Cormac:
In addition to the specific items Erik has mentioned some other things to consider:

A) Do you have a wiring error? Address lines shorted together?

B) Write a simple program that writes one external memory location in a tight loop forever and study the write cycles with an oscilloscope. Make sure that all the RAM handshake lines are working correctly and that that the address and data arriving at the RAM correctly.

C) Write a simple program that reads one external memory location in a tight loop forever. Then again use the oscilloscope to study the read cycles. Ensure the handshake lines are correct, the addressing is correct and that the data lines from the RAM get to the same data levels to the corresponding pins on the processor.

D) Look at the data sheet carefully. Many derivative parts have bits in SFRs that must be programmed in a particular manner in order to enable the external RAM bus to function. Some even have timing control to regulate the length of the external memory cycles.

Michael Karas