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William K.
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Platinum
But are the functions really separate?
William K.   1/24/2014 9:46:45 AM
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I keep seeing the claims that somehow safety functions and control functions all using the same processor can offer complete separation and 100% reliability. Of course there are a few hardware parts that are common to both of them and it is a challenge to imagine that if excessive enclosure temperature causes one part of the processor to lock up that the other part will continue to function as it should. And a power supply failure would probably shut the whole package down, as well. So while automation has become simpler and less expensive it does not make sense to go to those extremes of combining everything, even if it does reduce the initial prices.

Rob Spiegel
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Blogger
Re: But are the functions really separate?
Rob Spiegel   1/24/2014 10:21:35 AM
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You're not alone with this skepticism, William K. Certainly for years it was unheard of to combine both functions on the same network. But the safety folks are saying these combined networks are safer than the old single-duty safety networks.

William K.
User Rank
Platinum
Re: But are the functions really separate?
William K.   1/24/2014 3:00:46 PM
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I know that they are saying that, but I have not heard any explanation of facts that would validate the claim. It is one thing to make an assertion and a somewhat different thing to be able to explain in detailwhy it is true. My feeling is that they have not covered all of the failure modes that I have seen. So if they would like to place a really detailed article in Design News about the actual mechanism of why their combined system is safer I would certainly be happy to read it.

taimoortariq
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Gold
making lives easy
taimoortariq   1/25/2014 10:46:35 PM
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Nice Article Rob, Smarter PLC's certainly makes lives much easier. To get stuck with the setup and configuration problems can be quite time consuming and equally frustrating. To have every thing combined in one PLC is a treat for a control engineer.

taimoortariq
User Rank
Gold
Re: But are the functions really separate?
taimoortariq   1/25/2014 11:00:17 PM
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William, I understand your point. But I am sure that they must have done alot of rigourous experiments to check and finally verify the device, as SAFE. I mean I dont exactly know the internal architecture of the PLC security module, but if they are selling the product all around the world, they must have made it very robust under the fail conditions. Obviously, no system is 100% fool proof, but when they say that relatively this system is more reliable and safe then the other one. Than they must have made it more safer.

William K.
User Rank
Platinum
Re: But are the functions really separate?
William K.   1/26/2014 4:48:28 PM
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I am certain that some of the potential failure modes have been avoided on the safety section of the all-in-one-system PLC packages, but short of being truly triple redundant it is not reasonable to anticipate that they are 100% safe. And it would be very worthwhile to read the actual user documentation and see what disclaimers and warnings are there that don't make it to the advertising areas. 

In short, when a set of claims are simply too good to be true, I usually doubt that they are true. That attitude has saved me from a couple of disasters over the years. 

Rob Spiegel
User Rank
Blogger
Re: But are the functions really separate?
Rob Spiegel   1/27/2014 10:26:45 AM
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That's a great article idea, William K. I'll look into that.

William K.
User Rank
Platinum
Re: But are the functions really separate?
William K.   1/27/2014 3:37:36 PM
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Thanks, that is why I suggested it. Probably quite a few engineers would want to see what the reality is, aside from the hype. How are those two functions kept independant and yet in the same box.

Rob Spiegel
User Rank
Blogger
Re: But are the functions really separate?
Rob Spiegel   1/28/2014 11:47:24 AM
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Good point, WilliamK. The hype has been universal. It's been years since I have heard anyone argue for separate networks for control and safety. When I first covered this subject 10 years ago, the majority of plant operators I spoke with wanted separation. Now I don't hear anyone say that.

William K.
User Rank
Platinum
Re: But are the functions really separate?
William K.   1/28/2014 4:26:53 PM
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Ten years is probably a long enough time for that constantly repeated assertion to have taken on an " aura of correctness", no matter what. But all of those mechanisms for keeping the systems independant should be covered by patents now and so be something that can be discussed with the engineering community. And I am one who would really like to hear how a hardware fault in a processor only affects part of that processor. Of course, over the years, I have come across digital ICs that had partly failed, but not very many of them.

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