Hi,
Andy, you said "No setting up - just read for input; write for output."

It is not always true for this MCU original poster works with. The datasheet says that internal pullup resistors may be disconnected; it is controled by SFR PUR_3. It recommends to keep pullup resistors connected but:
Each GPIO pin has an associated internal pullup resistor. It is strongly recommended that the pullup resistor remain connected to the pin to prevent oscillations in the input buffer. The only exception is if an external source always drives the input.

By other words, what I have been trying to say from yesterday morning:
- if pin is being used as output then user should pay attention that TUSB3410 has not built-in strong pFET3; datasheet says: The MCU treats the outputs as open drain types in that the output can be driven low continuously, but a high output is driven for two clock cycles and then the output is tristated. So user probably needs to use external pullup resistor. It is the first difference from ordinary C51s.
- if pin uses as input then user should look on his hardware: how MCU input is drove by external hardware? If external hardware has produced two-level (0/1) output then internal pullup resistor may be disconnected. But if it has, for example, an open drain then internal pullup resistors must be connected with PUR_3 register. It is the second difference - many C51s have not such possibility to control pullups.
For me this datasheet describes it all with clean way.

Regards,
Oleg