>The hardware (USB-Serial adaptor) is the same

I am not totally sure this is the case - I suspect some of the problem may
well be revision levels of the chip itself, and changes made to the driver
to suit. Think in terms of the errata we become aware of for PICs, and then
think in terms of not being aware of similar documents for the Prolific (or
other) chips, and what errata information they may contain.

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