Programmable read-only memory

From Free net encyclopedia

Image:D23128C PROM.jpg A programmable read-only memory (PROM) or field programmable read-only memory (FPROM) is a form of digital memory where the setting of each bit is locked by a fuse or antifuse. Such PROMs are used to store programs permanently. They are frequently seen in computer games, or such products as electronic dictionaries, where PROMs for different languages can be substituted.

Contents 1 Programming 2 Invention 3 EPROM and EEPROM 4 See also

Programming

A typical PROM comes with all bits reading as 1, burning a fuse during programming causes its bit to read as 0. The memory can be programmed just once after manufacturing by "blowing" the fuses (using a PROM blower), which is an irreversible process. Blowing a fuse opens a connection while blowing an antifuse closes a connection (hence the name). Programming is done by applying high-voltage pulses which are not encountered during normal operation (typically 12 to 21 volts). Read-only means that, unlike the case with conventional memory, the programming cannot be changed (at least not by the end user).

Invention

PROM 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 come up with a more flexible and secure way of storing the targeting constants in the Atlas E/F ICBM's airborne digital computer. The patent and associated technology was held under secrecy order for several years while the Atlas E/F was the main operational missile of the United States ICBM force. The term "burn," referring to the process of programming a PROM, is also in the original patent, as one of the original implementations was to literally burn the internal whiskers of diodes with a current overload to produce a circuit discontinuity. The first PROM programming machines were also developed by Arma engineers under Mr. Chow's direction and were located in Arma's Garden City lab and Air Force Strategic Air Command (SAC) headquarters.

EPROM and EEPROM

Wen Tsing Chow and other Arma engineers followed up on this success by designing the first programmable Non-Destructive Read-Only (NDRO) memories for missile guidance applications, based on two-aperture magnetic core technology. These memories, again originally designed to hold targeting constants, were used in the United States' Atlas F ICBM and MMRBM (Mobile Medium Range Ballistic Missile) weapons systems. The primary motivation for this invention was the United States Air Force's need to reduce the cost of manufacture of PROM based targeting boards that needed to be constantly changed as new information became available about targets in the Communist bloc nations. As these read-only memories were both erasable, programmable and re-programmable, they constitute the first production implementation of the concept of EPROM and EEPROM memories, in manufacture prior to 1963.

It should be noted that the modern terms for these devices, PROM, EPROM and EEPROM were not created until some time after these nuclear ballistic missile guidance applications had already become operational. The original Arma PROM implementation was referred to as a "constants storage matrix" and the original Arma EPROM and EEPROM implementations were simply called "NDRO memories."

Modern commercial implementations of the PROM, EPROM and EEPROM based on integrated circuits, ultra-violet light erasure and various transistor properties would appear some ten years later in the 1970s as noted in the article on EPROM and EEPROM linked below. Until these new implementation methods were developed, outside of military applications, it was cheaper to simply manufacture ROMs than to use any of the very expensive programmable technology developed and manufactured by the Air Force missile contractors.

However, in missile, spacecraft, satellite and other ulta-high reliability applications many of the original 1950s implementation methods are often still in use.

See also

• Erasable programmable read-only memory, or EPROM.
• Electrically erasable programmable read-only memory, or EEPROM.bg:PROM

cs:ROM da:Rom (hukommelse) de:Read-Only-Memory es:Memoria ROM fr:Mémoire morte gl:ROM ko:롬 hr:ROM id:Read-only Memory it:PROM he:ROM nl:ROM ja:PROM no:ROM pl:PROM pt:PROM ru:Постоянное запоминающее устройство sr:ROM fi:Lukumuisti sv:PROM tr:ROM