How do you define requirements and constraints for a PLC system?

Inputs and outputs are the eyes and ears of a process, providing all the necessary information to the PLC from field devices and relaying the correct signals to control their behavior. However one thing which is crucial in selecting the hardware (I/O) modules is to group them according to their types. Analog, digital inputs and outputs are more common, but there are TC modules, RTD or frequency input modules which directly accept these type of signals. To keep things simple it is advisable to use minimum type of IO cards, by using field transmitters for RTD, TC or frequency signals which homogenize the system. Fieldbus system if in use further simplifies the system. But as far as possible IO uniformity greatly helps in keeping it simple.

This points back to the first step. Did you properly identify the problem? If so, there should now be a quantitative way to verify success. Whether you measure production, quality, efficiency, or something else, does the new system meet your requirements? Along with the business justification, safety should always be taken into consideration and measured. Automation, just for the sake of automation, does not really add any benefit. Just like gathering data into a historian and making fancy trends will not accomplish anything by itself. How do you use the data and new functionality?

In PLC architecture, addressing single points of failure (SPOF) is crucial. A thorough risk assessment identifies SPOFs, leading to a robust mitigation plan. Strategies like redundancy, diversified pathways, and fail-safes are employed to counteract these risks. This proactive SPOF management ensures high system reliability and safety, aiming to minimize operational interruptions and hazards. It's a foundational practice for resilient and sustainable PLC design.