Combinational gates are the way to represent logic functions. A combinational gate is a function of the inputs, creating a specific output. The truth tables correspond to the combinational gates. And if the combinational gate inputs correspond to valid values, then the combinational gate outputs will correspond to the valid output values.

So the Combinational gate can be characterised by two properties:

1. Output value depending on the input value.
2. The combinational gate values which belong to the static properties.

Figure 1 shows the most the simple symbols that are used in the combinational
logic.

Figure 1. Simple logic symbols
Figure 1. Simple logic symbols

Combinational gates can have more then two logical inputs, and they may have multiple inputs. Combinational gates can also be simple or complex. Complex combinational gates are combined with simple gates, using simple Boolean functions. A combinational gate physically consists of real semiconductor components, and represent the logic function of the input, creating specific output.

Table 1 represents the truth table for simple logical functions depending on the inputs.

Figure 2 depicts the logic diagram for the function Y=(AB+C+D) And this function can be represented in three ways: a. Using AND, OR and inverter functions; b. Using AND and NOR functions; c. Using AND, OR and NOR functions.

Figure 2. Three logic diagrams of the logic function Y=¯((AB+C+D))
Figure 2. Three logic diagrams of the logic function Y=¯((AB+C+D))
Figure 3. Logic diagram for the function Y=A+B¯B.
Figure 3. Logic diagram for the function Y=A+B¯B.

#3 Sum of products. Symplification of logic expressions. Binary numbers.

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