Programmable Logic Array (PLA)

· PLA is similar to a ROM in concept, but the PLA does not provide full decoding of the variables and does not generate all the minterms as in the ROM. In PLA, the decoder is replaced by a group of AND gates.
· PLA consists of n inputs, m (OR gate) outputs, K (AND gate) product terms and m sum terms.
· The PAL is the opposite of the ROM, having a programmable set of ANDs combined with fixed ORs.
· Compared to a ROM and a PAL, a PLA is the most flexible having a programmable set of ANDs combined with a programmable set of ORs.

Advantages:

o A PLA can have large N and M permitting implementation of equations that are impractical for a ROM (because of the number of inputs, N, required

o A PLA has all of its product terms connectable to all outputs, overcoming the problem of the limited inputs to the PAL Ors

o Some PLAs have outputs that can be complemented, adding POS functions

Disadvantages:

o Often, the product term count limits the application of a PLA.

o Two-level multiple-output optimization is required to reduce the number of product terms in an implementation, helping to fit it into a PLA.

o Multi-level circuit capability available in PAL not available in PLA. PLA requires external connections to do multi-level circuits.

3-input, 3-output PLA with 4 product terms.