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Memory devices; PAL vs PLA

Short answer

PAL has a programmable AND array and a fixed OR array; PLA has both arrays programmable. PAL is faster and cheaper but less flexible, as product terms cannot be shared among outputs. PLA can share product terms, offering more flexibility for multi-output functions.

What is a memory device? Distinguish between PAL and PLA memory devices.

A memory device is a device that stores binary information (bits) and lets it be read back, and in some types written. Each stored word is reached by an address. Examples: ROM, PROM, EPROM, RAM. Programmable logic devices such as PROM, PAL and PLA use the same array structure (an AND array that decodes the inputs and an OR array that holds the pattern), so they are studied with memories.

PAL vs PLA

PLA (Programmable Logic Array): both the AND array and the OR array are programmable. PAL (Programmable Array Logic): the AND array is programmable but the OR array is fixed (each OR gate gets a fixed group of product terms).

PointPALPLA
AND arrayProgrammableProgrammable
OR arrayFixedProgrammable
Sharing of product termsNot possible; each output has its own termsAny product term can feed any output
FlexibilityLessMore
SpeedFaster (fewer programmable links in path)Slower
Cost and complexityCheaper, simpler to makeCostlier, more complex
Terms per outputLimited to a fixed numberLimited only by total AND gates
Typical useSimple glue logic, widely used (e.g. PAL16L8)Multi-output functions with common terms
 PAL:  inputs -> [programmable AND] -> [fixed OR] -> out
 PLA:  inputs -> [programmable AND] -> [prog. OR] -> out

Example: if F1=AB+CF_1 = AB + C and F2=AB+DF_2 = AB + D, a PLA generates ABAB once and feeds it to both outputs; a PAL must generate ABAB twice, once in each OR group.

✓ Answer checked by Claude Opus 5.5 (high effort)

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