Programmable Read-Only Memory (PROM) is a special type of ROM because it can be programmed. Very few of today’s programmable controllers use PROM for application memory. When it is used, this type of memory is most likely a permanent storage backup for some type of RAM.
Although a PROM is programmable and, like any other ROM, has the advantage of nonvolatility, it has the disadvantage of requiring special programming equipment. Also, once programmed, it cannot be easily erased or altered; any program change requires a new set of PROM chips.
Erasable Programmable Read-Only Memory (EPROM) is a specially designed PROM that can be reprogrammed after being entirely erased by an ultraviolet (UV) light source. Complete erasure of the contents of the chip requires that the window of the chip be exposed to a UV light source for approximately twenty minutes.
EPROM can be considered a semipermanent storage device, because it permanently stores a program until it is ready to be altered. EPROM provides an excellent storage medium for application programs that require nonvolatility, but that do not require program changes or on-line data entry.
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