Programmable logic arrays – is an OR array programmed by blowing fuses to remove selected connections?

Introduction / Context:
Classic programmable logic devices include PROM, PLA, and PAL. Understanding which array (AND or OR) is user-programmable and how programming occurs (e.g., fuse blowing) is key to legacy digital design knowledge and reverse engineering older boards.

Given Data / Assumptions:

• PROM: typically fixed AND (minterm generator), programmable OR.
• PLA: both AND and OR arrays programmable.
• PAL: usually programmable AND, fixed OR (device-dependent variants exist).
• Historical devices used fusible links to make or break connections.

Concept / Approach:
In PROM-style architectures, programming the OR array involves blowing fuses to disconnect certain minterm lines from an output OR plane, effectively removing product-term connections so that only the desired minterms contribute to each output function. Describing this as “eliminating selected variables/connections from the output functions” is a common summary for beginners, even though what is actually eliminated are product-term connections rather than literal algebraic variables.

Step-by-Step Solution:

Verification / Alternative check:
Textbook block diagrams show a fixed decoder (AND) generating all minterms and a programmable OR matrix with fusible links to outputs. Modern non-volatile cells (EPROM/EEPROM/Flash/antifuse) replace literal fuses but the logical idea of connect/disconnect remains.

Why Other Options Are Wrong:
“Incorrect” would deny well-documented PROM behavior. “Only the AND array is programmable” applies to many PALs, not PROMs. “Only EEPROM cells are used” is historically false; many devices used metal fuses. “Depends solely on fan-in limits” is off-topic; fan-in affects capacity, not the programming principle.

Common Pitfalls:
Taking the word “variables” literally; in hardware you program connections to product terms, which correspond to minterms in Boolean expressions. Another pitfall is assuming all PLDs have the same programmable plane; families differ.

Final Answer:
Correct