Programmable Read Only Memory

PROM D23128C on the base plate of a Sinclair ZX Spectrum.

The programmable read-only memory or PROM (from English programmable read-only memory) is a digital memory where the value of each bit depends on the state of a fuse (or antifuse), which can be blown only once. For this reason the memory can be programmed (data can be written) only once through a special device, a PROM programmer. These memories are used to record permanent data in amounts less than ROMs, or when the data must change in many or all cases.

Small PROMs have been used as function generators, usually in conjunction with a multiplexer. Sometimes they were preferred to ROMs because they are bipolar, usually Schottky, achieving higher speeds.

Programming

A common PROM is found with all bits set to 1 as the factory default; the burning of each fuse changes the value of the corresponding bit to 0. Programming is done by applying high voltage pulses that are not found during normal operations (12 to 21 volts). The term read-only refers to the fact that, unlike other memories, the data cannot be changed (at least by the end user).

History

PROM memory was invented in 1956 by Wen Tsing Chow, working for the "Arma Division" of the American Bosch Arma Corporation in Garden City, New York. The invention was conceived at the request of the United States Air Force, to achieve a more secure and flexible way to store target constants in the MBI digital computer Atlas E/F.

The patent and associated technology were kept secret for several years while the Atlas E/F was the United States' primary missile. The term "burn", referring to the process of burning a PROM, is also found in the original patent, because as part of the original implementation the internal diodes had to be literally burned with excess current to produce a circuit discontinuity. The first PROM programming machines were also developed by Arma Division engineers under the direction of Mr. Chow and were located at the Arma laboratory in Garden City, and at the Air Force Strategic Air Command headquarters.

EPROM and EEPROM

Wen Tsing Chow and other engineers from the Armored Division followed this success by designing the first "non-destructive read-only memory" (non-destructive read-only memory', NDROM) to apply it to guided missiles, based on a double magnetic aperture base. These memories, originally designed to hold target constants, were used for mobile medium-range and intercontinental ballistic missile weapon systems.

The main motivation for this invention was that the US Air Force needed to reduce the costs of manufacturing target pads based on PROMs that needed constant changes as new targeting information came in from the bloc of communist nations. As these memories are erasable, programmable and re-programmable, they constitute the first implementation of a production of EPROM and EEPROM memories, manufactured before 1963.

It should be noted that the modern terms for these devices, PROM, EPROM, and EEPROM, were not created until some time after guided nuclear missile applications had been operational. The original implementations of Arma refer to PROMs as "constant storage array"; and EPROMs and EEPROMs were simply called "NDRO memories".

Modern commercial implementations of PROMs, EPROMs, and EEPROMs based on integrated circuits, ultraviolet erasure, and various properties of transistors, appear some ten years later. Until these new implementations were developed, outside of military applications, it was cheaper to make ROMs than to use one of the new expensive technologies developed and manufactured by air force missile contractors.

However, in missiles, spacecraft, satellites, and other high-reliability applications, many of the methods of the original 1950s implementation are still in use.