filter 26 to one input of nand gate 28 a corresponding low signal from switch 18 is coupled through emi filter 30 and is taken as one input to nand gate 32 it should be evident that a logical high could equally as well be used to indicate trouble this is a matter of design choice filters 26 and 30 are placed in the circuit as a part of a hardening process to protect the circuit against damage or interference from external electromagnetic fields and from radiation both nand gates 28 and 32 receive inputs from the output of nand gate 34 which supplies an inhibit signal the origin of which will be described later an output from nand gate 28 is taken to a set terminal of latch 36 the output of latch 36 is taken through inverter 38 and resistor 40 then trough led light emitting diode 42 to an unregulated voltage source typically 24 volts led 42 is energized when latch 36 is set applying a ground or logical low at the output of inverter 38 the logical high from the output of latch 36 is also coupled through diode 44 to supply both inputs to nand gate 34 this.