Programmable Array Logic (PAL) is a commonly used programmable logic device (PLD). It has programmable AND array and fixed OR array. Because only the AND array is programmable, it is easier to use but not flexible as compared to Programmable Logic Array (PLA). PAL’s only limitation is number of AND gates.
What is Programmable Array Logic (PAL)?
The Programmable Array Logic (PAL) is a programmable logic device that allows the implementation of the combinational logic circuits with the fixed OR array and a programmable AND array. This architecture enables the design of the specific logic functions by the configuring the connections in the AND array while the OR array remains static. The PALs are commonly used in digital systems for the creating custom logic circuits efficiently. PAL consist of small programmable read only memory (PROM) and additional output logic used to implement a particular desired logic function with limited components.
Key Components of Programmable Array Logic (PAL)
Programmable AND Array
The AND array consists of the multiple AND gates whose connections can be programmed by the designer. This allows for the creation of the various logic functions by the combining different input signals.
Fixed OR Array
The OR array is a set of fixed OR gates that combine the outputs from programmable AND gates. The outputs from the AND gates feed into the OR gates allowing for the final logic outputs.
Inputs and Outputs
The PAL devices have a number of inputs that feed into the AND array and a defined number of the outputs that represent the result of the logic operations.
Programming Mechanism
The PALs are programmed using the hardware description language (HDL) or through the special programming tools. This programming defines how the inputs are connected to the AND gates.
Comparison with other Programmable Logic Devices
Main difference between PLA, PAL and ROM is their basic structure. In PLA, programmable AND gate is followed by programmable OR gate. In PAL, programmable AND gate is followed by fixed OR gate. In ROM, fixed AND gate array is followed by programmable OR gate array. Describing the PAL structure (programmable AND gate followed by fixed OR gate).
Example: Realize the given function by using PAL: Any form from sum of product (SOP) form or product of sum (POS) can be used for realization of a Boolean function.
There are three inputs A, B, C and three functions X, Y, Z. Using sum of product (SOP) terms to express the given function as follows:-
[Tex]X(A, B, C)=\sum(2, 3, 5, 7) [/Tex]
[Tex] Y(A,B,C)=\sum (0,1,5)[/Tex]
[Tex] Z(A,B,C)=\sum (0,2,3,5)[/Tex]
Following Truth table will be helpful in understanding function on number of inputs:
Here ,place 1 as we take in part of the question( for example there is given that X=2,3,5,7 place 1 in the column of X for this values)
| A | B | C | X | Y | Z |
|---|
| 0 | 0 | 0 | 0 | 1 | 1 |
|---|
| 0 | 0 | 1 | 0 | 1 | 0 |
|---|
| 0 | 1 | 0 | 1 | 0 | 1 |
|---|
| 0 | 1 | 1 | 1 | 0 | 1 |
|---|
| 1 | 0 | 0 | 0 | 0 | 0 |
|---|
| 1 | 0 | 1 | 1 | 1 | 1 |
|---|
| 1 | 1 | 0 | 0 | 0 | 0 |
|---|
| 1 | 1 | 1 | 1 | 0 | 0 |
|---|
Finding X, Y, Z: Look for high min terms (function value is equal to 1 in case of SOP) in each function output:
X = A’B + AC
Y = A’B’ + B’C
Z = A’B + A’C + AB’C 
AND array has been programmed but have to work with fixed OR array as per requirement. Desired lines will be connected in PLDs.
Advantages of PAL
- Highly efficient
- Low production cost as compared to PLA
- Highly secure
- High Reliability
- Low power required for working.
- More flexible to design.
Disadvantages of PAL
- Limited Flexibility: Once programmed, the configuration of the AND array cannot be altered limiting the flexibility of design compared to the more advanced programmable devices like Field Programmable Gate Arrays (FPGA).
- Size Constraints: The PAL devices may have a limited number of logic gates and inputs which can restrict the complexity of the logic circuits that can be implemented.
- Propagation Delay: The fixed structure of the OR array may introduce propagation delays that can affect the performance of the high-speed applications.
Applications of PAL
- Digital Signal Processing: The PALs are used in the various digital signal processing applications where custom logic is required for the filtering and signal manipulation.
- Control Systems: They are employed in the control systems to implement specific logic functions such as the state machines and control logic for the automation.
- Embedded Systems: The PALs are commonly found in embedded systems for the implementing simple logic functions without the requiring extensive hardware resources.
- Communication Systems: Used in communication systems for the routing signals and implementing encoding/decoding functions.
Conclusion
Programming Array Logic (PAL) devices are valuable tools in the digital circuit design offering a programmable approach to implementing combinational logic. While they have some limitations in flexibility and scalability their applications in the various fields highlight their importance in the modern electronics. Understanding PALs is essential for the engineers and designers working with the digital systems.
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