Failsafe power-up/power-down switch

Claim 1

A power-up/power-down logic control switch for use with a logic control circuit comprising: a source of power supply voltage; logic circuit means having a power supply input and first and second logic inputs and an output, wherein said power supply input is connected to said source of power supply voltage and said first and second logic inputs are common inputs connected through resistance to a source of logic supply voltage and to said output via a feedback resistor, and wherein said logic circuit means is further characterized by having a minimum input threshold voltage and a maximum input threshold voltage which vary with said power supply input during power-up and power-down of said power supply voltage and function in a manner such that when said source of power supply voltage is turned on, as long as said first and second logic inputs exceed the minimum threshold, the output will be in its maximum value or higher state, and when the value of said first and second logic inputs fall below the minimum input threshold, the output will switch to its minimum value, and when said source of logic supply voltge is turned off, said output of said logic circuit means will switch to its maximum value when said maximum input threshold drops below the voltage of said first and second logic inputs; a voltage-limiting circuit means connected between ground and said first and second logic inputs thereby limiting the maximum value of said first and second logic inputs; and wherein the output of said logic circuit means is connected to the inhibit input of a logic control circuit.