2681 UART Interfacing

A0-A3 must be metastable for 10ns prior to assertion of RDN/WRN.

A0-A3 must remain metastable for 100ns after assertion of RDN/WRN.

CEN may be concurrent with RDN/WRN assertion.

D0-D7 are latched on WRN↑. WRN ⇐ /Wr