In interlocking, positive logic and negative logic are discussed

Positive and negative logic refer to the internal logic. As you mentioned, for the level interlock valve, whether it operates based on positive logic or negative logic, the valve will act once the level reaches the interlock value. Explanation: In normal logic, when the liquid level is 0, the logical output is 0; the solenoid valve does not receive power, and the valve remains in its normal state. Level interlock input 1, logical output 1: the solenoid valve is energized and the valve moves to the safe position. In negative logic, the normal liquid level is 0; when inverted, the output is 0, and as a result the solenoid valve is powered, allowing the valve to function properly. During level interlock, level 1 is inverted, logic output 1 is inverted, the solenoid valve is not powered, and the valve moves to a safe position. I hope this can help you understand

Strictly speaking, there is no such thing as positive logic/negative logic. If an event A occurs, then A is 1; otherwise, it is 0. Logical operations can take the value ‘1’ or ‘0’, depending on the specific application. In safety interlocks, a fail-safe design is often used, meaning that the detection switch closes when the event occurs, which is equivalent to the negation of event A. The purpose of doing this is to treat switch failures in event detection (faulty disconnection) as system failures as well.

The reason is the same, the outcome remains unchanged, but the process is reversed.

This post was last edited by jlshnlhj on 2023-12-12 at 18:43. First, we need to understand the key question: what does negative logic action refer to? Why propose negative logical actions? Negative logic action refers to the interlock action that occurs when the DI/DO signals are disconnected; it relates to the DI/DO signals connected to the field, not the 0/1 values inside the DCS or ESD, nor analog signals. Why propose negative logical actions? Because if the contacts fail or the cable is broken (due to construction work, rodent damage, etc.), the DI inputs of the DCS and ESD will no longer receive a closed \"1\" signal ； The DO“1” signal also does not reach its destination. It should be specifically noted that negative logic operation refers to DI/DO: when the field DI is 0, an interlock action is triggered, and the output DO is set to 0 and sent to the actuator. Negative logic has nothing to do with analog quantities. As for whether there is a 0/1 conversion inside the program, that is a matter of programming. Take a high liquid level as an example (high-level switch): In normal positive logic, when the liquid level is normal, the level switch is off; for the interlock program, DI=0 and the output DO=0. When the condition of high liquid level is met, the level switch closes, DI=1 is input, which triggers the interlock action, and DO=1 is output. Here is a problem: if the contacts for input DI/output DO fail, or if the cables are broken (due to construction work, rodents, etc.), then the “1” signal from DI will not reach the logic controller, and the “1” signal from DO will also not reach its destination. Moreover, such faults are often difficult to detect. It generally does not move during this action. It is only discovered through a post-event investigation, or before work begins during interlock checks. Therefore, a negative logic action is proposed: when the liquid level is normal, the level switch is closed; for the interlock program, DI=1 and the output DO=1 ； When the high liquid level condition is met, the level switch turns off, DI takes a value of 0, which triggers the interlock action, and DO becomes 0. In this way, if there is a fault or disconnection in the contacts for input DI/output DO, it will activate, meaning it operates in a \"fail-safe\" mode. There is also a problem here: a broken connection or malfunction can lead to incorrect operations. Between the two evils, the lesser one should be chosen – it’s better to have incorrect operations than no operation at all when it’s necessary to act. This is the crux of the issue: negative logic actions refer to interlock actions that occur when the DI/DO signals are disconnected.

I’ve learned that. Besides, Jiangsu is truly a major province for the chemical industry. . .

The choice between positive and negative logic depends on the requirements; negative logic is recommended according to the specifications in SIS systems, and it is also commonly used in DCS systems. The advantage of negative logic is that it allows for some action to be taken in case of a wire break

The above content is excerpted from \"SIL Classification and Verification\" by Zhu Dongli (1st edition, 3rd printing). This book is available on this forum; if you are unable to download it, post a reply with your email address and the author will send it to you.