Here you find schematics for a full-duplex party-line intercom. FAQ: How do you build "Quad fork box?" Answer: Just connect COMM+ from all of your units, likewise COMM-. I should write a new index.html but I'm too lazy. Sorry for the inconvenience... Kaoru "Mad Player" Maeda
Here are some photos on the way building.
Mount jumpers, laying resistors first.
A complete unit.
Parts side close up.
Patterns side. Two white jumpers are for on-board Mic-Vols.