Subscriber subset for pcm telephone system

Claim 2

A subscriber subset for a PCM telephone system of the type employing a continuous loop comprising means for generating different binary code groups each representing the number of another subscriber, means for examining all the signals in the loop when the subset is not already engaged to ascertain the presence in any one channel of the binary code group representing its own number, means for altering the code group representing its own number and retransmitting in the same channel the altered code group if the subset is not engaged, and means for determining the altered or unaltered condition of a binary code group sent from said subset when that code group is subsequently returned to said subset.

Claim 3

A subset according to claim 1 wherein the means for examining the signals in the loop includes a shift register into which the signals for each channel are fed and gating logic arranged to sample the contents of the shift register during the occurrence of each channel and to respond to the Appearance in the shift register of the code combination relating to the subset.

Claim 4

A subset according to claim 2 wherein the means for altering and retransmitting the code group includes means for inverting one bit of information in the shift register if the subset is not already engaged by a connection occurring in another channel.

Claim 5

A subset according to claim 1 including a store, a shift register, means for inserting in the shift register a binary code combination held in the store representing the number of another subscriber, means for transmitting that code combination in a channel seized by the subset, means for inserting into the shift register at the same time any code combinations received in the same channel, coincidence gating logic arranged to compare the received code combination in the shift register with the code combination held in the store and means responsive to the logic to indicate whether the combination in the register is the same as the combination is the store or is different in respect of one predetermined bit of information.

Claim 6

A subset according to claim 4 in which the means for indicating includes two oscillators of different frequencies, one of the oscillators generating an audible tone and the other oscillator generating a signal at a lower inaudible frequency and gating logic whereby the audible tone alone is continuously present in the subset receiver when one condition is indicated or the audible tone is gated by the inaudible signal to produce an interrupted tone in the receiver when the other condition is indicated.

Claim 7

A subset according to claim 5 including means for deriving from the lower frequency oscillator a frequency being a submultiple thereof and means for gating these frequencies with the audible tone when the other condition is indicated.