So you decode (@C0XX-C7XXH) by assuring P2.7,P2.6=1; P2.5,P2.4=0; P2.3 is "Don't Care"?
- The address pins of to FDC are A0(P2.1),
A1(P2.2) and A2(P2.3), the pin P2.0 is not used .

Are you using external ROM?
- Yes, the 80C31 is romless.

If so have you qualified your I/O with PSEN?
- No

You address C0XXH and write 55H twice with interrupts disable (critical region any intervening I/O causes trouble) and then perform config?
Because this device using indirect addressing (you write your address to Port 0, then your data to Port1) you are using a serial port to trace dumping IO address and IO data?
After config you write "AAH" to Port 0?
- I wrote wrong, the data port is P0. It's necessary the access the configuration mode ?