Hi Mark.

EPROM means "erasable programable read only memory". OTP means "one time programmable". Once you program an OTP chip that's it! You can't program it again. These types of chips are generally used for production devices where the chip will never be re-programmed again.

An EPROM based chip can be re-programmed only after exposing the window on the chip to ultraviolet light in a chip eraser for the required amount of time then checking to make sure it is blank. This takes time, like more than 20 minutes sometimes. Actual time is usually specified by the chip manufacturer.

The newer Flash memory chips are what you should be using to create projects. In seconds you can blank the chip and download a new program into the chip and then test and retest your project. It's really the way to go today!

BobK