PROMs and Programmable Logic Devices

The question asks to identify which type of device has fixed AND gates and programmable OR gates. Let's explore the characteristics of various programmable logic devices (PLDs) to find the correct answer.

Understanding Programmable Read-Only Memories (PROMs)

PROMs, or Programmable Read-Only Memories, are a type of digital memory. While primarily used for storing data, their internal structure perfectly matches the description given in the question.

• A PROM consists of a decoder (which is a fixed array of AND gates) and a programmable OR array.
• The fixed AND gates (decoder) are responsible for generating all possible minterms (product terms) for the input addresses. For example, if there are 'n' input lines, the decoder will generate \$2^n\$ unique minterms.
• The programmable OR gates allow for the selection of specific minterms to be combined, thereby producing the desired output data or function. The connections from the AND gate outputs to the OR gate inputs can be programmed (blown fuses or similar technology) to establish the desired logic.
• Therefore, PROMs indeed have fixed AND gates (the decoder) and programmable OR gates (the output array where data is stored).

Comparing with Other Logic Devices

Let's briefly look at other options to understand their structures:

• PALs (Programmable Array Logic): These devices feature a programmable AND array and a fixed OR array. This means users can program the AND gates to create specific product terms, but the OR gates are pre-wired to sum a fixed number of these product terms.
• FPLAs (Field-Programmable Logic Arrays): These are more flexible than PALs. FPLAs have both a programmable AND array and a programmable OR array. This allows for greater customization of both the product terms and their summation.
• MBM: This is not a standard acronym for a common programmable logic device in this context.

Conclusion on Gate Structure

Based on the analysis, devices that possess fixed AND gates and programmable OR gates are PROMs. The fixed AND array decodes the input addresses, generating all minterms, and the programmable OR array then selects and combines these minterms to produce the desired output, making them suitable for storing data or implementing specific logic functions.