Editing PLA Replacement Options

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The '''PLA''' is the single most-replaced chip in a Commodore 64. This page covers what the part actually is on each board, and which replacements can be verified from their makers' own documentation.

'''Every product claim below is cited to the maker's own page.'''<ref name="icomp">Individual Computers, ''SuperPLA V3'', https://wiki.icomp.de/wiki/SuperPLA_V3 . Source for what SuperPLA is and is not, the jumper settings, and the exact list of machines and sockets it supports. The page is filed under Individual Computers' '''EOL''' category.</ref><ref name="mantione">Daniël Mantione, ''PLA20V8 — The Commodore 64 PLA that you can build yourself'', https://www.freepascal.org/~daniel/c64pla/ . Source for the design being two GAL20V8B devices in DIP, programmable with a TL866; for the named disadvantages of EPROM-based substitutes (timing, bus contention, switching noise, voltage levels); for the attribution of the original reverse-engineering and of SuperPLA to Jens Schönfeld; and for the list of other replacements in circulation.</ref><ref name="cerebrum">''C64 Hardware Reference and Repair Guide'' (open_cerebrum), https://opencbm.org/doc/c64/hardware_reference_and_repair_guide/ . Community reference. Board-by-board component placement tables for assemblies 326298, 250407, 250425, 250466 and 250469, and the pinout and function of the 251715 / 252535 PLA/MMU. Secondary source, but the placement tables agree with Commodore's own schematics.</ref>

== Which part is in your machine ==

This matters more than anything else on the page, because the "PLA" is a different device on the short board and most replacements do not fit it.<ref name="cerebrum" />

{| class="wikitable styled-table" style="width:95%; text-align:left;"
|+'''The PLA by board assembly'''
! Assembly !! Year !! Device !! Socket !! Notes
|-
| 326298 || 1982 || 82S100 / 906114-01 || '''U17''' || 5-pin video connector board
|-
| 250407 || 1983 || 82S100 / 906114-01 || '''U17''' || Eight 4164 DRAMs
|-
| 250425 || 1984 || 82S100 / 906114-01 || '''U17''' || Reduced oscillator circuit
|-
| 250466 || 1986 || 82S100 / 906114-01 || '''U17''' || Two 41464 DRAMs; colour RAM is still a separate 2114 at U6
|-
| 250469 Rev.A / 3 / 4 || 1987 || '''251715-01''', 64-pin || '''U8''' || Colour RAM is a separate 2114 at U19
|-
| 250469 Rev.B || || '''252535-01''', 64-pin || '''U8''' || Colour RAM integrated; no 2114 at U19
|}

The 251715 and 252535 are not PLAs in the 82S100 sense — they integrate the PLA '''and''' a set of discrete TTL functions into one 64-pin device, which is why they are often called the "SuperPLA". The 252535 additionally absorbs the 4 K colour RAM.<ref name="cerebrum" />

'''A 28-pin DIP replacement for U17 will not fit U8, and vice versa.''' Replacements for the 64-pin part are a separate and much smaller field.

== Replacements whose specification can be checked ==

=== SuperPLA V3 — Individual Computers ===

Jens Schönfeld reverse-engineered the C64 PLA and produced the SuperPLA; V3 unifies the earlier versions, with '''jumpers on the underside''' selecting the target machine.<ref name="icomp" /><ref name="mantione" />

It '''adds no functionality''' — it is a repair part.<ref name="icomp" />

{| class="wikitable styled-table" style="width:75%; text-align:left;"
|+'''SuperPLA V3 jumper settings, from Individual Computers'''
! Machine !! Socket !! Jumpers
|-
| C64 assy 326298, 250407, 250425, 250466 || U17 || 2 and 4 (as shipped)
|-
| CBM 610 || U18 || 1 and 4
|-
| CBM 710 || U18 || 1 only
|-
| C16 || U16 || none
|-
| C116 || U101 || none
|-
| plus/4 || U19 || none
|-
| 1551 drive cartridge || || 1 and 3
|-
| PET || UE5 only || 2
|}

Two limits stated by Individual Computers themselves:

* '''Assembly 250469 is not supported.''' SuperPLA V3 does not replace the 64-pin part.<ref name="icomp" />
* '''On a PET, only socket UE5 is supported.''' The second PLA cannot be replaced with SuperPLA V3.<ref name="icomp" />

The product is listed in Individual Computers' '''EOL''' category, so treat it as a second-hand purchase rather than a current stock item.<ref name="icomp" />

The jumpers sit in the socket adapter on the underside and are, in Individual Computers' own words, tiny — fit them with tweezers, with good light, and take care not to break the adapter pins.<ref name="icomp" />

=== PLA20V8 — Daniël Mantione ===

A deliberately low-cost, self-buildable design: '''two GAL20V8B devices''' from Lattice, in '''DIP''' packages, on a small PCB. One GAL cannot hold all the logic, hence two. The GAL20V8B is out of production but widely available second-hand, and can be programmed with the inexpensive '''TL866''', so a repairer can keep several spares on hand without paying for someone else's assembly labour.<ref name="mantione" />

The GAL's AND-then-OR array is structurally close to the original PLA; in the "simple" and "complex" modes its flip-flop is bypassed.<ref name="mantione" />

=== Other replacements in circulation ===

Mantione, who surveyed the field while designing the PLA20V8, names '''RealPLA, PLAnkton, PLAdvanced''' and '''Ultimate PLA''' as existing replacements, and says that many of them are as good as or better than the original part.<ref name="mantione" />

'''This wiki has not verified the logic device, current draw or compatibility list of any of those four against its maker's own documentation''', and therefore states none. Do not assume that a specification you have seen attributed to one of them belongs to it — several of these products are described with each other's parts in online summaries.

=== EPROM-based substitutes ===

The earliest replacements used an EPROM holding the PLA truth table. Mantione lists the known disadvantages plainly: '''timing, risk of bus contention, switching noise and voltage levels'''. They work, but they are the weakest option and the one most likely to fail with cartridges.<ref name="mantione" />

== Fitting a replacement ==

# Power off and unplug the supply.
# The part at '''U17''' on a long board is a '''28-pin DIP''' — lift it with a DIP extractor or by easing each end alternately with a small flat blade. It is not a PLCC.
# Inspect the socket for oxidation and for pads lifted by earlier extraction attempts.
# Match pin 1 to the notch on the socket and the board silkscreen.
# Press home squarely; do not rock the part side to side, which bends pins under the body where you cannot see them.
# Power on and confirm a BASIC screen, then exercise the decode paths — load from disk, run a cartridge, and run a diagnostic. See [[Commodore Test Cartridge Index]].

If you are fitting a replacement in a machine that also has a [[JiffyDOS Kernal Upgrade|JiffyDOS]] or [[Jump-less Dual-Kernal Switcher|kernal switcher]] board, fit one thing at a time. Two changes at once turns a five-minute diagnosis into an afternoon.

== Related pages ==
* [[Commodore 64]]
* [[Commodore 64 Troubleshooting Guide]]
* [[Commodore Test Cartridge Index]]
* [[C64 Reloaded MK2]]
* [[SixtyClone Bare-Board Project]]

== References ==
<references />

[[Category:Commodore 64 Modifications and Enhancements]]
[[Category:Commodore Systems]]