Programmable Read Only Memory:
Fig. 3.71 shows four byte Programmable Read Only Memory (PROM). It has diodes in every bit position; therefore, the output is initially all Os. Each diode, however has a fusible link in series with it.
By addressing bit and applying proper current pulse at the corresponding output, we can blow out the fuse, storing logic 1 at that bit position. The fuse uses material like nichrome and polycrystalline. For blowing the fuse it is necessary to pass around 20 to 50 mA of current for period 5 to 20 vs.
The blowing of fuses according to the truth table is called programming of ROM. The user can program PROMs with special PROMs programmer. The PROMs programmer selectively burns the fuses according to the bit pattern to be stored. This process is also known as burning of PROM. The PROMs are one time programmable. Once programmed, the information stored is permanent.