IEC 61508 functional safety certification began in the 1990s. As I recall, the only organizations doing this work in those days were TÜV Rheinland and TÜV Product Services (now TÜV Sud). These two companies remain competitors. 

Functional safety certification started with logic solvers. Some engineers and regulators at the time felt these devices were a clear threat to functional safety compared to the traditional solution of using racks of relays. The first safety certified product by TÜV Rheinland that I remember is the Triconex Tricon, a Triple Modular Redundant (2oo3) design. Another logic solver architecture called 1oo2D came into the market in the late 1990s, with certifications mostly done by TÜV Sud. The designs typically used a 16- or 32-bit microprocessor. Scan times ranged from a few hundred milliseconds to a few seconds. These are expensive machines ($50,000 USD) with I/O counts in the thousands in order to keep the cost per I/O down.     

Obviously for anyone who follows technology, integrated circuits and microprocessors have gotten much more powerful and far less expensive. Today, complex microcomputers are used in most smart transmitters and many final element devices. And these products are also getting IEC 61508 certification. 

As the organization that has certified most products used in automation, exida is pleased to be a key part of these new times.