The communication otherwise works fine, with other processors. HC00 is just gating, by which RS232 or RS485 is selected, but RS232 comes from MAX232, and is phase correct (idle-high).

I haven't read about FLIP, but this here is a pure HW problem. "I am at the beginning" - means only with T89C... :-)

It seems like shorted to some output standing on high, I've just checked, it is not shorted to any other processor pin. It's the print where I replaced AT89S53 with T89C and it was functionally OK. That's the only operation I made, and PSEN connected to GND.

Regards