Patent US 3,978,290

A private automatic branch exchange utilizing PCM (pulse code modulated) digital technology with a four wire non-blocking PCM network having an ability to provide a double (three port) connection for each inlet, on a digital basis is disclosed. As a result of the network various new functional arrangements are possible. The application of PCM for implementing PABX systems provides an opportunity to interface directly with a PCM trunk thereby eliminating the PCM to analog code conversion.