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and
and
( F, F ), ( F′ , F ), ( F, F′ ), ( F′ , F′
)
Considering the four possible pairs 1 2 1 2 1 2 1 2
The minimum number of product terms is (4) and is obtained if we implement F 1 in
complement form and F2 in true form. The following PLA programming table is used.
Programmable Array Logic (PAL)
PAL has a fixed array of OR gates and programmable array of AND gates. It is easier
to program than the PLA but is less flexible. It is suitable to implement a number of
Boolean functions which have no or a few overlapping product terms.
When we implement Boolean functions with PAL, we must simplify the functions to
fit the sections of the PAL. A product term cannot be shared among two or more OR
gates. If the function cannot fit in one section, then we may use two sections to
implement the function.
A typical PAL integrated circuit may have 8 inputs, 8 outputs, and 8 sections, each
consisting of an 8-wide AND-OR array. The output terminals are sometimes driven by
three-state buffers or inverters.
The logic diagram of a typical PAL is shown in the following Figure.
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