THISS voter-switch analysis

THISS is a hybrid combination of majority voting (originated by Von Neumann) and classical standby redundancy. The acronym THISS stands for `t.m.r./hybrid/single/single¿, where t.m.r. is an abbreviation of triple modular redundancy, or 2-out-of-3 majority voting of three parallel channels or convertors. The last two letter of THISS describe the terminal old age of the THISS system, which employs each of two surviving single channels, instead of the `2-out-of-3¿ majority voting used in the system´s youth. Exact formulas for system reliability and unreliability are given, as well as good approximations for short missions and long missions. Both the effect of the redundancy itself and the effect of imperfect coverage can be described by formulas given for the mission-time-improvement factor. Any approach to the architecture of ultrareliable systems is very sensitive to uncoverage, or the discrepancy between the actual value of coverage and the ideal of unity. For the THISS system the simple formulas given for unreliability clearly indicate the quantitative effect of uncoverage and its importance.