DA conversion

The 74LS08A AND port enables the AD7869 by making input #math26##tex2html_wrap_inline845#
low as soon as a read or write takes place on address $301. A DA conversion is initiated by writing a byte to $301, because #math27##tex2html_wrap_inline847#
of the 74LS139A decoder is tied to the #math28##tex2html_wrap_inline849#
line of the converter.