Richard, that is useful.

Actually, if u see on the schematic of teh controller board, there are about a maximum of 6 ICs connected to P0 os the uC. Now, AT89c52 datasheet states fan-out of this port to be able to handle 8 LS TTL loads. I have only connected the following 6:

1) 74HC573 (For address Latch with ALE),
2) DS12887 RTC (takes multiplexed data & addr) - this is probably not shown on schematic,
3) 6264 (8K RAM)
4) 62256 (32K RAM)
5) AT28c256 (32K FLASH ROM)
6) 8259A

Now, if i connect the data cable from this board to BOARD-2 (IO Board) the U14 buffer chip is connected to the data bus. This makes a total of 7 ICs connected to P0 in all. If i do this, the RAMS cant be written-to or read from. While the rest of the circuit works fine, that is i can read/write to/from 9255s 8279, aux io ports, etc. I can even access th 8259 on the main board - its only the RAMS that create problem.

Now, when i disconnect the data cable from BOARD-2, i can read/write RAMS and all the functions on BOARD-1.

Do u really think this is a fan-out problem ? Secondly, i was also able to access all the functions of BOARD-2 without U14. The U14 came later on when i thought just incase not to overload teh data bus too much.

Just cant think straight now. At one time, it looks like fan-out problem, the other times, it looks like faulty RAM chips, though i have replaced the RAM chips with fresh parts.