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.

Spraying fluxes.
Mount jumpers, laying resistors first.
A complete unit.
Parts side close up.
Patterns side. Two white jumpers are for on-board Mic-Vols.