1 Aug
2000
1 Aug
'00
10:31 a.m.
Chuck wrote:
The term PAL was trademarked by the parent company
(whose
name escapes me at the moment MMD?, but AMD bought them)
MMI, Monolithic Memories Inc.
and so the generic
term was "PLA" (programmable logic array).
PLA predated PALs by many years. The PAL is a *less* flexible version
of a PLA.
In a PLA, both the AND and OR matrices are programmable. In a PAL, only
the AND matrix is programmable. The OR matrix is fixed. For instance,
a PAL16L8 has eight OR gates (with output inverters, so they're effectively
NOR), but each OR gate has exactly eight AND terms.
In a PLA, there are a large number of AND terms, which may each be wired
to any combination of the OR gate inputs, and may be connected to more than
one OR gate.
Many later PLDs (Programmable Logic Devices, a fairly generic term) have
added flexibility to the OR matrix, allowing some amount of product term
sharing. But generally they have not gone back to the same degree of
flexibility as the PLA.