Question 16.16: Explain how a programmable read only memory is formed?...

Despite the flexibility of the PLA provided by its two programmable arrays it is often more efficient to use a less complex configuration with a single programmable array. A PAL arrangement can be formed by replacing the programmable OR array with a fixed series of inter connections. An alternative method of simplifying the PAL structure is to remove the programmable AND array to form a structure as shown in Fig. 16.13. This forms a programmable read only memory or PROM.

A PROM may be visualized as a PLA that has one fixed product term (AND gate) for every possible combination of the input variables. Thus, a device with eight inputs would have 2^{8} or 256 product terms. Since all possible combinations of the inputs are represented by one of the product terms, there is no longer any need to program the AND array and this becomes a fixed decoder. Each input combination selects a single AND gate and the OR array is used to determine which of the various outputs is activated (taken HIGH) for that input combination. The pattern written into the OR array therefore determines the output pattern that will be produced for each possible set of inputs. PROMs were one of the earliest forms of array logic and predate both PLAs and PALs. However, the use of a full decoder is inefficient for most logic applications and they are more commonly used for the storage of program or data, rather than to implement logic functions. When used in this way the input pattern represents the address and the corresponding pattern in the OR array presents the stored data. When devices of this type are designed for program or data storage they are more often referred to as ROM.