If there is a possibility of a chip being slow to get off the bus after it becomes deselected it would have to be the relic 8255 part. You are also going to have to look carefully as to whether the MOVX instructions you use to access the 8255 generate a wide enough Write pulse for the chip. Get out the datasheet for the particular 8255 you have and look carefully at its timing specifications. They will have minimum width requirements for WRITE, minumum time of write data valid till trailing edge of WRITE (called data setup time), and also a minimum amount of time for keeping the write data valid after the WRITE pulse trailing edge (called data hold time). The faster new 8052 derivative parts very often make pulses from the MOVX instructions that are too fast to operate the 8255 correctly. This is one reason that these parts are not even recommended for new designs.

Michael Karas