EASA Part 66 Module 5.5 : Logic Circuit

5.5 LOGIC CIRCUIT

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LOGIC SYSTEMS

• System performs calculation or makes decisions • A logical operation on one or more logic inputs

and produces a single logic output to make decision.

• Using electronic component that can conduct electricity based on a rule either analog or digital.

• Primarily implement using diodes or transistor acting as electronic switch

POSITIVE AND NEGATIVE LOGIC

• Binary digits are use for the input in logic gate• Positive logic : Low voltage = 0

High voltage = 1• Negative logic : Low voltage = 1

High voltage = 0* Normally use positive logic

OR GATE

X

X = A + B

A B X0 0 00 1 11 0 11 1 1

IF GOT 1 = RESULT MUST BE 1

AND GATE

X = A.B

A B X0 0 00 1 01 0 01 1 1

IF GOT 0 = RESULT MUST BE 0

NOT GATE

A X0 11 0

NAND GATE

A B X0 0 10 1 11 0 11 1 0

X

NOR GATE

A B X0 0 10 1 01 0 01 1 0

X

EXCLUSIVE OR GATE

A B X0 0 00 1 11 0 11 1 0X

EXCLUSIVE OR GATE

C B A X0 0 0 00 0 1 10 1 0 10 1 1 01 0 0 11 0 1 01 1 0 01 1 1 1

Any ODD number of Inputs gives X

X

EXCLUSIVE NOR GATE

A

BX

X

A B X0 0 10 1 01 0 01 1 1

GATE TABLE

A B OR NOR AND NAND EX-OR EX-NOR

NOT A

0 0 0 1 0 1 0 1 10 1 1 0 0 1 1 0 11 0 1 0 0 1 1 0 01 1 1 0 1 0 0 1 0

UNIVERSAL GATE

• NAND gate is universal gate• It can make another gate

NOT GATE

OR GATE

B

AND GATE

IC - INTEGRATED CIRCUIT

Quad 2-input AND Gate 7408

ADDERS

• An adder or summer is a digital circuit that performs addition of numbers.

• For new processor, adder are use to calculate addresses, table indices (database).

• The most common adders operate on binary numbers

HALF ADDER CIRCUIT

• Required 2 input and provide 2 output• Combination of an exclusive OR and an AND

gate

A B SUM CARRY0 0 0 00 1 1 01 0 1 01 1 0 1

SUM = A BCARRY = A . B

FULL ADDER CIRCUIT

• Required 3 input provide 2 output• Combination of 2 half adder and OR gate.

A B Cin Cout S0 0 0 0 01 0 0 0 10 1 0 0 11 1 0 1 00 0 1 0 11 0 1 1 00 1 1 1 01 1 1 1 1

INPUTS OUTPUTS

AIRCRAFT LOGIC CIRCUIT

• LANDING GEAR WARNING CIRCUIT

AIRCRAFT LOGIC CIRCUIT

• FLIGHT DIRECTOR GYRO FLAG - drop when gyro system malfunction either loss of vacuum or electrical signal

Error Flag

GYRO FLAG SYSTEM

SERVO ALTIMETER OFF FLAG• Correct operating when sin outputs (‘0’) and the cos

outputs (‘1’).• When total output is 1, it will energising the OFF Flag

coil, removing the flag from view.