Comparison Between PLA and PAL: Which One is Right for You?

Integrated circuits known as programmable logic devices, or PLDs, can be set up to perform a variety of digital logic functions. They are utilized in many different applications, ranging from simple logic gates to complex systems.

The two main kinds of PLDs are programmable logic arrays (PLAs) and programmable array logic (PALs). PALs are a more complex type of PLD than PLAs, which are the most basic. 

What are Programmable Logic Arrays (PLAs)?

A programmable logic array (PLA) is a kind of programmable logic device and it is used to implement combinational logic circuits. One way to generate an output in the PLA is to conditionally complement a set of programmable AND, a set of programmable OR, and a set of programmable AND gate planes. For N input variables, it has two N AND gates. For M PLA outputs, it should have M OR gates with programmable inputs from all of the AND gates. This pattern can be used in the sum of products' canonical forms to generate a variety of logic functions. PLAs include programmable AND and OR gate planes, as opposed to programmable array logic devices (PALs and GALs).

What is Programmable Array Logic (PALs)?

Similar to a PLA, a Programmable Array Logic (PAL) is a PLD (Programmable Logic Device) circuit. PAL lacks programmable AND gates in favor of fixed OR gates, unlike PLA. It makes use of two straightforward functions, the number of linked AND gates to each OR gate determining the maximum number of product terms that can be created in the SOP representation of the function. Even though the AND gates are always connected to the OR gates, the resulting product term is not shared with the output functions.

PLAs vs PALs

The OR gate array is the primary way that PALs and PLAs differ from each other. Whereas the OR gate array in PLAs is programmable, the OR gate array in PALs is fixed.

PALs are easier to program than PLAs because of this distinction. Compared to PALs, PLAs can perform a greater range of logic operations.

In addition to the aforementioned issue, PLA and PAL differ in the ways listed below.

• Compared to PLA, PAL responds faster. On the other hand, PAL operates faster than PLA.
• Compared to PAL, PLA has more complexity. In contrast, PLA is more complex than PAL.
• The PLA moves forward gradually. In contrast, PAL moves swiftly.
• PAL is more expensive than PLA. Price-wise, PAL is less expensive than PLA.
• The PLA is more versatile than the PAL. Despite this, PLA is more adaptable than PAL.
• More production and accessibility convenience is provided by the PAL. On the other hand, getting into the PLA is difficult.
• There aren't many functions implemented in PLA. However, the great majority of PAL's features are currently functional.

Features of PLAs and PALs

Features of PLAs

• A PLA, or programmable logic device, is composed of an AND array and an OR array (PLD).
• Using the AND array, the desired logical function's product terms are generated.
• The product terms are combined using the OR array to create the PLA's output.
• Numerous logical operations can be implemented thanks to the programmable connections between the gates in the AND and OR arrays.
• PLAs are widely used in applications where flexibility is essential, such as computer CPUs and other digital logic circuits.
• PLAs are more flexible than PALs, but PALs are faster and less expensive.

Features of PALs

• A PAL is a specific type of PLD that consists of an AND array and a fixed OR array.
• Unlike the OR array, the AND array is programmable.
• The AND array can be used to generate the product terms for the desired logical function.
• A fixed OR array is used to combine the product terms and produce the output for the PAL.
• PALs are commonly used in low-cost, small-scale applications such as embedded systems, where cost or speed are more important than flexibility.
• PALs are faster and less expensive than PLAs, but they are less flexible.

What are PLAs and PALs used for?

PLAs Applications

Compared to PALs, PLAs are more flexible but also slower and more expensive. They are widely used in applications such as computer CPUs and other digital logic circuits where flexibility is essential.

Here are some specific examples of applications for PLAs:

• Computer CPUs: PLAs implement the control logic of computer CPUs. Since the control logic is responsible for selecting the next instruction to be executed, it needs to be able to implement a broad variety of logical functions.
• Digital logic circuits: PLAs are used to implement a variety of digital logic circuits, such as registers, multiplexers, and decoders.
• Industry uses programmable logic controllers, or PLCs, to run machinery. PLAs are a great option for this application since they can carry out a variety of logical operations and are trustworthy and adaptive.

PALs Applications

PALs have less flexibility than PLAs, but they are also faster and less expensive. They are widely used in applications like embedded devices and other small and inexpensive software where speed or cost are more important than flexibility.

Here are some specific examples of PAL applications:

• Embedded systems: Embedded systems are found in a wide range of products, such as toys, appliances, and automobiles. Because they must be inexpensive and both small and fast, PALs are a great choice for this application.
• Instrumentation: Physical processes are monitored and controlled using instrumentation. For this application, PALs are a great choice because they are easy to use, accurate, and dependable.
• Telecommunications: Telecommunications networks have to manage a wide range of traffic. PALs are a great choice for this application due to their speed and scalability.

PLAs and PALs Examples

PLA Example

The example below illustrates the PLA that could be used to carry out the logical operation XOR. Two inputs (A and B) and four product terms (AB, A'B, AB', and A'B') make up the AND array. One output and four inputs (the four product terms) make up the OR array.

AND Array

------------

A | B | AB | A'B | AB' | A'B'

-- | -- | -- | -- | -- | --

1 | 1 | 1 | 0 | 0 | 0

1 | 0 | 0 | 1 | 1 | 0

0 | 1 | 0 | 1 | 0 | 1

0 | 0 | 0 | 0 | 0 | 1

OR Array

------------

AB | A'B | AB' | A'B'

-- | -- | -- | --

1

It is possible to program this PLA to perform the XOR function by correctly connecting the AND and OR arrays. To implement the XOR function, for example, we would create a link between the output of the OR array and the AB product term.

PAL Example

The example below illustrates the PAL that can be used to implement the logical function AND. The two product terms are AB and A'B, and the two inputs are A and B. One output and two fixed inputs (the two product terms) make up the OR array.

AND Array

------------

A | B | AB | A'B

-- | -- | -- | --

1 | 1 | 1 | 0

1 | 0 | 0 | 1

OR Array

------------

AB | A'B

-- | --

1

This PAL can be configured to perform the AND operation by correctly connecting the AND array to the fixed OR array. For example, to create the AND function, we would link the AB product term and the output of the OR array.

Pros and cons of PLAs and PALs

Pros and cons of PLAs

Pros:

• PALs can adapt better
• Able to perform a wider variety of logic operations
• Suitable for use in more complex applications

Cons:

• More expensive than PALs
• More challenging to program

Pros and cons of PALs

Pros:

• Less expensive than PLAs
• Simpler to write
• Suitable for simpler applications

Cons:

• less flexible than PLAs
• Capable of performing a limited number of logical operations

Which One is Right for You?

Which PLD is best for you will depend on your specific application. If you need to implement a simple logic function and want a PLD that is easy to program, a PAL is a good choice. If a complex logic function needs to be implemented, a PLA is a better choice.

When deciding between PLA and PAL, you should take the following into account:

• The level of complexity of the logic function you have to apply
• PLD programming's ease of use
• The PLD's cost
• The accessibility of the PLD

Also, you can check Ampheo to have a better understanding of PLAs, PALs, and even more.

Conclusion

In conclusion, a vast array of digital logic operations can be performed by both PALs and PLAs. The best PLD for you will depend on your specific application.