I understand there is a special version of FF designed for the use in safetey instrumented systems. Is this common used? What about other field buses? Are hard wired systems still most used for safety instrumented systems?
Hard wired systems are preferred, as they are much easier to control and thus have less variations that need to be considered during the safety certification process. There are a lot of variants of FieldBus. Some are communication standards, others are power distribution standards.