Programmable ROM

D23128C PROM on the board of ZX Spectrum

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. The key difference from a strict ROM is that the programming is applied after the device is constructed. They are frequently seen in video game consoles, or such products as electronic dictionaries, where PROMs for different languages can be substituted.

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).

Advantages

• Reliability
• Stores data permanently
• Moderate price
• Built using integrated circuits, rather than discrete components.
• Fast: speed is between 35ns and 60ns.

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.

External links

• View the US "Switch Matrix" Patent #3028659 at US Patent Office or Google
• PROM vs. Other Memory Types