Prior to 1968, most safety-instrumented systems, also known as emergency shutdown systems, were implemented using relays. The advent of the programmable logic controller (PLC) changed all that. Hard-wired solid-state systems—designed to replace relays without using software—were also popular for several decades. Software-based systems pose the majority of safety-instrumented systems installed today.
General purpose PLCs have a variety of weaknesses in safety applications that have been recognized for many years, primarily the lack of thorough diagnostics.1 Users, vendors and integrators have understood these weaknesses and some have engineered customized solutions to overcome man
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