> a dozen Intel 8755A-2 eprom/I/O chips that appear to
> be faulty.  As long as the erase window is exposed to
> light I can erase,verify the erase, program and even
> use the chips in my device.  When I cover up the erase
> window the devices revert back to pattern like this
> 0000FFFF 0000FFFF

Did you cover the window while they were programmed?  If the programmer
is using an "intelligent" algorithm, they only program it with enough
pulses so that they can read it back, plus some extras for margin.  But
the parts behave differently when exposed to light, so they might not
get programmed enough.

Eric