I heard about a “safety certified” PLC in the late 1980s at an ISA SP84 standard committee meeting. The “logic solver” (as they later called it) was the focus of attention in the field of functional safety back then. Many engineers even said, “My system is safe because I used a safety certified logic solver.” Redundancy was considered the only valid technique for high safety and most designs used three sets of equipment in a triple modular redundant system.  

How the technology has changed since then. Changes started when logic solver designers started adding self-diagnostics to their products. Probabilistic modeling has shown this approach can lead to equal or superior safety with higher reliability (lower false trip rate) when compared to redundancy alone. Over the years, new designs, driven by high production volume industries like automotive, have added far more sophisticated diagnostics, some built into the integrated circuits.

Logic solver designs have been IEC 61508 certified based on FPGA designs. These have no online software—an attribute that might be useful in the context of cybersecurity.

It is a pleasure to see the engineering advancements in logic solver design. As exida has done most of the certification work for these new logic solver designs, we get to see the work of some clever engineers. This is fun.