One Out of Two (1oo2) and Two Out of Three (2oo3) Logic Schemes

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.

2 Likes