At last, all boards done, all seem to function properly.

Here is an overview of all boards for this project:

There was no way to place all circuitry for multiplexor on a single board, so I decided to extend it by one small board. As T-buffers occupy only half of this board, I placed some input capability. So the small board contains group of 4 tri-state buffers with common enable, which are enabling first nybble (most significant one) to the bus.

On the big board there are three groups of four tri-state buffers with common enable. Also two small circuits -- one is group of four invertors, which invert bus value. They are needed because t-buffers have inverted output.

Also there are four 2-NAND gates which are part of decoder.

Below the two boards of multiplexor connected together:

Part count:

part name                      |  small board  | big board  |

---------------------------------|--------------------|-----------------|

2n2222a transistors    |   20                |  68              |

1n3138 diodes              |   8                  |   36             |

red LEDs                      |   8                  |   32             |

10k resistors                 |  24                 |  32             |

20k resistors                |  8                   |  32             |

47k resistors                |   8                   |  32             |

dpdt switch                  |    1

DIP 8x spst switch       |    2

pin header 4p              |    4 

-------------------------------------------------------------------------

Total parts:                   |   83                 |  232           |

Also one 5*7cm and and one 7*9cm perfboards, and wires, of course.