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Programmable read-only memory (PROM) is non-volatile semiconductor memory that is supplied blank and programmed after it is manufactured. A conventional fuse PROM can be programmed only once: its contents remain available without power, but its programmed bit pattern cannot be erased and rewritten.
What does “read-only” mean in PROM?
“Read-only” describes how a PROM is used after its initial programming, not an inability to write data at all. A device programmer writes a bit pattern to a blank PROM; afterward, the chip is used to read that stored data. Because PROM is non-volatile, it retains its contents when power is removed. Computer History Museum
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How is a conventional PROM programmed?
A common one-time PROM stores bits using fuse links. A PROM programmer applies controlled electrical pulses to selected cells, changing links to encode the desired data. In a fuse PROM, those physical changes are irreversible, so the chip cannot be erased and programmed again. The precise materials and whether a blown link represents zero or one depend on the device; neither convention is universal. Electronics Notes
Programming is distinct from normal reading: the programmer is used to make the initial change, while the stored data can be read without repeating that operation. The correct programmer depends on the exact chip and its requirements, so the term “PROM programmer” alone is not enough to establish compatibility. Electronics Notes
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- Manufacturer Part Number: BMBS
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- Do not replace a prom with a non matching unit.
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How PROM differs from other ROM types
The key distinction is when the data is set and whether the chip can be erased for reuse. “PROM” is sometimes used broadly when discussing related programmable ROM families, but a conventional one-time PROM is not interchangeable with EPROM or EEPROM.
| Memory type | How data is programmed | Can it be erased and reused? |
|---|---|---|
| Mask ROM | Data is set during chip manufacture. | Not through ordinary user programming. |
| One-time PROM | Programmed after manufacture; a common design uses fuse links and an external programmer. | No. Fuse changes are irreversible. |
| EPROM | Programmed electrically and erased with ultraviolet light in the erasable design described by the sources. | Yes, after erasure. |
| EEPROM | Programmed and erased electrically. | Yes. |
Computer History Museum; Electronics Notes
Why the distinction matters
- Choose a one-time PROM when the data is meant to be fixed after programming and the device’s specifications match the application.
- Choose an erasable type when the stored data must be updated: EPROM is erased with ultraviolet light in the design described above, while EEPROM is erased electrically.
- Check the exact part before programming: package, programming requirements, and device compatibility matter. A programmer that supports one PROM does not necessarily support another.
A brief history of PROM
The Computer History Museum records that Radiation Inc. introduced a 512-bit bipolar TTL user-programmable ROM in 1970. Designers could enter code themselves by burning metal fuse links with an external programming unit, but the device could be used only once. Computer History Museum
Rank #2
- Description: IC MICROs GRAB BAG: Microprocessors, RAMs, DRAMs, SRAMs, PROMs, EPROMs, CPUs, 8000 Series
- Piece Count: 50
The museum also documents Intel’s 2048-bit 1702 EPROM from 1971. Unlike a one-time fuse PROM, it could be reused after ultraviolet erasure—an early example of the practical difference between one-time and erasable programmable memory. Computer History Museum
Quick Recap
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- 10PCS W27C512-45Z DIP W27C512 DIP-28
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- Manufacturer Part Number: BMYU
- Powertrain Control Module (PCM) Programmed to Your VIN,Easy Installation
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- Manufacturer Part Number: ATSX
- Powertrain Control Module (PCM) Easy Installation,Programmed to Your VIN
- Do not replace a prom with a non matching unit.
- Plug and Play, Instructions Included
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