Proper decision of output signals is extremely
important in redundant system processing in faulttolerant
systems. Namely, the output decision circuit
must be a single redundant system and a faultinduced
error directly becomes an output signal
error. For this reason, a fault decision circuit that
can immediately report a fault is indispensable for
output decision.
Particularly when control on the safe side is
required, the above-mentioned output decision circuit
must allow errors on the safe side alone and must be
fail-safe.
This paper proposes a threshold operation
device with fail-safe output characteristics as logic
device for output decision. The authors call this
operation device the window comparator. The
window comparator allows not only fail-safe AND
operations but also multiple-valued threshold logic
operations. The fail-safe window comparator can
thus comprise a fail-safe multiple redundant system
output decision circuit (majority Operation Circuit)
that does not produce any output signals in the faulty
condition.
The window comparator with the function of
threshold logic operation and the signals handled by
the window comparator will be defined first. Then,
the method of constructing the fail-safe window
comparator, examples of circuits constructed by
using the fail-safe window comparator and
characteristics of the circuits will be explained.
Lastly, an example in which the window comparator
is applied to a 2-out-of-3 circuit will be discussed.
Since the 2-out-of-3 circuit makes the majority
decision by summing the coincidence signal output of
two input signals and performing the threshold logic
operation accordingly, it can detect faults not only in
one of the three coincidence detection circuits but
also in one of the three logic OR operation circuits.
Namely, a triple redundant system realizes a fail-safe
voter by the fail-safe, fault-tolerance and multiplevalued
techniques.