Reading a byte from $301 starts an AD conversion, because the
#math29##tex2html_wrap_inline852# and #math30##tex2html_wrap_inline854# (start conversion) lines become low
simultaneously. The byte obtained from the next read (more than
#tex2html_wrap_inline856# 2 μs later) is the conversion result, and a new conversion
is started immediately. This works out because the result of an AD conversion
is stored in a latch on the AD7569. Only after completion of the next
conversion the latched result is replaced by the new result.
The least significant bit of the byte read from $300 is the by the 74LS240B
inverted #math31##tex2html_wrap_inline859# signal of the AD7569. So bit 0 of address
$300 becomes 0 upon completion of an AD conversion. The 3-state 74LS240B
is enabled (puts its data on the bus) as soon as a read from $300 takes place.