News You Can Bruise for 2002 May 5 (entry 2)

Sun May 05 2002 11:38: As long as I'm drawing block diagrams, I'll explain the mux and demux here.

A mux is a black box which takes multiple inputs and lets you control which of those inputs you want to be the output. A n-bit mux takes as its inputs n real input bits and log2(n) additional selection bits. Out of the mux comes one of the input bits, and its value at any point in time corresponds to the value of the input designated by the selection bits.

Here's a 4-bit mux. It has two selection bits and one output bit.

          +---+
 input00->|MUX|-->output0
 input01->|   |
 input10->|   |
 input11->|   |
          +---+
 selector0-^ ^-selector1

If selector0 is 1 and selector1 is 0, then the mux will tie input10 to output0.

Most pieces of equipment I can think of that use mux technology actualy use demuxes. A demux is the opposite of a mux. It takes one input and sends it off to one of n places. Here's a four-bit demux.

          +---+
 input0-->|DE |-->output00
          |MUX|-->output01
          |   |-->output10
          |   |-->output11
          +---+
 selector0-^ ^-selector1

If selector0 is 0 and selector1 is 1, then the sole input bit will go to output01, and all the other outputs will be left alone.

My four-track is basically a tape recorder, some mixer stuff, and two demuxes:

          +---+
    Mic-->|   |-->Track 1
          |   |-->Track 3
          +---+
            ^-Track selector

          +---+
 Guitar-->|   |-->Track 2
 (or other|   |-->Track 4
  input)  +---+
            ^-Track selector

The "A/B box" you buy to connect two printers or two monitors to the same computer (but only one at a time) is also a demux:

          +---+
Computer->|   |->Printer1
          |   |->Printer2
          +---+
            ^-A/B switch

If you care about truth tables, here's a truth table for a two-bit mux.

In0 In1 Sel | Out
------------+----
 0   0   0  |  0
 0   0   1  |  0
 0   1   0  |  0
 0   1   1  |  1
 1   0   0  |  1
 1   0   1  |  0
 1   1   0  |  1
 1   1   1  |  1

You can chain muxes and demuxes to ridiculous extents, and in doing so sink into a delusion that your muxes and demuxes form a system of aqueducts and that you are a Sumerian tyrant who controls the flow of water throughout your land. Be careful!