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Negative Logic Diagram

process control control narrative negative logic

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#1 mes127


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Posted 02 November 2017 - 11:15 AM

Hello Everyone,


Our EPC company asked to provide a control logic diagram based on process control narrative for a hydrocarbon processing plant. However, I was asked specifically the diagram should be prepared in "negative logic" format. 



I did some internet search but there are limited sources on this, plus as a chemical engineer it makes more sense to me to see similar approach on a chemical process rather than other disciplines such electrical engineering.

I just wonder if someone could share with me with such a diagram for a typical unit operation with a few instruments?


Thank you



#2 Pan Nata

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Posted 03 November 2017 - 04:12 AM

Hi Max,


I've never heard any kind of that document. For control logic, process/chemical eng. mostly deal with S.A.F.E charts, causes & effects matrix, and ESD Logic diagram.

#3 mes127


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Posted 03 November 2017 - 05:30 PM



The documents you mentioned are also part of the package, but Control Logic is also a stand alone document.


Thank you



#4 Saml


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Posted 03 November 2017 - 09:18 PM

As far as I know, "negative logic" means that low voltage means true, while high voltage means false.

This makes sense when you want to consider the loss of signal as a "true" value signal for an alarm or a switch that activates a logic.


But it may also mean writing the logic in the negative forms like A_AND_B = Neg(NegA_OR_NegB)



Positive AND

True-True = True

True-False = False

False-True =False

False-False = False


equals to

Negative OR

False-False (Negative of True-True) = False (Negative is True)

False-True (Negative of True-False) = True (Negative is False)

True-False (Negative of False-True) = True (Negative is False)

True-True (Negative of False-False) = True (Negative is False)


Also Check This


#5 breizh


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Posted 05 November 2017 - 07:53 AM

hi ,

training materials are available on Youtube .

Good luck.


#6 mes127


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Posted 06 November 2017 - 03:26 PM

Thank you everyone for your feedback.



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