It is very easy to control an ISA card from an 8051. You just connect to the data bus (and it is easier for 8 bit cards) and the I/O Read, I/O Write, (and/or MEM Read , MEM Write) strobes and the necessary address lines. I say necessary address lines because many ISA boards (especially the I/O type) only use a few of the lowest ordered address lines to select the ports on the board. In this case you would just hard wire the rest of the upper address bus lines to highs and lows that select the range of addresses used by the ISA card. For example if you were connecting an ISA parallel port board to an 8051, that board, as an LPT1: board, uses I/O addresses 0x378 to 0x37F. The 8051 would only need to drive SA2-SA1-SA0 low order address lines and the upper lines SA9 -> SA3
can be tied to HHLHHHH respectively. If the particular ISA board your interfacing has interrupt handshake and you desire to connect that to an 8051 you would have to invert the signal becasue ISA bus interrupts trigger going high whilest the 8051 extenal interrupt pins trigger low (at least for the generic 8051). I have used ISA I/O boards in this manner many times. (Not always using an 8051, but the concept is the same).

Have Fun
Mike