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Cadenti
logic family
medium scale integration
metal oxide semiconductor
ac register
accumulator logic
adder circuit
basic computer design
branch unconditionally
flowchart
input and output communication
input output instruction
input register
interrupt cycle
logic adder circuits
logic gates
output register
register and memory
binary code
binary number
clock pulse
data types
decimal numbers
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For small quantities it is more economical to use a second type of ROM called a programmable read-only memory or PROM. When ordered, PROM units contain all the fuses intact, giving all 1's in the bits of the stored words. The fuses in the PROM are blown by application of current pulses through the output terminals for each address. A blown fuse defines a binary 0 state, and an intact fuse gives a binary 1 state. This allows users to program PROMs in their own laboratories to achieve the desired relationship between input addresses and stored words. Special instruments called PROM programmers are available commercially to facilitate this procedure. In any case, all procedures for programming ROMs are hardware procedures even though the word "programming" is used.
The hardware procedure for programming ROMs or PROMs is irreversible, and once programmed, the fixed pattern is permanent and cannot be altered. Once a bit pattern has been established, the unit must be discarded if the bit pattern is to be changed. A third type of ROM available is called erasable PROM or EPROM. The EPROM can be restructured to the initial value even though its fuses have been blown previously. When the EPROM is placed under a special ultraviolet light for a given period of time, the shortwave radiation discharges the internal gates that serve as fuses. After erasure, the EPROM returns to its initial state and can be reprogrammed to a new set of words.