Dual SIS Technologies do not cost less than TMR; they almost always cost more.
Many companies advertise their Dual SIS technology (1oo2D (Dual), 1oo2DR (Dual Redundant), 2oo4D) as a lower-cost alternative to Triple Modular Redundant (TMR) systems. This is an unfortunate misrepresentation of the capabilities of Dual SIS architectures. Dual PLCs in a 1oo2 (1 out of 2) configuration were the initial solution of choice for "fail safe" applications, but they cannot overcome an inherent problem with false trips.
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