EPLDs

Erasable Programmable Logic Device (EPLD) is a type of programmable logic device that uses erasable memory to implement complex logic functions. EPLDs are programmable after manufacture and can be erased and reprogrammed by exposing the device’s EPROM memory to ultraviolet light. They were designed to offer more logic capacity and flexibility than simple programmable array logic devices while remaining more approachable than full custom designs, making them suitable for a variety of glue logic and control applications.

Architecturally, EPLDs typically consist of a programmable array of logic resources organized into terms that feed into a programmable OR (or similar) network, with routing resources that connect terms to outputs. The configuration data resides in EPROM cells, which determine the logic terms and interconnections. Programming is performed with an external programer, while erasing is accomplished by UV exposure. Because EPROM cells retain data without power, EPLDs enjoy non-volatile configuration once programmed, though erasing requires a UV step to reset the device for a new program.

In practice, EPLDs found use in the 1980s and early 1990s for embedded logic, interface control, and glue logic where a fixed-function approach was insufficient but full ASIC development was not warranted. They offered a middle ground between fixed logic and more modern non-volatile programmable devices. With the advent of EEPROM-based EPLDs, CPLDs, and eventually FPGAs, the popularity of UV-erasable EPLDs declined. Today, they are largely of historical interest, though they contributed to the evolution of programmable logic and provided an accessible path for customers to customize logic circuits before newer technologies emerged.