Learn everything about Programmable Logic Arrays (PLA) in digital logic, including architecture, working, design procedure, applications, and advantages. Master how PLAs are used for efficient combinational circuit implementation.
Introduction to Programmable Logic Array (PLA)
In digital electronics, flexibility in implementing combinational logic functions is highly desirable. Programmable Logic Arrays (PLAs) are versatile devices that allow engineers to design custom logic circuits without using a fixed set of gates.
A PLA is a type of programmable logic device (PLD) that can implement multiple Boolean functions by programming its internal AND and OR planes. It is widely used in digital systems, microprocessors, and embedded applications where custom logic functions are required.
Key Features of PLA
- Programmable AND-OR array: The logic functions can be programmed according to the design requirements.
- Multiple outputs: Each output can implement a different Boolean function.
- Reduced hardware complexity: Combines multiple gates in a single array.
- Flexible combinational logic implementation: Any logical expression can be programmed in SOP (Sum of Products) form.
- Reconfigurable: PLAs can be programmed once, or in some cases, reprogrammable variants exist.
PLA Architecture
A typical PLA consists of three main components:
- Input Lines: Carry the binary inputs to the device.
- AND Plane: A programmable array of AND gates used to generate product terms (minterms).
- OR Plane: A programmable array of OR gates used to sum the selected product terms to generate outputs.
Illustration of PLA Structure:Learn everything about Programmable Logic Arrays (PLA) in digital logic, including architecture, working, design procedure, applications, and advantages. Master how PLAs are used for efficient combinational circuit implementation.
Introduction to Programmable Logic Array (PLA)
In digital electronics, flexibility in implementing combinational logic functions is highly desirable. Programmable Logic Arrays (PLAs) are versatile devices that allow engineers to design custom logic circuits without using a fixed set of gates.
A PLA is a type of programmable logic device (PLD) that can implement multiple Boolean functions by programming its internal AND and OR planes. It is widely used in digital systems, microprocessors, and embedded applications where custom logic functions are required.
Key Features of PLA
- Programmable AND-OR array: The logic functions can be programmed according to the design requirements.
- Multiple outputs: Each output can implement a different Boolean function.
- Reduced hardware complexity: Combines multiple gates in a single array.
- Flexible combinational logic implementation: Any logical expression can be programmed in SOP (Sum of Products) form.
- Reconfigurable: PLAs can be programmed once, or in some cases, reprogrammable variants exist.
PLA Architecture
A typical PLA consists of three main components:
- Input Lines: Carry the binary inputs to the device.
- AND Plane: A programmable array of AND gates used to generate product terms (minterms).
- OR Plane: A programmable array of OR gates used to sum the selected product terms to generate outputs.
Illustration of PLA Structure:
Inputs → Programmable AND Plane → Programmable OR Plane → Outputs- Each input line can be connected to multiple AND gates.
- Each AND gate output can be connected to multiple OR gates.
- Outputs represent custom Boolean functions in SOP form.
Working Principle of PLA
- The inputs are applied to the PLA.
- The programmable AND plane generates all necessary product terms for the Boolean functions.
- The programmable OR plane sums the selected product terms to generate final outputs.
- Each output can implement a different Boolean function simultaneously.
Example:
Suppose we want to implement two Boolean functions:
- F1 = A·B + A’·C
- F2 = B·C + A·C
Steps:
- Program the AND plane to generate minterms: A·B, A’·C, B·C, A·C
- Program the OR plane:
- F1 = (A·B) + (A’·C)
- F2 = (B·C) + (A·C)
All outputs are generated from the same PLA using a single device.
Design Procedure for PLA
- List all Boolean functions required for the outputs.
- Convert each function to SOP form.
- Identify all product terms (minterms) across all functions.
- Program the AND plane to produce the required minterms.
- Program the OR plane to sum the appropriate product terms for each output.
- Connect inputs and outputs according to design requirements.
- Simulate the design using digital logic tools like Logisim, Proteus, or Multisim.
Advantages of PLA
- Implements multiple Boolean functions in a single device.
- Reduces circuit size and hardware cost.
- Flexible and suitable for custom combinational logic design.
- Simplifies logic function updates in certain programmable versions.
- Can replace multiple discrete gates with a single programmable array.
Disadvantages of PLA
- Limited by the number of inputs and outputs supported by the device.
- Speed may be slower than fixed combinational logic circuits due to gate propagation delays.
- For very large logic functions, PLAs may become impractical due to hardware size.
Applications of PLA
- Digital System Design: Implementing custom combinational logic in CPUs, ALUs, and controllers.
- Embedded Systems: Custom logic for microcontroller peripherals.
- Address Decoding: Used in memory-mapped circuits.
- Signal Processing: Generate specific logic functions in DSP circuits.
- Learning & Prototyping: Ideal for students and engineers to test combinational logic designs.
PLA vs PAL
| Feature | PLA | PAL |
|---|---|---|
| AND Plane | Programmable | Fixed |
| OR Plane | Programmable | Programmable |
| Flexibility | High (AND & OR programmable) | Medium (AND fixed, OR programmable) |
| Complexity | Can implement complex functions | Simpler logic functions |
| Applications | Complex combinational circuits | Simpler combinational circuits |
Conclusion
Programmable Logic Arrays (PLAs) provide an efficient and flexible solution for implementing combinational logic. Their ability to program multiple outputs from a single device reduces hardware complexity, making them essential in digital system design, embedded electronics, and prototyping.
Call to Action:
Practice designing a PLA for 3-input, 2-output Boolean functions and simulate it using Logisim or Multisim to gain hands-on experience with programmable logic devices.
Frequently Asked Questions (FAQ)
1. What is the main function of a PLA?
A PLA implements multiple Boolean functions using a programmable AND-OR array.
2. How is PLA different from PAL?
In PLA, both AND and OR planes are programmable; in PAL, only the OR plane is programmable.
3. Can a PLA implement multiple outputs?
Yes, PLAs can simultaneously implement multiple Boolean functions with different outputs.
4. Where are PLAs commonly used?
PLAs are used in ALU design, embedded systems, memory decoding, and combinational logic prototyping.
5. What type of Boolean expression is implemented using PLA?
PLAs typically implement Sum of Products (SOP) expressions.
