EPROM (Erasable Programmable Read-Only Memory) is a non-volatile memory technology that can be programmed by applying high voltage pulses to store charge in floating-gate transistors, and erased by exposing the chip's quartz window to ultraviolet (UV) light for several minutes. Once programmed, it retains data without power for decades under normal conditions, though charge retention is not literally infinite and can degrade over very long timescales or under environmental stress.
In practice
EPROM was a dominant code-storage technology from the 1970s through the 1990s, appearing in a wide range of embedded systems, industrial controllers, and early personal computers. Classic part families include the Intel 2716 (2 KB), 2732 (4 KB), 2764 (8 KB), 27128 (16 KB), 27256 (32 KB), and 27512 (64 KB), all sharing a broadly compatible parallel address/data interface; however, pinout and voltage differences between members can prevent direct interchangeability without board or configuration changes. Programming requires a dedicated EPROM programmer that applies a high-voltage VPP programming supply (commonly in the range of 12.5 V to 21 V, varying by family and generation) while cycling through addresses.
The defining characteristic of EPROM is its UV erase mechanism. The ceramic DIP package of a UV-erasable device includes a quartz window over the die. Erasing typically requires roughly 15 to 30 minutes of exposure to a shortwave UV lamp at approximately 253.7 nm, though actual erase time varies by device, lamp intensity, distance, and temperature. The entire array is erased at once; selective byte or page erase is not possible. Windowed packages are noticeably more expensive than one-time-programmable (OTP) versions of the same die, which are sold in opaque plastic packages for production use.
In modern embedded development, standalone EPROMs are largely obsolete, replaced first by EEPROMs (which erase electrically, byte by byte) and then by NOR Flash (which offers faster erase, in-system programmability, and high density). You may still encounter EPROMs in legacy industrial equipment, vintage computer restoration, or hobbyist retro-computing projects. When servicing such hardware, be aware that EPROMs left under direct sunlight or strong UV sources for extended periods can lose data, as UV exposure causes gradual charge bleed-off; even prolonged ambient light exposure can cause slow degradation over years.
A common pitfall when handling EPROMs is confusing OTP (one-time programmable) variants with windowed UV-erasable ones. OTP parts use the same die but cannot be erased; attempting to reprogram an already-programmed OTP cell partially works for bits that need to go from 1 to 0 (adding charge), but you cannot restore a 0 bit to 1 without erasure. Always verify whether your part has a window before assuming it is re-erasable.
Frequently asked
How is EPROM different from EEPROM?
EPROM is erased by UV light and can only be erased as a complete array, requiring removal from the circuit.
EEPROM (Electrically Erasable Programmable ROM) is erased by electrical signals and supports byte- or page-level erase in-system, without removing the chip. EEPROM is significantly more convenient for field updates and is the technology underlying modern data-storage EEPROMs and NOR
Flash.
Can you reprogram an EPROM without erasing it first?
Partially. Because programming only moves bits from 1 (erased/uncharged) to 0 (programmed/charged) in the typical floating-gate logic convention, you can overwrite locations that are still all-ones without a prior erase. However, you cannot change a 0 bit back to 1 electrically. If any target byte requires a bit to go from 0 to 1, a full UV erase of the entire device is required first.
How long does UV erasure take, and can regular light damage stored data?
A dedicated UV eraser at approximately 253.7 nm typically takes around 15 to 30 minutes for a full erase, though actual time varies by device, lamp intensity, and distance. Direct sunlight carries meaningful UV content and poses a real risk over extended exposure; office fluorescent lighting is generally a much smaller concern, but can cause slow degradation over very long periods. As a precaution, opaque labels are commonly placed over the quartz window during normal use.
What is the difference between a windowed EPROM and an OTP EPROM?
Both use the same floating-gate die. The windowed version is packaged in a ceramic DIP with a quartz window to allow UV erasure and is intended for development and low-volume use. The OTP (one-time programmable) version uses an opaque plastic package with no window; it can be programmed once but never erased. OTP parts are cheaper to manufacture and are used in production when the firmware is final.
Are EPROMs still used in new designs?
Rarely. NOR
Flash offers higher density, in-system erase and program capability, and no need for a UV eraser or a separate programmer for most devices, making it the standard choice for code storage in new embedded designs. EPROM is mostly encountered in legacy industrial systems, retro-computing restoration, and niche applications where the existing PCB footprint or supply chain is already committed to a 27-series part.
Differentiators vs similar concepts
EPROM is frequently confused with
EEPROM and with OTP ROM. EEPROM uses electrical erasure at the byte or page level and supports in-system reprogramming, while EPROM requires UV light to erase the entire array out of circuit. OTP ROM (including OTP EPROM) uses the same floating-gate cell as EPROM but is packaged without a quartz window, making erase impossible in practice. NOR
Flash is the modern successor to EPROM: it is electrically erasable in sectors, supports in-system programming via standard
SPI or parallel interfaces, and is available in much higher densities. The term "PROM" (Programmable ROM) is sometimes used loosely, but strictly refers to fusible-link one-time-programmable devices that predate floating-gate technology entirely.
