Suppose we have a multiplexor with 4 data inputs and 2 control signals

The inputs are numbered as follows:

The control signals is a binary number used to select the corresponding numbered input

Example: control signals = 00

The control signals = 00 will make output z = input i₀

Example: control signals = 01

The control signals = 01 will make output z = input i₁

Watch the behaior in the DEMO: /home/cs355001/demo/circuits/mux

I will construct a 4x1 mux (4 inputs to 1 output) - it's easy to generalize

Start with the input data signals and control signals:

Prepare to form all possible combinations of the control signals:

Let input i0 pass only when control signals are equal to: c1=0, c0=0:

Let input i1 pass only when control signals are equal to: c1=0, c0=1:

Let input i2 pass only when control signals are equal to: c1=1, c0=0:

Let input i3 pass only when control signals are equal to: c1=1, c0=1:

Exactly one of the inputs will be active - collate with an OR gate:

DEMO: /home/cs355001/demo/circuits/mux+lights

The EDiSim circuit program for the previous circuit is:

Switch aa   i3 '3' ZERO; // Data inputs
Switch ba   i2 '2' ZERO;
Switch ca   i1 '1' ZERO;
Switch da   i0 '0' ZERO;

Switch gb   c1 '4' ZERO; // Control inputs
Switch gc   c0 '5' ZERO;

Not    fb   c1 not_c1;
Not    fb   c0 not_c0;

And    dc   i0 not_c1 not_c0    out_0;
And    ec   i1 not_c1     c0    out_1;
And    bc   i2     c1 not_c0    out_2;
And    ac   i3     c1     c0    out_3;
Or     cd   out_3  out_2  out_1 out_0   z;

Probe  ce   z;        // Probes to show users  

DEMO: /home/cs355001/demo/circuits/mux