EPROM

An EPROM is a non-volatile memory chip that retains data after power is removed, yet allows the user to erase its contents and write new data into it. The term stands for erasable programmable read-only memory. Unlike mask-programmed ROMs, which are fixed at manufacture, an EPROM can be reprogrammed in the field, making it valuable for prototyping, firmware updates, and situations where flexibility is required before committing to mass production.

The core of an EPROM is an array of floating-gate transistors. Each cell stores a bit by trapping charge on an isolated polysilicon gate, usually for ten years or more. To erase the entire chip at once, the device is exposed to ultraviolet light (typically at 254 nanometers) for ten to twenty minutes through a quartz window on the package. The UV photons give trapped electrons enough energy to escape, resetting all cells to the logical 1 state. The chip can then be reprogrammed by applying higher voltages (around 12 to 21 volts, depending on design) to inject fresh charge into selected cells.

Standard EPROM packages came in familiar plastic or ceramic DIP, PLCC, and later surface-mount forms. Common capacities ranged from 2 kilobits to several megabits, though the practical limit hovered around 4 megabits before faster alternatives became dominant. Access time typically fell between 150 and 450 nanoseconds. Erasure cycles were not truly unlimited: the floating-gate oxide would degrade after roughly 100 to 1000 program-erase cycles, though this rarely mattered for development work.

Variants and replacement

EEPROM (electrically erasable programmable read-only memory) improved upon EPROM by allowing byte-by-byte electrical erasure without removing the chip from the circuit, eliminating the UV lamp entirely. Flash memory further refined this approach with faster block erasure and higher density, eventually displacing both EPROM and EEPROM from most roles. Today EPROMs are encountered mainly in legacy systems and as educational examples of how non-volatile memory works.

In practice, the UV window made EPROMs inconvenient for field service and left them vulnerable to accidental erasure if exposed to sunlight or powerful fluorescent lamps over years. Engineers sometimes covered the window with tape or opaque stickers to lock the device once programmed. The need to pull the chip into an external programmer, wait for UV exposure, and reinstall it created friction that disappeared once EEPROMs reached similar density and cost in the 1990s. EPROM nonetheless became the workhorse of embedded systems during the 1980s, appearing in microcontroller boards, industrial controllers, and early personal computers.