>Is there a difference ( or several differences)
>between a 509 OTP and a 509 Eprom?

People have reported problems like this in the past, but typically in the
reverse direction, I.E. JW->OTP.

What is known to happen is that the light coming in the window of a JW part
will initialise RAM registers to different states to the OTP parts. It would
pay to check through your code to make sure that the RAM registers are being
properly initialised by your program. You may have just got away with it in
the OTP part, and another one may work differently.

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