EPROM short for Erasable Programmable Read-Only Memory is a relict from the early days of computing. It is a from of memory used to store program code for CPU and micro computer. As the name states it is a read-only memory this means that in normal operation the memory content can only be read but not altered. To erase then the memory the chip needs to be exposed to UV light. For this propose the housing of the chip has a built in quartz glass window. I used a so called flash gun which is basically a mains powered fast firing strobe light which is pointed on to the window.
The process of erasing the memory takes some time. I interrupted the erasing process to monitor the progress erasing process by dumping the chips memory into a file. During the first 60 seconds of the chip being exposed to the light no data change occurs. After that the memory fades pretty fast. Even the placement and structure of the memory cells on the chip is somewhat recognizable.
I have been collecting old EPROM chips for many years. Now I was curious what data these hold exactly. I am not looking for program data but more for character maps, hidden “Easter eggs” . Something like this Hack a day ((Hack a day on hidden photographs in old Mac ROM chips )) post.
Recently I installed this interesting plugin ((Civil Footnotes plugin page https://defomicron.net/projects/civil_footnotes)). It makes it easy to put web links to sources to the bottom of the page. Similar to the Wikipedia citation style.