What are PAL and PLA, Design and Differences?

Earlier, the designing of logic circuits can be done using SSI (small scale integration) components like logic gates, multiplexers, de-multiplexers, FFs, etc. But, now a PLD can replace all these SSI components. So this is the reason to decrease the SSI industry compared with PLD, and these are used in several applications. The programmable logic device or PLD is one kind of chip used to implement the logic circuit. It includes a set of logic circuit elements that can be modified in several ways. A PLD is looked like a black box that consists of programmable switches as well as logic gates. The main function of the switches is to let the logic gates within the PLD to be associated mutually to execute logic circuits. PLDs are classified into different types such as SPLD-simple PLD (PLA & PAL), CPLD-complex PLD, FPGAs-field programmable gate arrays. This article discusses what is a PAL and PLA, design and their differences.

What are PAL and PLA?

Both Programmable Array Logic and Programmable Logic Array are types of PLDs (programmable logic devices), and these are mainly used for designing combination logic mutually by sequential logic. The main difference among these two is that PAL can be designed with a collection of AND gates and fixed collection of OR gates whereas PLA can be designed with a programmable array of AND although a fixed collection of OR gate. A programmable logic device offers a simple as well as flexible logic circuit designing.

Programmable Array Logic
Programmable Array Logic

Previous to programmable logic devices, the combinational logic circuits can be designed with multiplexers, and these circuits were rigid as well as compound, then PLDs are developed. The initial programmable logic device was ROM, but it was not successful due to the hardware wastage issues as well as exponential growth enhancement in the every hardware application. To overcome this issue, PAL and PLA were used. These two are programmable, and efficiently uses the hardware.

Programmable Logic Array
Programmable Logic Array

Design of Programmable Array Logic (PAL)

The definition of term PAL or Programmable Array Logic is one type of PLD which is known as Programmable Logic Device circuit, and working of this PAL is the same as the PLA. The designing of the programmable array logic can be done with fixed OR gates as well as programmable AND gates. By using this we can implement two easy functions wherever the associates AND gates with each OR gate denote the highest number of product conditions that can be produced in the form of SOP (sum of product) of an exact function.

As the logic gates like AND is connected continually toward the OR gates, and that indicates that the produced product term is not distributed with the output functions. The major notion behind PLD development is to fabricate a compound Boolean logic onto a single chip by removing the defective wiring, avoiding the logic design, as well as decreasing the consumption of power.

Example of PAL

Implement the following Boolean expression with the help of programmable array logic (PAL)

X =AB + AC’
Y= AB’ + BC’

The above given two Boolean functions are in the form of SOP (sum of products). The product terms present in the Boolean expressions are X & Y, and one product term that is AC’ is common in every equation. So, the total required logic gates for generating the above two equations is AND gates-4 OR programmable gates-2. The equivalent PAL logic diagram is shown below.

PAL Logic Circuit
PAL Logic Circuit

The AND gates which are programmable have the right of entry for normal as well as complemented variable inputs. In the above logic diagram, the available inputs for each AND gate are A, A’, B, B’, C, C’. So, in order to generate a single product term with every AND gate, the program is required.
All the product terms are obtainable at the inputs of an each OR gate. Here, the programmable connections on the logic gate can be denoted with the symbol ‘X’.

Here, the OR gate inputs are fixed. Thus, the required product terms are associated with each OR gate inputs. As a result, these gates will generate particular Boolean equations. The ‘.’ The symbol represents permanent connections.

Design of Programmable Logic Array (PLA)

The definition of term PLA presents the Boolean function in the form of a sum of product (SOP). The designing of this programmable logic array can be done using the logic gates like AND, OR, and NOT by fabricating on the chip, that makes every input as well as its compliment obtainable toward every AND gate.

An every AND gate’s output is connected to the every OR gate. Finally, the output of the OR gate generates the output of the chip. Thus, this is how an appropriate association is finished to use the expressions of the sum of the product. In the programmable logic array, the connections of logic gates like AND & OR are programmable. PLA is expensive and difficult to compare with PAL. The PAL uses two dissimilar developed methods can be used for a programmable logic array for enhancing the effortlessness of programming. In this kind of method, every connection can be done using a fuse on each intersection point wherever the unnecessary connections can be detached by the fuse blowing. The final technique engages the making of connection while the process of the fabrication using the suitable cover offered for the precise interconnection model.

Example of PLA

Implement the following Boolean expression with the help of programmable logic array (PLA)

X = AB + AC’
Y = AB’ +BC + AC’

The above given two Boolean functions are in the form of SOP (sum of products). The product terms present in the Boolean expressions are X & Y, and one product term that is AC’ is common in every equation. So, the total required logic gates for generating the above two equations is AND gates-4, OR programmable OR gates-2. The equivalent PLA logic diagram is shown below.

PLA Logic Circuit
PLA Logic Circuit

The AND gates which are programmable have the right of entry for normal as well as complemented variable inputs. In the above logic diagram, the available inputs for each AND gate are A, A’, B, B’, C, C’. So, in order to generate a single product term with every AND gate, the program is required.
All the product terms are obtainable at the inputs of an each OR gate. Here, the programmable connections on the logic gate can be denoted with the symbol ‘X’.

Difference between PLA and PAL

The Difference between PAL and PLA in Tabular Form mainly includes PAL and PLA full form, construction, availability, flexibility, cost, number of functions, and speed which are discussed below.

Programmable Array Logic (PAL) Programmable Logic Array (PLA)
The full form of PAL is programmable array logic The full form of the PLA is a programmable logic array
The construction of PAL can be done using the programmable collection of AND & OR gates The construction of PLA can be done using the programmable collection of AND & fixed collection of OR gates.
The availability of PAL is less prolific The availability of PLA is more
The flexibility of PAL programming is more The flexibility of PLA is less
The cost of a PAL is expensive The cost of PLA is middle range
The number of functions implemented in PAL is large The number of functions implemented in PLA is limited
The speed of PAL is slow The speed of PLA is high

Thus, this is all about PAL and PLA. From the above information, finally, we can conclude that these are the programmable logic devices (PLDs) where the programmable logic array is more flexible than programmable array logic. But, programmable array logic can effortlessly generate a combinational logic circuit. Here is a question for you, what is the role of PAL and PLA in Digital Electronics?

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