Detecting aircraft 28V discrete inputs

1 . A circuit for detecting discrete inputs, the circuit comprising:

a first line replaceable unit (LRU); and

a second LRU including an LRU circuit, the LRU circuit including:

a power source;

a diode set coupled to the power source;

a first resistor connecting from a node to ground, wherein the node is in series between the diode set and the first LRU; and

a logic detection and processing circuit operably connected to the diode set and configured to detect a state of the first LRU based on a detected voltage, wherein the detected voltage is either open or 28-Volts; and

wherein the diode set includes at least a first Zener diode and a second Zener diode connected in series with one another and connected in series between the power source and the logic detection and processing circuit, the second Zener diode having an anode connected to the logic detection and processing circuit.

2 . The circuit of claim 1 , wherein:

the first Zener diode is oriented to inhibit current below a Zener voltage of the first Zener diode from the power source to the logic detection and processing circuit; and

the second Zener diode is oriented to inhibit current below a Zener voltage of the second Zener diode from the power source to the logic detection and processing circuit.

3 . The circuit of claim 2 , wherein:

at least one diode of the diode set is a protection diode; and

the protection diode is reverse-biased during a negative over-current event.

4 . The circuit of claim 1 , wherein the power source includes a generator.

5 . The circuit of claim 1 , wherein the power source includes an external power source external from the second LRU.

6 . The circuit of claim 1 , wherein the first LRU is selectably connected to the logic detection and processing circuit using a switch.

7 . The circuit of claim 1 , wherein the second LRU further includes a diode configured to provide forward-bias protection to the logic detection and processing circuit.

8 . The circuit of claim 1 , wherein a cathode of the first Zener diode is connected to the diode set.

9 . A method comprising:

detecting a state of a line replaceable unit (LRU) using a logic detection and processing circuit;

providing power to a circuit;

receiving a discrete input;

detecting, by the logic detection and processing circuit, the discrete input over a path that maintains a constant voltage, wherein the discrete input is either open or 28-Volts; and

processing, by the logic detection and processing circuit, the discrete input;

wherein processing the discrete input comprises determining that the discrete input is open by detecting, using the logic detection and processing circuit, voltage that is below a specified threshold at a first node, the voltage at the first node based on leakage current multiplied by a resistance of a first resistor that connects between the first node and ground, the leakage current including a sum of leakage currents in the logic detection and processing circuit and at least two Zener diodes in the path, one of the at least two Zener diodes having an anode connected to the logic detection and processing circuit at the first node.

10 . The method of claim 9 , further comprising switching the circuit from a first mode to a second mode.

11 . The method of claim 10 , wherein the first mode is powered by a generator.

12 . The method of claim 11 , further comprising maintaining a constant voltage drop over a range of operation of the generator.

13 . The method of claim 10 , wherein the second mode is powered by a power source external from the LRU.

14 . The method of claim 13 , further comprising maintaining a constant voltage drop when operating in the second mode.

15 . The method of claim 9 , further comprising protecting the circuit during a negative over-current event.

16 . The method of claim 9 , further comprising protecting the circuit during a positive over-current event.

17 . The method of claim 9 , wherein:

providing the power to the circuit includes providing power from a power source connected to the path at a second node;

processing the discrete input comprises determining that the discrete input is at 28-Volts during a first case where a first diode is forward-biased and a second case where the first diode is reverse-biased;

the first diode is connected between the second node where the power source connects to the path and a third node; and

a second resistor connects the path to ground at the third node.

18 . The method of claim 17 , wherein, in the first case, voltages at the first, second, and third nodes are defined as:

Vc

=

(

V

⁢

1

*

R

R

⁢

3

)

/

(

R

R

⁢

3

+

IMP

)

,

Va

=

Vc

+

Vf_D1

,

and

Vb

=

Va

-

Vf_D3

-

VZ

⁢

1

-

VZ

⁢

2

,

wherein:

Vc is the voltage at the third node;

Va is the voltage at the second node;

Vb is the voltage at the first node;

V1 is the discrete input at 28V;

R R3 is a resistance of the second resistor;

IMP is an input filter impedance of an input filter connecting the LRU to the path;

Vf_D1 is a forward voltage of the first diode;

Vf_D3 is a forward voltage of a second diode in the path in series between the second node and the at least two Zener diodes;

VZ1 is a Zener voltage across one of the Zener diodes;

VZ2 is a Zener voltage across another of the Zener diodes;

the second diode is oriented to inhibit flow in the path in a direction from the at least two Zener diodes to the second node;

* indicates multiplication; and

/ indicates division.

19 . The method of claim 18 , wherein, in the second case, the voltage at the first node is defined as:

Vb

=

{

(

VDC_SOURCE

-

Vf_D3

-

VZ

⁢

1

-

VZ

⁢

2

)

*

R

R

⁢

2

}

/

(

R

R

⁢

2

+

R

R

⁢

1

)

wherein:

VDC_SOURCE is a voltage of the power source;

R R2 is the resistance of the first resistor; and

R R1 is a resistance of a third resistor that connects between the power source and the third node.

20 . The method of claim 9 , wherein a cathode of another Zener diode of the at least two Zener diodes is associated with the discrete input.