Control signal transmission system for key telephone system

Claim 1

In a key telephone system comprising a key service unit, a plurality of key telephone sets connected to the key service unit each key telephone set having keys operable for developing key information signals and indicators for indicating the busy status of a plurality of CO lines corresponding to said keys, a plurality of pairs of parallel communication lines each connecting a corresponding key telephone sets individually to the key service unit; the improvement comprising: a plurality of pairs of parallel control lines, each pair connecting a corresponding key telephone set individually to the key service unit; each key telephone set having a parallel-serial converter for converting key information signals to a serial signal configuration for transmission through a corresponding one of said pairs of parallel control lines to said key service unit; a random-access memory in said key service unit for receiving and storing the key information signals of the key telephone sets; said key service unit having means effective for connecting and disconnecting said pairs of parallel communication lines to selected ones of said CO lines in reponse to said key information signals and to store status signals representative of said busy status of each CO line and transmit said status signals to each key telephone set over said corresponding parallel control lines. control means in the key service unit for controlling said storage of the status signals for accessing all of the pairs of parallel control lines simultaneously for key information signals and applying them in a first synchronized access time into the key service unit random-access memory, said control means including additonal control means for reading out from said random access memory stored status signals and applying them at a second synchronized access time through corresponding parallel control lines to the parallel-serial converters of the corresponding key telephone sets as control signals for activating the corresponding indicators of said key telephone sets.