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Section 5.9

Multiplexing

Operation, application and identification in logic diagrams of multiplexers and demultiplexers; time-division and frequency-division multiplexing. Examined for Category B2.

Notes

Multiplexing — summary notes

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Main ideas
  • Multiplexing lets many signals share one transmission channel, saving wiring weight — the reason a modern aircraft carries data buses rather than a wire per parameter.
  • TDM (time-division multiplexing) gives each signal its own repeating TIME SLOT on the same channel. All signals use the full bandwidth, but only in their turn. This is the method used by digital data buses.
  • FDM (frequency-division multiplexing) gives each signal its own FREQUENCY BAND on the channel simultaneously — the method used by radio and analogue carrier systems.
  • A multiplexer (MUX) is a many-to-one selector: 2ⁿ data inputs, n select lines, one output. A demultiplexer (DEMUX) is the reverse, one input routed to one of 2ⁿ outputs.
  • The MUX and DEMUX select lines must stay synchronised, otherwise data is delivered to the wrong destination — synchronisation words in the data stream keep the two ends aligned.
  • ⚠ Exam trap: TDM = time slots, FDM = frequency bands. A MUX has ONE output and many inputs; do not reverse it.
Key formulas
Multiplexer size
2ⁿ inputs need n select lines
TDM
same frequency, different time slots
FDM
same time, different frequency bands
Solved examples

  1. How many select lines does a 16-to-1 multiplexer require?

    Four. 2⁴ = 16, so four select lines address all sixteen inputs.

  2. A TDM system delivers each parameter to the wrong display. What is the most likely cause?

    Loss of synchronisation between the multiplexer and demultiplexer — the receiving end is sampling the wrong time slot, so every channel is offset. Recovery depends on re-acquiring the frame synchronisation word.

Mind map

Multiplexing concept map

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Multiplexing

Quiz

Multiplexing quiz

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