The core of this design was inspired by the early TRS80 "put a DAC on an I/O port and do it all in software" designs, along with the Covox and similar devices. The CPU side is a latch that then feeds into a resistor ladder and an small amplifier.

The twist is that it also has a 7474 flip flop on it. This is cleared when the port is written and can be set again by a clock pulse on an external line. Why - well if you wire the set side of it into a spare CTC channel and you plug the ready/notready signals into a Z80DMA card you get something akin to an early soundblaster for RCbus/RC2014