I've just tried to add GND to pin 39 (it's a VQFP package). It would of course be a surprise (a discovery !) if it worked after that, but no surprise and no change. I'll have to put another chip T89C and see how it behaves.

To Joseph: the output that comes to RXD is a standard 74HC00 output and it must drive the pin from 0 to 5V, as it does with AT89S53's RXD input. And as I told, the board has otherwise a good RS232 communication.

If I manage to pass this first step with another chip, I'll post it here, if not I'll have to try with the AT89C51ED2. Thanks to all !

Regards, Slobodan