Page 204 - Bebop to The Boolean Boogie An Unconventional Guide to Electronics Fundamentals, Components, and Processes

Programmable ICs 185

Programmable Read-only Memories (PROMs)

In the case ofprogrammable read-only memory (PROM) devices, the AND
array is predefined and only the OR array is programmable (Figure 16- 10).

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Address 0

Address 1

Address 2

Address 3

Address 4

Address 5

Address 6

Address 7

Figure 16-10, Generic structure of a PROM
(predefined AND, programmable OR)

PROMs are useful for equations requiring a large number of product terms,
but tend to have fewer inputs because every input combination is decoded and
used. The internal structure of a PROM is similar to that of a ROM, the main
difference being that a ROM’s contents are defined during its construction.
PROMS are generally considered to be memory devices, in which each address
applied to the inputs returns a value programmed into the device.

11 PROMs were introduced in Chapter 15.