What is a Programmable Logic Array

A programmable logic array (PLA) is a large Scale Integrated programmable logic device which is used for synthesizing combinational logic functions.  It consists of a programmable AND gate array followed by a programmable OR gate array. To synthesize the output logic functions, first of all these are needed to be reduced to their minimum sum of product expressions. In PLA implementation, a designer should try to deduce these expressions in such a way that maximum number of common product terms exist between them. All these product terms are then generated in the AND gate array. From there, these product terms are fed in to the OR gate array where they are added according to the deduced logic expressions in order to get the output functions.

The size of a Programmable logic array is defined in terms of:-

Number of Inputs Number of Product terms Number of Outputs

A 4 input, 8 Product terms and 4 output PLA will be defined as an (4, 8, 4) PLA. As the logic functions are synthesized almost in terms of their minimum sum of product expressions, the digital hardware required in a PLA is much less than that of a PROM. Also the costly decoder circuitry present in a PROM is replaced in the PLA by a more economical AND gate array. Use of less hardware also provides advantages in terms of speed, power consumption etc.

The biggest disadvantage of a Programmable Logic Array is the two step programming involved. A designer has to first of all program the AND gate array and then program the OR gate array according to the programming of the AND gate array. Also PLA implementation requires the designer to try and deduce logic expressions with as many common terms between different expressions as possible. As this is an intuitive skill rather than a set procedure, it makes it very hard to automate or computerize the design process.

To counter this problem, another line of programmable logic devices known as PAL’s have been devised. These involve only one step programming while also having similar hardware advantages of a PLA. In fact, PAL’s (Programmable Array Logic) have become the most popular LSI programmable logic devices today, far surpassing PROM’s and PLA’s.

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