I have found a programmer for the AT89C2051 microcontroller here:
I have created a version similar to it. What's common with both versions is that none have a diode between the VPP pin and 12V. Just a resistor only.
My programmer does work but after several programming attempts (way less than 1000 per chip), the chip decides to no longer be programmable.
I remember that someone suggested that I should use a diode and a resistor between VPP pin and 12V.
So I'm curious as to what diode is best and what makes it so beneficial?
In the future, I want to program the AT89C4051 with the same programmer.