FAQ
TL;DR: Sunlight can erase up to 90 % of a 27C256 EPROM in 14 days [Elektroda, TechEkspert, post #15866935]; “first errors showed after two weeks” [Elektroda, TechEkspert, post #15867500] Outdoor UV-C is weak, so full erasure varies with weather and chip design. Why it matters: Field devices that still use windowed EPROMs can lose code unexpectedly if left unshielded.
Quick Facts
• EPROM erase spec: 15 Ws/cm² at 253.7 nm, 12 mW/cm² for 20 min [Elektroda, TechEkspert, post #15866935]
• Data-retention rating: >200 years when shielded [Elektroda, TechEkspert, post #15866935]
• First bit flips in sunlight: ≈14 days; avalanche after ≈21 days [Elektroda, TechEkspert, post #15867500]
• Atmospheric UV-C at ground level: <0.5 % of space value *EPA UV Factsheet*
• Soviet 2716 clones needed several hours of UV to clear [Elektroda, deus.ex.machina, post #15871604]
Can direct sunlight really erase a windowed EPROM?
Yes. A 27C256 left on a roof for two weeks showed most cells changed to 1 (state FF) [Elektroda, TechEkspert, post #15866935] The quartz window passes the UV-B/UV-C that accumulates charge and clears the floating gate.
How long does solar erasure take?
In the cited test, first single-bit errors appeared after 14 days and near-total erase after 21 days of summer sun [Elektroda, TechEkspert, post #15867500] Wikipedia estimates one week in peak sunlight, three years under a fluorescent lamp [Elektroda, TechEkspert, post #15866935]
What wavelength and intensity does a professional EPROM eraser use?
Manufacturers specify 253.7 nm (mercury line). The required dose is 15 Ws/cm², achievable with 12 mW/cm² UV-C for 20 minutes [Elektroda, TechEkspert, post #15866935]
What early symptoms show radiation damage before full erase?
Single ‘0’ bits flipping to ‘1’ appear first. In the monitored chip these isolated errors started after two weeks, long before bulk erase [Elektroda, TechEkspert, post #15867500]
Why place an opaque sticker over the EPROM window?
A paper or aluminium foil label blocks ambient UV, preserving data for the rated >200 years [Elektroda, TechEkspert, post #15866935]
Does visible light or office lighting matter?
Yes, but slowly. A common fluorescent lamp can wipe an un-stickered EPROM in about three years [Elektroda, TechEkspert, post #15866935]
Are modern Flash or EEPROM memories also vulnerable?
Flash lacks a window, yet high-energy particles cause single-event upsets. Space tests record SEU rates around 10⁻⁹ errors/bit-day in low-earth orbit [Ferlet-Cavrois, 2015]. Ground-level UV has negligible effect because the package is opaque.
Why were some Soviet EPROMs ‘impossible’ to erase?
Users reported several-hour UV exposures needed for 2 KiB Soviet chips due to thicker oxide or different doping [Elektroda, deus.ex.machina, post #15871604]
How can I safely monitor solar erasure at home?
- Address the EPROM with two 4040 counters and an Arduino Mini [Elektroda, TechEkspert, post #15866935]
- Log 32 KB blocks to an SD card every hour.
- Convert binary dumps to 512 × 512 PNG frames using Python + Pillow [Elektroda, TechEkspert, post #15866935]
What protection steps stop accidental field erasure?
- Cover the window with a UV-opaque label.
- Store boards inside opaque enclosures.
- Insert leads into conductive foam to avoid ESD; sunlight can generate surface charge that damages inputs [Elektroda, deus.ex.machina, post #15868080]
Have real-world failures been traced to radiation?
Forum members cite tower audio systems misbehaving when IR sunlight hit the remote-sensor IC [Elektroda, cb656, post #15873280] and CRT monitors losing I²C EEPROM contents after lightning-induced surges [Elektroda, Jacekser, post #17873346]
Can software like the CIH virus overwrite an EPROM BIOS?
No. Classic desktop BIOS chips after 1995 were Flash, not UV-erasable EPROM. CIH corrupts Flash electrically; true windowed EPROMs need high-voltage VPP pulses unavailable on a running PC [Elektroda, HD-VIDEO, post #15877235]
Edge case: what happens if only part of the chip is lit?
Shadow from the ceramic lid left border rows almost intact while the center erased, creating uneven yellow zones in the image map [Elektroda, krru, post #15867279]
