Erik,

You are right, lots of differences. Like the second serial port on the P89C51RD2 actually WORKS! Apparently the latest die revision of the T89C51RD2 has a second serial port that works, but I really hate spending hours debugging software only to find out none of the chips I have are 100% functional...

Other than the things noted in the data sheet, like XRAM size, EEPROM, etc, the "Gotcha" that surprised us was the Clock-Doubling technology. I forget which part is which, but one device has a SFR that controls the 12-clock or 6-clock mode. The other has an OTP bit that can be cleared when programming the device (in a device programmer) to change from the default (6-clock) mode to the traditional 8051 12-clock mode. Since the bit is OTP, it can not be reset.

-- Tom