48. Programmable Logic Devices (PLDs): PLA and PAL

Programmable Logic Devices (PLDs) are integrated circuits that allow users to customize the logic function of the device after manufacturing, offering a flexible middle ground between fixed ICs and microprocessors.

General PLD Structure

PLDs implement SOP expressions using two arrays:

1. AND Array (Product Term Generation): Generates all necessary product terms (minterms or simplified products).
2. OR Array (Summation): Sums the required product terms to form the final outputs.

1. PLA (Programmable Logic Array)

• Characteristics: Both the AND array and the OR array are programmable.
• Advantage: Extremely flexible, capable of minimizing complex expressions and sharing product terms among multiple outputs.
• Disadvantage: Complex to manufacture and slow due to two large programmable arrays.

2. PAL (Programmable Array Logic)

• Characteristics: The AND array is programmable, but the OR array is fixed.
• Advantage: Simpler, faster, and cheaper than PLAs because only the AND array needs configuration. Easier to implement in hardware.

PLDs paved the way for modern, vastly more complex devices like FPGAs (Field-Programmable Gate Arrays).