| Digital Signals and Logic Gates |
Engineers know that it is easier
to design
devices than multi-state devices. In logic systems, variables, circuits,
statements, etc., can be treated in one of two distinct states: true
or false,
yes
or no, on
or
off, present
or absent, energized
or not energized,
conducting
or non-conducting,
high
voltage or low
voltage, and so on.
In digital electronics, we distinguish
two distinct values of voltage, VH corresponding to the higher
of the two voltages and VL corresponding to the lower of the
two voltages. There are three ways in which we can assign binary values
to these voltages :
1. Positive logic assignment : True [ 1 ] : VH
False [ 0 ] : VL
2. Negative logic assignment : True [ 1 ] : VL
False [ 0 ] : VH
3. Mixed logic assignment : Allow the designers to use positive
or negative logic at any point in
their design, as they desire.
We shall use positive logic
throughout this course. The description of the other two assignments can
be found in elsewhere [Ref.1].
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