There is a significant difference between the programming algorithm of the EPROM based 87C52H and the FLASH based AT89C52. The first uses a series of programming pulses ("quickpulse"), while the second issues a "programming command" and then loops, until the chip responds "done". See the corresponding datasheets. You would need to write your own firmware to the "easy downloader".

Also note, that while the EPROM programming in 87C52H uses V_PP as the power source for the programming so it needs to supply significant current (50mA) in pulses, while keeping stable voltage; for the AT89C52 it serves only as an "unlocking" logic level, so it draws negligible currents in uA range.

So there certainly are pitfalls in EPROM-based '51 programming. I don't say it is not viable, but still the question remains - why?

Jan Waclawek