Positive logic versus negative logic: the differences between national standards and industry standards
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Regarding the positive and negative logic issues of safety interlocks, someone at Haichuan mentioned them, but not in depth. According to national standards: \"GBT 50770-2013 Code for Design of Safety Instrumented Systems in Petrochemical Industries\" was issued on February 7, 2013, and came into effect on September 7 of the same year; paragraph 11.1.1 states that the logical design of application software should preferably use positive logic. Provisions of chemical industry standards: \"HG_T20511-2014 Design Specifications for Signal Alarm and Interlock Systems\" was issued on May 6, 2014, and came into effect on October 1. 4.3.1 Negative logic may be used for the logical design of safety interlocks. Explanations to the provisions of HG_T20511-2014: 4.3.1 ...... 1. When digital instruments are used as sensors, normally closed switches are generally selected; that is, they remain closed under normal conditions and open when the interlock set point is reached, meaning that the boolean value is “0” when the interlock input signal is triggered” ； (Comment: Not bad. ) 2. In the initial state or fault state of the logic controller, the boolean value in the software is “0”. (Comment: It’s not bad, but it’s absolute; “0” and “1” can be set.) ) The provisions of the two standards differ. In my opinion, although logically speaking, there is no problem with implementing either positive logic or negative logic, the simultaneous use of the two (as explained in HG industry standards: non-safety logic can use positive logic, while safety logic can use negative logic) will inevitably lead to confusion in operations, and may even result in accidents. I believe that, in line with the working traditions of the process engineering and instrumentation fields, using positive logic uniformly is more beneficial for the work.
I think industry standards should be followed, with reference to ISA standards as well.
Are the standards absolute? Is there an absolute truth? When it comes to standards, I believe that national standards are already highly authoritative and beneficial for engineering work ； It is the industry standards that confuse people’s minds; I’m completely confused myself.
Regarding project execution, both positive and negative logic are acceptable; the key is to maintain consistency. Don’t have one project where this logic is positive logic and that logic is negative logic; this will cause a lot of difficulties in configuration and maintenance.
Generally speaking, negative logic is safer. Being active on the falling edge makes it easier to avoid interference than being active on the rising edge.
Considering fault safety, negative logic is more reasonable. In comparison to this standard, industry standards are more up-to-date and therefore more reasonable. In our practice, positive logic is used in the logic diagrams, which is simple to understand and avoids complexity ； Negative logic is used in programming. Accordingly, negative logic is also required for the construction wiring.
Sorry! I still don’t see sufficient reasons for using negative logic. It would be best to explain in detail how the falling edge can help avoid interference better than the rising edge. Moreover, newer standards do not necessarily mean they are more suitable than earlier ones, as standards are created by humans; those who develop new standards may not be as skilled as those from before, and this phenomenon is common in many design firms – is it possible to fail to create good designs? Go to the meeting to develop standards.
Besides, that logic of being positive one moment and negative the next is confusing enough; is it really necessary to complicate something that’s already simple?
When it comes to things related to safety, the highest level of safety must be prioritized
Haha, our company encountered this situation too – DCS uses true logic while SIS uses negative logic. I was really confused when I first started working here