Hi,
CEN means chip enable (active low).The DUART is to be a small part in the memory space of the 80xx. (data memory)This is called memory-mapped I/O.The device needs a 16-bit base adress in the data-memory. If you choose (e.g.) F000 (hex) to be the base adress then you have to filter out that adress (a15,a14,a13,a12 =1 and a11,a10,a09,a08 =0). Only on this combination the CEN is active (the device is selected)the low 8-bit adress select the registers from the DUART. For the adress decoding the 74ls138 is often used.If the DUART is selected there must be no!! other device (e.g. data-RAM) selected.
hope this will help,
good luck henk.