On my early work with the 8031, I inserted the 80C31 and no longer the serial transmit works. The reason was, that CMOS devices driver lower high current, than the NMOS devices.
This time I changed from using transistors (e.g. BC550, BC560) to using MOSFETs (e.g. BS170, BS250) and no longer the driver current was an issue.

Maybe Rustu meet also not the worst case conditions and there are small differences between 89C51 and 89C55WD, which meet both the values given on the data sheet, but can cause different working on such an application.
On such a case pull up resistors may help.


Peter