27C256, 32-kilobyte EPROM. A typical 1980s device.

A type of memory that is ROM when it is installed in a circuit. However, its contents can be changed by removing the EPROM from the circuit (it has to be installed in a socket, obviously), putting it under a special ultraviolet light for about 20 minutes, and then placing it in a device called a PROM programmer which generates special voltages and signals to re-program it. (This is often called "burning" the EPROM, an apt name since some types get quite warm when being programmed, and the machine itself is sometimes called a "PROM burner".) In the past, EPROM devices were widely used for non-volatile memory in low-quantity electronics manufacturing; the cost for short production runs was less than the setup cost of having a mask ROM made, and the erasable nature of EPROM allowed for revisions during the production run.

PROM programmer

EPROMs were widely used in 1980s sample playback synths and drum machines to hold the recorded samples. A cottage industry sprung up offering alternate, pre-programed EPROMS with different samples, and knowledgeable users could buy a programmer and burn their own. Oberheim actually offered a customized PROM programmer as an accessory to its DMX drum machine. A user connected the PROM programmer to a computer, which had to convert the data to be loaded into the format needed by the programmer and the particular EPROM part to be programmed. Programming the EPROM took up to 30 minutes, depending the EPROM's memory capacity.

Classic EPROM parts can easily be distinguished by the window in the top of the integrated circuit body, which exposes the circuitry inside. Light from the ultraviolet erasing lamp passes through this window and falls on the memory cells in the circuitry, which is what performs the erasing action. The window is often covered by a sticker, which serves two purposes: (1) it notes the contents and version of whatever data is in the EPROM, and (2) it prevents inadvertent erasure should the EPROM be exposed to the sun or mercury-vapor lighting for a long time. Note that most EPROMs being manufactured today are "one-time programmable"; they lack the window and so, once programmed, cannot be erased.

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