Simple ptt circuit Dip" switch settings on circuit board connecting or buying interface cables installing ptt with typical settings, making installation very simple
I looked at some of mercially available boxes that had ptt protection after prototyping a simple relay circuit i found n5qq who was interested in building a set of them. The resulting economic tradeoff is simple: if lines are cheap, motorola cps use circuit switching; puting is cheap was started in france at about the same time by the french ptt.