What is a PROM

PROM stands for Programmable Read only memory. It is a large scale integrated circuit that can be programmed for realization of various combinational digital functions. A PROM consists of a decoder circuit that generates all the minterms corresponding to its input variables. The output lines of the decoder are then fed in to a programmable OR gate array. This gate array can be porgammed to add different combination of min terms to realize different logical functions. Each output line of a PROM is used for realization of a different output function.

For realization of a truth table, a PROM can be programmed such that the inputs of the truth table form the input lines of the PROM. The OR gate array is then programmed to match the outputs corresponding to the different input addresses. The outputs are obtained at the output lines of the OR gates. Thus the truth table is mapped on to the contents of the PROM and it is for this reason that a PROM is also known as a Hardware Truth Table or Look Up Table.

The size of a PROM is given as 2nxm bits where n is the number of inputs and m is the number of outputs. A PROM based implementation of a combination logic is considered better than a gate base or Multiplexer based one because of the fact that the whole truth table can be implemented by using just a single PROM IC which leads to a lot of saving in power and space.

There are a number of different types of PROM’s that are available such as:-

Erasable PROM which can be erased by exposing the programmable array to UV rays. Electrically Erasable PROM Electrically Adjustable PROM which is also known as flash memory.

A PROM implementation though is not the most efficient LSI implementation of a combinational logic because of the large decoder circuitry involved. The fact that many of the minterms generated are never used at all in the programmable OR array along with the synthesis of the logical function in its canonical sum of product form leads to the use of lot of extra hardware logic. Two better replacements in terms of hardware saving are available in the form of Programmable Logic Array (PLA) and Programmable Array Logic (PAL) Chips. The advantage though that a PROM possesses is that it is most easy to program from a designer’s point of view as compared to these other LSI chips.

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