During software development. A safety PLC requires additional software testing techniques. To verify the data integrity checking, a series of fault injection test software must be running. The program is deliberately broken to ensure that the PLC to respond in a predictable safe. Design and testing software is fully documented so that third-party inspector to understand the operation of the PLC. While most software development does not justify this activity, precisely how the errors of the most dangerous software design is revealed.
There are certainly many similarities between the safety of a conventional PLC and Safety PLC. Both have the ability to perform mathematical calculations and logic. Both usually have input and output (I / O) modules that provide them with the ability to interpret signals from sensors and process control elements running late. Both will scan the input, perform calculations and write the output. Both usually have a digital communication port. However, PLC was originally not designed to be a fault tolerance and fail-safe. That's the fundamental difference.
The realization that many users of conventional controllers cannot be relied upon in critical protection applications creates the need for safety PLC. High standards for design safety PLC manufacture, and installation. Something that is less those high standards will be considered irresponsible, if not negligent, from the point, professional business and social perspective.