Hobbies And Interests
History of PROMs
Wen Tsing Chow invented PROMs in 1956 while working at the American Bosch Arma Corporation. The chips were developed for the U.S. Air Force to store targeting data in the airborne computers of the Atlas missiles. Originally kept secret because of their military importance, PROMs did not become commercially available until 1969. Since then, advances in semiconductor technology have lead to PROMs growing in capacity and working with a range of voltages. Mass production has led to falling prices, making PROMs readily affordable.
Programming PROMs
During manufacturing, every bit on the PROM chip is initially set to a "1." The chips are then programmed later by using a higher than normal voltage to burn out the fuses on certain bits, setting the bits to "0." As the voltage burns out the relevant fuses during programming, the process is non-reversible, so it is not possible to reset zero bits back to one. This means that a PROM chip can only be programmed once. Programmers are often relatively inexpensive devices and usually require connection to a computer to operate.
PROM uses
PROMs can help to cut costs during the development and manufacturing of electrical devices. Developers can repeatedly alter programs and burn to a new PROM for testing as required until the code is bug free, allowing for a faster development cycle with lower costs. Instead of having to commit financially to having a large number of ROM chips specially manufactured, PROMs can be burned as required. Electronics hobbyists, for whom having a ROM manufactured would not be viable, can burn PROMs using low-cost programmers to create their own custom devices.
EPROMs and EEPROMS
The main disadvantage when developing an electronics project with PROMs is that these chips can only be programmed once. If a program has a bug and needs altering, the PROM chip becomes useless. Erasable programmable read-only memory, or EPROM, solves this problem by allowing the chips to be erased using exposure to ultraviolet light, and the chips can then be reprogrammed a number of times. An EPROM can be reprogrammed by shining ultraviolet light through a small window in the case of the chip, which erases the data. Electrically erasable programmable read-only memory, or EEPROM, is even more convenient, as the same programmer used to program the chip can also erase the contents for reprogramming. This allows for a faster development cycle when debugging custom software.