Question

(i) Compare

(a) Mask Programmable Logic Devices, (b) Field Programmable Logic Devices and (c) Field Programmable Gate Arrays.

(ii) Draw an appropriate schematic diagram to describe following programmable logic devices and discuss their advantages and disadvantages.

(a) Programmable Logic Array

(b) Programmable Array Logic

Answer 1

2 Programmable Logic Array(PLA) is a fixed architecture logic device with programmable AND gates which is followed by programmable OR gates.
PLA is basically a type of programmable logic device used to build a reconfigurable digital circuit. PLDs have an undefined function at the time of manufacturing but they are programmed before made into use. PLA is a combination of memory and logic.

Programmable Array Logic (PAL) is a type of Programmable Logic Device (PLD) used to realize a particular logical function. PALs comprise of an AND gate array followed by an OR gate array. However, we should observe that only the AND gate array is programmable. we cannot programme the OR gate arrays. This is because here the inputs are fed to the AND gates through fuses which act as programmable links.

Programmable-AND and fixed-OR structure of PALs make them less flexible from a programming point of view when compared with Programmable Logic Arrays (PLAs). But because of this same reason PALs are less expensive than PLAs.