Programmable Logic Array (PLA)

A programmable logic array (PLA) performs the same basic function as a ROM. A PLA with n inputs and m outputs (as shown in the figure below) can realize m functions of n variables

The internal organization of the PLA is different from that of a ROM. The decoder is replaced with an AND array which realizes selected product terms needed to form the output functions, so a PLA implements a sum-of-products expression, while a ROM directly implements a truth table. A PLA can also be expressed by AND-OR array as shown below.

The contents of a PLA can be specified by a PLA table as shown above. The input side of the table specifies the product terms. The symbols 0,1, and – indicate whether a variable is complemented, not complemented, or not present in the corresponding product term. The output side of the table specifies which product terms appear in each output functions. A 1 or 0 indicates whether a given product term is present or not present in the corresponding output function. Thus the first row of the table indicates that the term A’B’ is present in output functions F0 and F2, and the second row indicates that AC’ is present in F0 and F1.

By: Jonald Sarmiento