i'v faced the similiar problems with 89C51RB2, and i've follow the same instruction as belows;-
a) PSEN held low with R 2k Ohm
b) P2.6 or P2.7 are high or floating
c) ALE is high or floating
and i've check the signal from scope, and the char "U" is seen at the RXD of the MCU, but not signal is out from TXD signal.
As i read from Kanwal, Do we need to blank the chip before we start our ISP. ??
TQ