====== 2681 UART Interfacing ======

===== Reads =====

  * 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.

===== Writes =====

  * D0-D7 are latched on WRN↑. WRN ⇐ /Wr