Hi Ijaz,

Oooops... I forgot that 89S51 (unlike 89C51) let me also read directly its 3 lock bits. I'll give it a try and let you know.

You are right, ISP is a far better choice. I am already familiar to it since it is the only way to build a programmer for CAM cards (needed for some sat receivers as the FUNcard).

In my next post I'll write the exact voltages of all 40 pins for both 89C51 and 89S51 in the read mode (it needs indeed steady levels... right?).
By the way, the AT89S51 can run its code properly if put on its circuit (or on the prototype if 'RST' is made low, for a short time of course).

I just can't ignore I am facing an inexplicable fact that quasi-similar MCUs and under a simple condition (as read) give totally different result. I know something is still missing but I wonder how I can just skip knowing it... and live :))

I'll be back soon with the measurements.
Thank you for joining me.

Kerim