Seeking advice on the selection of positive and negative logic in DCS design
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1 Systems that have both DCS and SIS: the SIS already includes all safety interlocks and uses negative logic; in such cases, are the process interlocks on the DCS based on positive logic? In a system that uses only DCS, with both process and safety interlocks implemented through the DCS, should positive logic or negative logic be used? Seek guidance on standard engineering practices*.
I’m not quite clear about the concept of positive and negative logic here; generally, safety interlocks are designed to be fail-safe. For example, when the interlock condition is met and the on-site solenoid valve loses power as a result of interlocking is one such case. Perhaps the original poster meant to ask whether it is a 1 signal or a 0 signal during interlocking, right? This can definitely be unified; there is no issue with the positive or negative nature of DCS and SIS signals. The key is to ensure one thing: fail-safe operation.
I agree with the friend above; no matter what system is used or what type of interlock is employed, safety must be ensured (including equipment and process safety).
It is recommended to use positive logic uniformly, as in most of the processes I have encountered before, negative logic was not understandable. During configuration, this point should be communicated to the project engineer from the SIS manufacturer. Actually, if it’s a skilled person, they will understand even without you telling them.
Positive and negative logic are independent of system type; the key is the principle of fail-safe operation.
This post was last edited by superme84 on 2015-10-6 at 12:31. Does the way positive and negative logic is expressed in interlock logic diagrams affect the actual programming? For example, in the case of a level switch that is normally closed in normal conditions and switches to an alarm state when there is an issue, using positive logic means that an inversion operation must be performed during programming, so that 0 represents the normal state and 1 represents the alarm state. With this negative logic approach, it’s sufficient to simply input the corresponding contact status – 1 for normal conditions and 0 for alarm conditions. Is that correct? Please give some advice
If it is a protection logic, I think negative logic meets the criteria for fault safety better; because with positive logic, if a device or relay fails and loses power, the logic in which it is involved will become ineffective and unable to be triggered. In the case of negative logic, the above situation will cause the protection system to function properly. It’s just my personal understanding; I hope experts can give some guidance.
This post was last edited by zxb1990 on 2015-10-7 at 13:13. Since SIS is used for safety protection functions and has the capability to initiate a safe shutdown in case of failures, its safety interlocks generally use negative logic. DCS, on the other hand, has no specific requirement regarding whether positive or negative logic should be used; it depends on one’s own design preferences. I couldn’t find any guidelines in DCS regarding whether to use positive logic or negative logic.
It should be a fail-safe design approach; some safety interlock relays must be forcibly reset in order to be unlocked. High-level triggering can also be used, and positive logic or negative logic is generally not considered.
For input and output, normal is 1 and faulty is 0； Are there any specifications regarding whether positive logic or negative logic should be used in the internal logic of SIS? Logic modules usually operate on a 1-trigger basis, and flipping values back and forth is quite troublesome.