Multiplexing
Operation, application and identification in logic diagrams of multiplexers and demultiplexers; time-division and frequency-division multiplexing. Examined for Category B2.
Multiplexing, summary notes
- 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.
- Multiplexer size
- 2ⁿ inputs need n select lines
- TDM
- same frequency, different time slots
- FDM
- same time, different frequency bands
How many select lines does a 16-to-1 multiplexer require?
Four. 2⁴ = 16, so four select lines address all sixteen inputs.
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.
Multiplexing concept map
Multiplexing
Multiplexing quiz
Multiplexing, quiz
1. Multiplexing allows:
Several signals to share one transmission pathOne signal to use many pathsSignals to be amplified2. In time-division multiplexing, the signals are:
Given the channel in turn, each in its own time slotSent on different frequencies at onceCombined into one analogue value3. TDM shares a channel by allocating:
Time slotsFrequency bandsVoltage levels4. FDM shares a channel by allocating:
Frequency bandsTime slotsSeparate wires5. A multiplexer has:
Many inputs and one outputOne input and many outputsEqual numbers of inputs and outputs6. A 16-to-1 multiplexer requires how many select lines?
48167. The main airframe benefit of multiplexing is:
Fewer wires for the same number of signalsHigher signal voltageElimination of data errors8. If the multiplexer and demultiplexer lose synchronisation:
Data is delivered to the wrong destinationsThe channel bandwidth doublesOnly the last channel is affected