Hi David,

Thanks for your post. In my scheme I am manually enabling the boot mode. My scheme is not using any RTS/DTR signal but only RX and TX from pc com port.

In my scheme:

VCC = 5V
EA/VPP = +5V
PSEN = FLOAT (IN NORMAL OPERATION) and Receives GND when I want uC to go in boot mode
ALE/P = is left unconnected
RXD = PC-TXD (THROUGH MAX232)
TXD = PC-RXD (THROUGH MAX232)

A DIP-4 switch is connected at P1 lower four bits which can pulls down the port lower 4 pins. But i dont think P1 can impact in boot loader enabling in anyway.

The scheme works fine with AT89C51RD2 Devices I have been using. But this was firt time i tried P89C51RD2 chip which I dont know why is making these troubles.

P89C51RD2 Device that I have could only be programmed for just one time. Now it goes straight executing the old program without going in bootloader. In my program an Led is flashed when uC runs. (that tells me if the program is in bootloader or in normal execution)

At firt I had tried P89c51RC2 devices. Those devices were give different problem. They were straigthaway going in bootloader wether or not the PSEN was grounded. Unfortunately I dont have those P89C51RC2 chips with me. I had returned those chips. and In replacement I received another phillips device P89C51RD2.

Please suggest

Best regards

Asim