Introduction:
PLA (Programmable Logic Array) and ROM (Read-Only Memory) are two types of digital logic devices used in electronic systems. They have different characteristics and purposes. In this explanation, we will discuss the main difference between PLA and ROM.

PLA:
- PLA is a type of programmable logic device that can be used to implement combinational logic functions.
- It consists of a fixed AND array and a programmable OR array.
- The AND array generates product terms by combining the inputs with the complemented inputs.
- The OR array combines these product terms using programmable connections, creating the desired logic function.
- PLA can be programmed to implement any desired Boolean function by configuring the connections in the OR array.
- It offers flexibility and allows the designer to implement complex logic functions using a relatively small number of gates.
- PLA can be reprogrammed multiple times to implement different functions.

ROM:
- ROM is a type of non-volatile memory that stores data permanently, which can only be read and not modified.
- It is used to store pre-defined data or instructions that are required for the operation of a digital system.
- ROM has a fixed structure that consists of address lines, data lines, and a decoder circuit.
- The data stored in ROM is programmed during the manufacturing process and cannot be changed or erased by the user.
- The data stored in ROM is accessed by providing the appropriate address on the address lines, and the corresponding data is read from the data lines.
- ROM is commonly used to store look-up tables, program instructions, and other static data.

Main Difference: PLA vs ROM
The main difference between PLA and ROM is as follows:

PLA:
- PLA is used to implement combinational logic functions.
- It can be programmed and reprogrammed to implement different functions.
- PLA economizes on the number of minterms to implement Boolean functions.

ROM:
- ROM is used to store pre-defined data or instructions permanently.
- It cannot be modified or reprogrammed by the user.
- ROM has a fixed structure with a fixed set of data.

Conclusion:
In summary, the main difference between PLA and ROM lies in their purpose and functionality. PLA is a programmable logic device used to implement combinational logic functions and can be reprogrammed multiple times. On the other hand, ROM is a non-volatile memory used to store pre-defined data or instructions that cannot be modified.