FAIL-SAFE LOGIC CIRCUIT, FAIL-SAFE N INPUT AND GATE CIRCUIT AND FAIL-SAFE SELF-HOLDING CIRCUIT

摘要

PURPOSE: To provide a fail-safe logic circuit, a fail-safe.n input AND gate circuit and a fail-safe self-holding circuit capable of realizing fail-safe even when modulation signals modulated by a logical variable or the like enter through a demodulation means such as an envelope detection circuit. ;CONSTITUTION: The modulation signals 9 for which the AC of a frequency to which photocouplers PC1 and PC2 can respond is modulated by the logical variable X1 are provided. The modulation signals 1 for which the AC of the frequency to which the photocoupler PC 2 can not respond is modulated by logic signals X2 are provided. The modulation signals 1 are amplified in a transistor 2, demodulated in the envelope detection circuit A and impressed to the serial circuit of a light emitting element 12 and a light receiving element 11. Thus, when the modulation signals 9 are inputted while the modulation signals 1 are inputted, the modulation signals 9 are transmitted to a load resistor 14 and demodulated in the envelope detection circuit B and output is obtained in an output terminal 19. When the modulation signals 1 directly enter the serial circuit by a fault, since the modulation signals 9 are turned on and off by the modulation signals 1, the photocoupler PC2 can not respond and the output of the output terminal 19 is eliminated.;COPYRIGHT: (C)1996,JPO