Abstract

Privacy by way of excluding all other key set talking circuits (T-R) from the seized line in a standard key set or conference key set system is obtained by providing a privacy exclusion circuit at each key set in the system. The exclusion circuit at the key set which seizes a particular T-R line first causes the voltage on the A lead associated with that line to drop below a certain critical value, thereby causing all the other exclusion circuits in all the other key sets to open their respective talking circuits if an attempt is made to access the occupied line. In order to permit other key sets to access the occupied line, the key set first to seize the line can, without relinquishing the line, permit the voltage on the respective A lead to rise above the critical value, permitting another key set access. Upon such access occurring by the other key set all other key sets are again excluded because the voltage on that particular A lead again drops below the critical value. A third key set may be allowed access to the line in the same way. Each time access is permitted, the light under the hold button on the key set turns on to indicate to the key set user that the exclusion function has been deactivated.In the case of an intercom key set system, the exclusion circuit of the present invention provides an artificial A lead. This A lead is required to prevent the exclusion circuit from opening the T-R leads when using intercom. Privacy on intercom is not provided by this arrangement. However, by a slight modification, the present invention will provide privacy on the intercom circuit.