Combinational Logic Devices Overview | Generated by AI
Overview
The video from “The Learning Circuit” explains combinational logic devices, which generate outputs based only on current inputs (no memory of past states). It covers four key types: multiplexers (MUX), demultiplexers (DEMUX), encoders, and decoders. These are essential for signal routing, data compression/expansion, and digital control in electronics.
Multiplexers (MUX)
- Purpose: Like a digital switchboard—selects one of many inputs to send to a single output, controlled by select lines.
- Basic Example (74LS157 Quad 2-to-1 MUX):
- 4 channels, each with inputs A and B, select pin (S), and enable pin (E).
- S high: Selects A inputs; S low: Selects B inputs.
- E low: Enables output; E high: Disables (outputs go low).
- Larger Examples:
- 4-to-1 MUX: 2 select lines choose from 4 inputs.
- 8-to-1 MUX: 3 select lines; only one input passes through.
- Tip: General form is \(2^n\)-to-1, where \(n\) is select lines.
Demultiplexers (DEMUX)
- Purpose: Opposite of MUX—routes one input to one of many outputs, based on select lines.
- Basic Example (1-to-2 DEMUX):
- One select line (S): S low → input to Y0; S high → input to Y1.
- General Rule: \(1\)-to-\(2^n\) outputs, matching MUX’s select lines (e.g., 2 selects → 4 outputs).
Encoders
- Purpose: Compresses multiple inputs into a coded (e.g., binary) output.
- Basic Binary Encoder:
- Example: 4 inputs → 2-bit binary output (e.g., input 3 active → output 11).
- Issue: Multiple active inputs can cause conflicts (ambiguous output).
- Priority Encoder (Fixes Conflicts):
- Assigns priority (highest first overrides others).
- Example (8-input, active-low): Input 7 (highest) active → output 111, ignores lower inputs.
- No inputs active → output 000.
Decoders
- Purpose: Expands coded input into multiple outputs (e.g., activates specific lines).
- Basic Binary Decoder:
- Like a DEMUX: Binary input selects one output (e.g., 2-bit input 10 → activates output 2).
- BCD to 7-Segment Decoder:
- 4-bit BCD input (0–9) → Controls 7 LED segments for digit display.
- Examples: “0” lights 6 segments; “1” lights 2 (right-side bars).
- BCD to Decimal Decoder:
- 4-bit input → Activates one of 10 outputs (for digits 0–9).
Key Takeaways
- MUX/DEMUX: Data routers (traffic directors).
- Encoders: More inputs than outputs (data compression, like zipping).
- Decoders: More outputs than inputs (data expansion, like unzipping).
- All are “combinational”—output changes instantly with inputs, no clocks or storage.
How Combinational Logic Devices Work - The Learning Circuit