Why Are Rails and Pull Down Resistors Essential in Digital Circuits?

[treehouse]

Homework Statement

8. Describe why rails are used in building a digital logic circuit.

9. Describe why a pull down resistor is needed for a switch in a digital circuit.

Homework Equations

8. I've found nothing on rails.

9. A resistor with one end connected to a LOW voltage level and the other end connected to an input or output line so that when that line is in the float condition (not HIGH or LOW), the voltage level on that line will, instead, be pulled down to a LOW state.

The Attempt at a Solution

I don't know enough about such architecture to approach this.

 

[MisterX]

In this context, "rails" probably refers to connections to a DC power supply. For example there might be a +5 V "rail."