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AS Level Logic Algebra Bistable Latch Karnaugh Maps >Multiplexing< NOR Array Propagation Delay

Logic Multiplexing

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AS Level Combinational Logic

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Time Division Multiplexing

Four telephones share one cable.
In real life, this technique works for hundreds of phones.
For intelligible audio, 300 to 3000 Hz must be sent down the line.
The switch must visit each phone at least 6000 times per second (Double the highest audio frequency).
This is called BASEBAND SIGNALLING because the original signal is sent down the cable without a carrier signal.
Ethernet, used in most local area networks, uses baseband signalling.
This is somewhat simplified because it's not a good idea to try to multiplex an analogue signal.
The audio from the phone is passed through an ADC, analogue to digital converter, and the digital signal is then multiplexed.
The animation and circuit diagrams show one way links. All this has to be repeated for the other direction to give full duplex two way communication.

Time Division Multiplex Logic for Two Channels

Time Division Multiplex Logic for Four Channels

S₀ and S₁ count repeatedly from 0 to 3. This is address information.
The AND gates are connected as address decoders.
Each AND gate is opened by one of the numbers between 0 and 3.
At the receiver, similar address decoding happens, opening the correct AND gate.
In this way, each channel is copied to the correct output while sharing the connecting cable.
This technique can scale up to hundreds of channels.
The clock signal S₀, must be synchronised at both ends of the link.
This is an example of synchronous communication where identical clock signals are used at both ends of the link.

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