One Out of Two (1oo2) and Two Out of Three (2oo3) Logic Schemes
The following is taken from Control, Safety Systems for Non-Engineers. SIS logic solvers use the input provided by the sensors and execute the program logic that eventually results in an automated emergency shut-down.
For example, a one-out-of-two logic design monitors two inputs. If either of those inputs changes states, the logic solver executes the shut-down sequence.
A two-out-of two logic design uses two inputs, and both must agree for the logic solver to initiate the shut-down. There are a number of different design types, one-out-of-one, one-out-of two, two-out-of two, two-out-of three, etc., and it is quite common to mix analog and discrete inputs. This is called using diverse technologies, and it is very useful in eliminating false or nuisance shut-downs resulting from common- cause failures.