PLAs (Programmable Logic Arrays): A Comprehensive Guide

In the world of digital electronics, the ability to customize logic circuits for specific applications has revolutionized the way we design and implement hardware systems. Programmable Logic Arrays (PLAs) represent one of the key components in this domain, offering flexibility in designing logic circuits while ensuring efficient use of hardware resources.

This blog will provide an in-depth look at PLAs, their structure, functionality, applications, and how they compare to other programmable logic devices. Whether you’re a student of electronics or a professional looking to deepen your understanding, this post will guide you through everything you need to know about PLAs.

What is a Programmable Logic Array (PLA)?

A Programmable Logic Array (PLA) is a type of digital logic device used to implement combinational logic circuits. It consists of two programmable planes: an AND plane and an OR plane. By configuring these planes, designers can create custom logic circuits that meet specific requirements.

The core idea behind PLAs is the ability to program the logic functions after the hardware has been manufactured, offering a degree of flexibility that traditional fixed-function logic gates don’t provide. This makes PLAs especially useful in situations where logic functions need to be adapted or modified without redesigning the entire circuit.

Key Characteristics of PLAs:

- ***Programmability*** : As the name suggests, PLAs are programmable, meaning their logic can be defined by the user. This allows for custom logic functions without needing to manufacture a new circuit for every design.
• AND-OR Structure : PLAs consist of a programmable AND plane followed by a programmable OR plane. This structure allows the device to realize any combinational logic function by forming the required sum-of-products (SOP) expressions.
• Customizable Logic : Designers can implement various Boolean functions within the same PLA by configuring the connections between the input lines, AND gates, and OR gates.
• Efficiency : PLAs allow for the implementation of multiple logic functions within a single device, reducing the need for large, complex circuits made up of many individual gates.

Structure of a PLA

To understand how a PLA works, it’s essential to dive into its internal structure. A typical PLA is organized into three main parts:

- ***Input Lines*** : These are the binary inputs to the PLA, which are used to define the logic that the device will implement.
• AND Plane : This is the first programmable layer of the PLA. In this layer, the input lines are connected to an array of AND gates. Each AND gate performs the logical AND operation on one or more inputs or their complements, allowing for the creation of product terms.
• OR Plane : The output of the AND gates is fed into the programmable OR plane, where these product terms are combined using OR gates to form the final output. This OR plane allows for the creation of a sum-of-products (SOP) expression for the desired Boolean logic function.

The general operation of a PLA can be represented as follows:

- The inputs (both true and complemented values) are fed into the AND plane.
• The AND gates in the AND plane generate product terms (AND combinations of inputs).
• The outputs from the AND plane are fed into the OR plane, where they are combined to form a sum of products (SOP) expression.
• The final output is produced by combining these SOP expressions.

Example of a PLA Implementation

To illustrate how a PLA works, let’s consider an example where we want to implement the following two Boolean functions:

- ***F1*** = A'B + AB'
• F2 = A’B + AB + AB'

In a PLA, the first step is to define the product terms. In this case, the product terms would be:

- A'B
• AB'

The next step is to configure the AND plane to generate these product terms, and then the OR plane combines these product terms to form the final outputs for F1 and F2 .

- For ***F1*** , we can create the SOP expression by OR-ing A'B and AB'.
• For F2 , we can create the SOP expression by OR-ing A’B and AB.

This illustrates the flexibility of PLAs: you can implement multiple Boolean functions with the same AND terms, saving space and increasing efficiency.

Advantages of PLAs

PLAs offer several advantages over traditional fixed-function logic circuits or gate-level implementations. Some key advantages include:

1. ***Customizability***

The primary benefit of PLAs is their programmability. Rather than relying on pre-designed logic gates, designers can create custom logic circuits that meet their specific requirements. This is particularly useful when working with complex combinational logic that would require numerous individual gates.

2. ***Efficiency***

PLAs allow multiple logic functions to be implemented within a single device. Instead of using several discrete logic gates for each function, a PLA can implement several Boolean functions with the same set of input variables. This reduces the overall complexity of the circuit and minimizes the space required on a printed circuit board (PCB).

3. ***Cost-Effectiveness***

Because PLAs are programmable, they reduce the need for creating custom hardware for every new logic function. This can save manufacturing costs, especially in prototyping or situations where the design may change frequently. PLAs are also widely available and inexpensive, making them a practical choice for many applications.

4. ***Faster Development***

When designing digital systems, the flexibility of PLAs speeds up the development process. Instead of building new circuits from scratch for every function, developers can reprogram the PLA to meet new requirements. This ability to make rapid changes is particularly valuable in early stages of development, where design specifications are subject to frequent revisions.

Disadvantages of PLAs

Despite their advantages, PLAs do have some limitations:

1. ***Scalability***

While PLAs are excellent for small to medium-sized logic circuits, they may not be as efficient for large-scale designs. The number of input and output variables in a PLA is limited, and increasing the number of logic functions can make the device bulky and inefficient.

2. ***Limited Sequential Logic***

PLAs are typically used for combinational logic rather than sequential logic . While they are efficient at implementing combinational circuits, more complex devices like Field Programmable Gate Arrays (FPGAs) or Complex Programmable Logic Devices (CPLDs) are often better suited for designs requiring sequential logic, such as state machines or memory-based designs.

3. ***Power Consumption***

PLAs, especially large ones, can consume significant power. For designs where power efficiency is critical, more modern solutions like FPGAs or application-specific integrated circuits (ASICs) may offer better power performance.

PLA vs. Other Programmable Logic Devices (PLDs)

PLAs are just one type of programmable logic device. Other common types include Programmable Array Logic (PAL) , Complex Programmable Logic Devices (CPLD) , and Field Programmable Gate Arrays (FPGA) . Here’s how PLAs compare to these alternatives:

1. ***PLA vs. PAL***

While both PLAs and PALs are programmable logic devices, the key difference lies in their structure. In a PLA, both the AND and OR planes are programmable, offering greater flexibility. In a PAL, only the AND plane is programmable, and the OR plane is fixed. This makes PALs simpler and less flexible than PLAs, but also faster and cheaper for simpler applications.

2. ***PLA vs. CPLD***

CPLDs are more advanced than PLAs and offer greater scalability. While PLAs are generally used for small to medium-scale logic functions, CPLDs are designed for more complex logic circuits and can handle both combinational and sequential logic. CPLDs also offer non-volatile memory, meaning they retain their configuration even when powered off, unlike PLAs, which rely on external storage for configuration.

3. ***PLA vs. FPGA***

FPGAs represent the most advanced form of programmable logic devices. While PLAs and PALs are typically limited to combinational logic, FPGAs can handle highly complex designs involving both combinational and sequential logic. FPGAs also offer significantly more inputs, outputs, and programmable logic elements compared to PLAs. However, FPGAs are more complex and expensive than PLAs, making them overkill for simple logic circuits where PLAs can do the job.

Applications of PLAs

PLAs find their applications in various industries and electronic systems where flexibility in logic design is needed. Some common applications include:

1. ***Prototyping and Development***

In the early stages of digital design, engineers often use PLAs to prototype new logic circuits. Because PLAs are programmable, they allow for rapid testing and iteration without the need to manufacture new hardware for each revision.

2. ***Control Systems***

PLAs are used in control systems where combinational logic is required to manage inputs and outputs based on specific conditions. They are often employed in simple digital controllers, switches, and signal processing systems.

3. ***Embedded Systems***

In embedded systems, PLAs can be used to implement custom logic functions for various tasks, such as signal processing or decision-making algorithms. They provide a low-cost solution for logic implementation without needing more advanced devices like FPGAs or microcontrollers.

4. ***Automotive Electronics***

PLAs are also used in automotive electronics, where they perform simple but essential tasks such as managing sensors, signals, and control systems. The programmability of PLAs makes them a good fit for automotive applications where flexibility is needed to accommodate different system requirements.

Conclusion

Programmable Logic Arrays (PLAs) represent a powerful and flexible solution for implementing combinational logic circuits. They offer the ability to customize logic designs while maintaining efficiency in terms of space and cost. Although PLAs may

not be the best choice for large or complex designs, they remain an essential tool for digital designers, especially in applications requiring simple, programmable logic.

As the demand for adaptable, reconfigurable hardware continues to grow, PLAs will likely remain a popular choice for prototyping, control systems, and embedded applications. For many, they provide an ideal balance between flexibility, efficiency, and cost-effectiveness in digital design.