2:1 Selector

The 2:1 selector outputs one of two 8 bit inputs. Also known as a multiplexer.

Interface and Operation

This is the interface of a 2:1 selector:


This is how it operates:

Name Bit width Description
A 8 First input
B 8 Second input
out 8 Output, either A or B.
A/B 1 When low, A is output, when high, B is output


  • Two 74HCT157s are used to choose between the two inputs.
  • LEDs with current limiting resistors are used to display the output.

Due to 2:1 selectors being required in different parts of the computer, the layout on the breadboard of A, B and out sometimes need to differ.

This is the A B out layout:


This is the out A B layout:


Different coloured wires are used in the Fritzing images above to help track the A, B and out bits. When creating the computer only green wire was available :(.