>I thought for the EEPROM devices, 10+ Volts was good enough.  For EPROM
>devices, 12.5+ Volts is required.

The 16C84 and maybe others will not enter programming mode unless the
voltage is high enough. The early Picstart Plus programmers wouldn't work
on all of them, so Microchip upped the programming voltage a little and
there is a note somewhere telling which resistor to change if you have an
old programmer. All the EEPROM parts have an internal programming supply,
so the exact voltage is not critical.

-Bob