9.1 Encoders — turning many lines into few

An encoder is the opposite of a decoder. It has 2^N inputs and N outputs; whichever single input is active, the output produces its binary index.

9.1.1 Simple 8-to-3 encoder

    A0 = D1 + D3 + D5 + D7
    A1 = D2 + D3 + D6 + D7
    A2 = D4 + D5 + D6 + D7

Just three OR gates. The problem: what if more than one input is active at the same time? The simple encoder produces an incorrect output that is the OR of the encoded values — usually meaningless. We need a smarter version.

9.1.2 Priority encoder — the 74LS148

A priority encoder picks the highest-numbered input that is currently active and outputs its index. All lower-priority inputs are ignored.

Priority encoder truth table (active-low inputs, 74LS148-style)

EI̅ is the enable input, GS̅ asserts "any input active and chip enabled", EO̅ cascades into the next 74LS148's EI̅ — giving a 16-to-4 priority encoder out of two chips, and 32-to-5 out of four.

Priority encoders are central to interrupt controllers. Inside a microcontroller, every peripheral has an interrupt request line; a priority encoder converts them to a single vector telling the CPU which one to service first.

9.2 Decoders — turning few lines into many

A decoder is the inverse of an encoder. Its N inputs select exactly one of 2^N outputs.

74LS138 — 3-to-8 line decoder truth table

Decoder applications

• Memory chip-select decoding. A microprocessor's address bus has high-order bits that pick which memory chip to enable. Wire those bits to a 74LS138; each Yi enables one chip. Address ranges are partitioned automatically.
• Function realisation as minterms. Any Boolean function can be written as the OR of its minterms; a 3-to-8 decoder produces all 8 minterms in parallel, and a single OR gate at the output gives the function. Useful when the function has many minterms.
• Demultiplexer. Tie one of the enable inputs to a single data line and you have a 1-to-8 demultiplexer — routing one input to one of eight outputs.

9.3 Display drivers — the BCD-to-seven-segment decoder

A seven-segment display shows decimal digits using seven LED segments labelled a–g. Each digit is a particular pattern:

     a a a
    f     b      Segments lit for "5":   a, f, g, c, d
    f     b
     g g g
    e     c
    e     c
     d d d

Full BCD-to-7-segment truth table (common-cathode, active-HIGH)

A BCD-to-7-segment decoder/driver has four BCD inputs and seven outputs that drive the segments directly through current-limiting resistors. Two flavours:

7447/7448 extras

• LT (lamp test) input — forces all seven segments on for a visible self-test.
• BI/RBO (blanking input / ripple blanking output) — turns the display off when forced LOW; chained between digits to suppress leading zeros (a "01234" 5-digit display shows " 1234").
• RBI (ripple blanking input) — paired with the previous digit's RBO so the leading-zero suppression "ripples" across the digits automatically.

9.4 Drivers vs decoders — what is the difference?

A pure decoder (74LS138) can output only standard logic-level currents — typically 4 mA, not enough to light an LED segment. A driver has a higher-current output stage, often open-collector or open-drain, capable of sinking 25–50 mA. The 74LS47 is a true driver — you wire its outputs directly to the LED cathodes through a small resistor and that is it. The "decoder" in BCD-to-7-segment "decoder/driver" tells you it has both functions in one chip.

9.5 Putting it all together — a digital voltmeter front panel

A complete 4-digit decimal display panel uses:

1. Four BCD digits (from an ADC's output register).
2. Four 74LS47 BCD-to-7-segment decoder/drivers — one per digit.
3. Four common-anode 7-segment displays — wired in multiplex mode for low pin count.
4. A 4-bit BCD counter (74LS90) and a small clock to scan one digit at a time, plus a 1-of-4 decoder (74LS139) to pulse the common anodes in sequence.

Everything in that block diagram is an MSI chip we have just met — and that is exactly the design style used on every cheap multimeter, calculator and panel meter you have ever owned.