Logic gates
A logic gate is a circuit whose output depends only on the present state of its inputs, each of which is either 1 (HIGH, true, on) or 0 (LOW, false, off). Gates are the building blocks of every digital system — and the same AND/OR/NOT reasoning appears in relay circuits, warning systems and the logic diagrams of the aircraft maintenance manual. Module 4 introduced the gates; at B1 level 2 for Module 5 you must identify each gate from its symbol, truth table or equivalent circuit, and recognise how gates are used in aircraft systems.
| Gate | Rule — output is 1 when… | Remember it as |
|---|---|---|
| NOT (inverter) | the input is 0 | always the opposite |
| Buffer | the input is 1 | same logic, more drive current |
| AND | all inputs are 1 | series switches |
| OR | at least one input is 1 | parallel switches |
| NAND | not all inputs are 1 (0 only when all are 1) | AND + NOT |
| NOR | no input is 1 (1 only when all are 0) | OR + NOT |
| XOR | the inputs are different | odd number of 1s |
| XNOR | the inputs are the same | equality / comparator |
- A truth table lists the output for every combination of inputs: 2n rows for n inputs (4 for two inputs, 8 for three, 16 for four).
- Boolean algebra writes the functions: AND as a dot (A·B), OR as a plus (A + B), NOT as a bar over the term (Ā), XOR as ⊕.
- A bubble on an output means the output is inverted; a bubble on an input means that input is active-low (it does its job when it is 0).
Equivalent circuits
Every gate has equivalent circuits built from switches, relays, diodes or transistors. Recognising them lets you read relay logic in a wiring diagram as easily as gate symbols in a logic diagram.
- Switches and relay contacts: series = AND; parallel = OR; a normally closed (NC) contact of a relay driven by A = NOT A. NC contacts in parallel = NAND; NC contacts in series = NOR. Normally open contacts operated by relays A and B in series and in parallel give AND and OR.
- Aircraft wiring diagrams show relays and switches in a defined normal (shelf) state — usually de-energised, aircraft on the ground, systems off. Check the diagram's notes before deciding whether a contact is open or closed.
- Diode logic: with a pull-up resistor and the diodes pointing to the inputs, any input at 0 V pulls the output low — an AND gate. With a pull-down resistor and the diodes pointing to the output, any input at +V lifts the output — an OR gate. Aircraft wiring often uses diodes the same way, so that either of two switches can operate a light without the switches feeding each other.
- Transistor inverter: a 1 at the base turns the transistor on and pulls the collector (output) to 0 V; a 0 turns it off, so the output rises to the supply. Add the inverter to a diode AND and you have a NAND (the basis of early DTL logic); TTL and CMOS integrated circuits refine the same idea.
Universal gates and De Morgan
- NAND and NOR are universal gates: any function can be built from either type alone. NAND with its inputs tied is a NOT; NAND followed by NOT is AND; NANDs on both inputs of another NAND give OR.
- De Morgan's theorem: A·B = A + B, and A + B = A·B. So a NAND gate can be drawn as an OR with bubbles on its inputs, and a NOR as an AND with bubbles on its inputs. Logic diagrams choose whichever symbol shows the designer's intent ("any input low gives a high output").
- Positive and negative logic: in positive logic the higher voltage is 1; in negative logic the lower voltage is 1. The same physical gate changes its name: a positive-logic AND gate is a negative-logic OR gate, and a positive NAND is a negative NOR.
Reading a simple logic diagram
To find what a combination of gates does, label every gate output, then work through the truth table one row at a time. For example, the circuit Y = (A·B) + C — an AND gate and an inverter feeding an OR gate:
| A | B | C | A·B | C | Y |
|---|---|---|---|---|---|
| 0 | 0 | 0 | 0 | 1 | 1 |
| 0 | 0 | 1 | 0 | 0 | 0 |
| 0 | 1 | 0 | 0 | 1 | 1 |
| 0 | 1 | 1 | 0 | 0 | 0 |
| 1 | 0 | 0 | 0 | 1 | 1 |
| 1 | 0 | 1 | 0 | 0 | 0 |
| 1 | 1 | 0 | 1 | 1 | 1 |
| 1 | 1 | 1 | 1 | 0 | 1 |
Gates whose output depends only on the present inputs form combinational logic. Circuits with memory — latches and flip-flops made from cross-coupled gates, counters and registers — form sequential logic; aircraft logic diagrams show them as boxes marked latch, memory, S/R, or with time-delay symbols. Detailed interpretation of logic diagrams is B2 knowledge (5.5 b).
Applications in aircraft systems
Warning, interlock and protection functions are logic, whether they are built from relays, from discrete gate ICs or — on modern aircraft — from software in a computer. The maintenance manual still documents them as logic diagrams or schematic diagrams using gate symbols, and fault isolation means working out which input is stopping (or causing) an output.
- Air/ground logic: weight-on-wheels (WOW) or proximity sensors on the landing gear are combined (for example "left AND right main gear compressed") to decide whether the aircraft is on the ground. Dozens of systems use the result.
- Landing gear warning: gear not down and locked AND (low altitude OR flaps in landing position) AND thrust levers at idle → warning.
- Thrust reverser interlock: deploy is permitted only if on the ground AND the reverse lever is selected AND the thrust levers are at idle.
- Stall warning, cabin pressure warnings, fire detection loops (two loops: alarm if loop A AND loop B detect, or one loop if the other has failed), door warnings and many BITE tests are all logic.
- Discrete inputs to computers are typically 28 V/open or ground/open; interface circuits convert them to logic levels. A ground/open discrete that is "true when grounded" is an active-low signal.
NAND gives 0 only when all inputs are 1; NOR gives 1 only when all inputs are 0. XOR detects a difference, XNOR equality. In IEC symbols, "=1" is XOR, "≥1" is OR and "&" is AND — a circle on the output inverts each of them.