EPROMs

EPROM stands for erasable programmable read-only memory. It is a non-volatile memory technology used to store data even when power is removed. EPROMs consist of floating-gate MOS transistors, where data is represented by the amount of charge on the floating gate, which shifts the transistor’s threshold and changes its conduction state. Information is initially written by applying high programming voltages that inject charge into the floating gate; the stored state remains without power.

Erasure of an EPROM is accomplished by exposing the device to ultraviolet light through a quartz window in the package. The UV exposure releases the trapped charge, returning the memory cells to their erased state and allowing the device to be reprogrammed. The classic EPROM package is a ceramic DIP with a transparent window; later variants used plastic packages or windowless designs that required removal from circuits for erasure or used alternative erasable technologies.

Programming and access characteristics vary by device, but EPROMs generally require higher voltages during programming and have slower erase and read cycles compared with modern memory types. Typical devices offered times in the hundreds of nanoseconds range for reads, with programming and erasure requiring more time. The front-end compatibility of EPROMs made them popular for firmware storage in computers and embedded systems during the 1970s through the 1980s.

Over time, EPROMs were largely supplanted by EEPROM and flash memory, which can be erased and reprogrammed electrically in-circuit without removing the device or using UV light. EPROMs remain of historical and niche use for legacy hardware and specialized applications.