CS355 Sylabus

Demultiplexor: One-to-many digital Switching Circuit

As I mentioned before, the decoder is not a true one-to-many switch, but we do not need a real one-to-many switch to construct the register update mechanism inside the computer. Instead, a related, more simpler device called decoder is used.
To give you the full scoop of the digital switching techniques, I will make a "detour" here and talk about the true one-to-many digital switch demultiplexor and show you how this can be combined with a mux to construct a many-to-many switch.
Note: the demultiplexor is not used inside a CPU...
Schematically, a one-to-many switch (demultiplexor) has 1 input, N control signals and 2^N output signals.
At any time, the input signal is directed (switched) to exactly one of the outputs (and all other outputs are equal to 0) and the output that receves the input signal depends on the current values of the control signals.
The following figure shows schematically what a 2x4 demultiplexor does. The 2x4 decoder has 2 control signals and 4 outputs:
The following figure shows how a 2x4 demux circuit can be constructed using elementary gates:
- The construction is very simple and similar to a decoder.
- You connect the inputs of the and-gates to every possible combinations of the input signals:
  - (c₁) (c₀)
  - (c₁) (NOT c₀)
  - (NOT c₁) (c₀)
  - (NOT c₁) (NOT c₀)
- Then connect the input to all and-gates.
- The filtering is done by the and-gates: exactly one of then will let the input signal pass.
- The right figure shows that when what happens when the control signals c₁ = 1 and c₀ = 0. In this case, c₁ = 1 and (NOT c₀ = 1), so that the second AND gate will receive all 1's as inputs PLUS the input signal that is variable. Only this and gate will let the input signal pass, and all other and-gates will output 0 (input signal is stopped).

Here is a logic-sim circuit file that implements a 2x4 demultiplexor: click here

/home/cs355001/bin/cs355sim /home/cs355001/demo/circuits/demux

You can combine a mux and a demux to construct a "many-to-many" switch....
First, what is a many-to-many switch.....
- a many-to-many switch is a device that can make connections between a set of inputs and a set of outputs
- depending on the number of internal connections (and cost) the number of simultaneous connections can range from "1 input to 1 output at a time" to "all inputs to all outputs".
We can use a mux-demux combo to construct a many-to-many switch that can connect 1 input to 1 output at a time.
The following figure illustrates this many-to-many switch:

The figure on the right depicts the fact that the second input is connected to the last output.
Note that the unconnected inputs and unconnected outputs will not be active:
- changes in the unconnected inputs will not have any effect on any of the the output signals
- the unconnected outputs will always have a value 0
You can see how the many-to-many switch work using the following logic-sim circuit file: click here
Here is how to make the many-to-many switch with a mux-demux combo: