Does the SIS system use positive logic or negative logic?
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The Design Code for Safety Instrumented Systems in the petrochemical industry, GBT50770-2013, states that positive logic should be used for software, while the Design Code for Chemical Process Automation, HGT20511-2014, recommends the use of negative logic. Which standard should be followed?
1. In GB-50770-2013, 11.1.1 states that the logical functions of application software shall utilize Boolean logic and the rules of Boolean algebra. Positive logic should be adopted for the logical design of application software. 2. In HG/T-20511-2014, positive logic can be used for the logical design of non-safety interlock systems, while negative logic can be employed for the logical design of safety interlocks. Note that positive logic refers to a high level or boolean value of “1” when the input signal is triggered” ； Negative logic refers to a low level or boolean value of “0” when the input signal is triggered. 3. Common definition: The convention in which a high level represents logic 1 and a low level represents logic 0 is called positive logic. Conversely, the convention of using a high level to represent logic 0 and a low level to represent logic 1 is called negative logic. 4. In both DCS and SIS logic, a high level (closed state) represents 1 ； Low level (off) indicates 0 ； It’s just that in the SIS, a “0” triggers interlock ； In DCS, a “1” triggers interlock. So, I think GB is right ； HG misunderstood the original definition of positive and negative logic, instead emphasizing the state at the time of interlock activation, and required that the SIS be fail-safe. Suggestion: Positive logic should be adopted for the logical design of SIS ； The inputs and outputs of SIS should be fail-safe.