Extension ringing arrangement key telephone system

Claim 1

An extension ringing arrangement for a key telephone system comprising main equipment including a main CPU, a plurality of key telephone sets interconnected in parallel to said main equipment respectively through an associated plurality of channel units, and an interface means including a sub-CPU interconnected to said main CPU and connected in parallel to said channel units, wherein said interface means comprises means for transmitting a ringing tone allowing a calling party to recognize a connection of said key telephone system with CO line in response to a terminating CO line call, and means for detecting an extension number of a selected one of said key telephone sets further dialed by said calling party to turn ON a corresponding one of said channel units connected to said selected key telephone set, said interface means further comprising a channel switch connected to said sub-CPU, and said ringing tone transmitting means comprising a tone source driven by said channel switch turned ON by said sub-CPU, said interface means further comprising a receiver which detects said extension number in response to said turning ON of said channel switch through said sub-CPU, a detection output of said receiver being provided, through said sub-CPU and main CPU to one of said channel untis which corresponds to said selected key telephone set so as to turn such telephone set on and connect it to said CO line.