It might be not related to Pinnacle at all - the behaviour of '51 port is, that if you write e.g. #1 into port 0 (mov p0,#1), you set bit 0 of port 0 (P0.0) to "not driven low" (assuming you have an external pullup it means it is high), all other bits of port 0 are driven low by the output transistors. It means, you cannot drive them high externally and if you perform mov a,P0 , you can read only either 1 (if nothing drives low P0.0) or 0 (if something drives P0.0 low).

I assume you planned to read back some of the port 0 pins, e.g. P0.4-P0.7; you then need to write #0F1h, #0F2h etc. to keep the pins P0.4-P0.7 in the "not driven low" state.

Also, please reread the "bible", the "ports structure" chapter.

Jan Waclawek