Mitsubishi PLC and have my ladder in no time. Debugging is still easier/quicker on a PLC than a Pascal or C++ program.

Anyway, is there any news on the status of this project?
Michel

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