http://web.archive.org/web/20120713012459/http://plextor.jp/pc/download/internal.html

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First resource, this official site contains many firmware updates for real Plextor drives. But a few of them are missing (for example, the firmware for my PX-4824TU/TA and PX-716AL drives):

http://www.plextoramericas.com/index.php/download?task=viewcategory&catid=29

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Second resource, a mirror managed by the Plextor Americas forum's administrator. It contains many firmware updates for older models:

http://www.mediafire.com/?ea15vuhnjpmtb

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Third resource, the original plextor.jp preserved by archive.org. Archive.org could preserve certain firmware updates (not every download unfortunately) . Just use the "Filter results" field text to refine your search and try the different snapshots taken by archive.org:

http://web.archive.org/web/*/http://plextor.jp/download/firmware/*

For example, archive.org fortunately could save securely the latest firmware (1.07) for my PX-W4824TU (actually a PX-W4824TA in a USB 2.0 enclosure)

http://web.archive.org/web/20061212073724/http://www.plextor.jp/download/firmware/pxw4824_107.zip

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Fourth resource. The original american Plextor website mirrored by archive.org. It seems that contains many firmware updates for the older models, even for the legendary UltraPlex 40x CD-ROM reader. As before just use the "Filter results" to refine the search.

http://web.archive.org/web/*/http://www.plextor.com/ENGLISH/support/downloads/*

For example, archive.org preserved the lastest firmware for my PX-716AL drive:

http://web.archive.org/web/20060409030853/http://www.plextor.com/english/support/downloads/716AL_102.exe

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Addendum:

If you own an PX-755(S)A or a PX-760(S)A, you could try (at your own risk) the «free your Plextor» firmwares, in order to remove the special and protected commands implemented by Plextor (like controlling the silent mode, the GigaRec mode, disc quality scannings). So that a third party program (like QPxTool, available for Linux, OS X and Windows) can control these functions.

http://www.oocities.org/de/linux_piewie/download/

1_07Xrpc1.bin: For PX-760(S)A drive.
1_08Xrpc1.bin: For PX-755(S)A drive.

To program the unlocked firmware:

-Download the original and official firmware for your PX-755/760(S)A drive.

-It's actually a SFX file, use 7-Zip or Winrar to unpack the SFX into a folder.

-Copy the unlocked firmware to that folder.

-Open the PlexFirm.ini through notepad (or any plain text editor) and replace (this is for my PX-755SA drive) FirmwareFileName=ess_108.bin by FirmwareFileName=1_08Xrpc1.bin. That is, replace the original filename of firmware to program by the unlocked one.

-Run PXFirm3.exe to program the firmware. As always, be careful when updating a drive's firmware.

-Freed Plextor drive, no more PlexTools (only for Windows) required to control certain functions.

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P.S.: If you own a Plextor drive which features the silent mode (for example: Premium, Premium 2 and PX-7xx drives [for this serie only the true ones, not the rebadged ones]), NEVER enable the silent mode. This funcion is somewhat buggy, and after of that your drive maybe remains stucked forever at 8x read speed for CD, even if you disable that mode.

P.S2: If any native english speaker can rewrite better any text I have written, please give feedback.