PROM - Programmable Read Only Memory

The first PLD is PROM was introduced in 1970. PROMs was introduced for use as computer memories in which to store program instructions and constant data values. PROM have fixed AND plane and programmable OR plane. PROM can be use to program any combinational logics with limited numbers of inputs and outputs. Given n variables, it would necessary to have 2n AND gates, one for each possible minterm. A figure below shows the unprogrammed PROM for 3 inputs and 3 outputs, where AND plane is fixed and OR plane is programmable. The programmable links in OR array can be implemented as fused link, or as EPROM transistor or E2PROM cells depend on vendors. PROMs are useful for equations requiring a large number of product terms, but they can support few inputs as every input combination is always decoded and used.

Two basic versions of PROM:
1) Mask-Programmable: can be programmed only by the manufacturer. Mask-programmable chip has less delay because connections within the device can be hardwired during manufacture.
2) Field-Programmable: can be programmed by the end-user .Field-programmable chips are less expensive, and can be programmed immediately. The Field Programmable PROM developed into two types, the Erasable Programmable Read-Only Memory (EPROM) and the Electrically Erasable Programmable Read-Only Memory (E2PROM). The E2PROM has the advantage of being erasable and reprogrammable many times.

Image Courtesy: the Design Warrior's Guide to FPGAs