Patent GB-1216759-A (GB)
Title: Subscriber subset for p.c.m. telephone system
Inventor: MCNEILLY JOSEPH HOOD
IPC: H04M9/02
Filed: 1968-05-23
Abstract: 1,216,759. Telecommunication systems. STANDARD TELEPHONES & CABLES Ltd. 23 May, 1968, No. 24694/68. Heading H4K. In a telecommunication system in which stations are distributed round a looped unidirectional transmission line time division multiplexed from a control station and in which a calling station circulates a pulse code identifying a wanted station, the code having a spare bit, a free station circuit on recognizing its own code breaks the loop in the time slot of the calling code to establish send and receive channels and sends its own code with the spare bit inverted so that the calling station can detect the ringing state, the calling code and the spare bit circulating round the loop with the spare bit unchanged if the wanted station is busy. Each time slot is available for transmission of eight bit PCM codes, one time slot in each system frame being set aside for station synchronization. Each station has a nine push-button keyset to set up wanted station codes in bits 1 and 3 to 8 of a time slot. Button number 9 is pressed first in making a call to provide an interlock which inhibits the station speech encoder. Buttons 1 to 6 and 8 are available to set up a wanted station code, button 7 (corresponding to bit 2 of the eight bits) being pressed last to establish a one in the bit 2 position and to initiate seizure of an idle channel round the loop. In the idle condition of a station the loop is shunted across the station, a stepping register inspecting the code stream as it passes by in order to obtain synchronization, own code recognition, and idle channel selection. On seizure of an idle channel the station breaks the loop repetitively in that time slot to terminate the channel for codes incoming round the loop and to be able to send codes forward round the loop. An idle station similarly breaks into the loop in a seized calling time slot when it detects its own code. Having been detected as an own code by an idle station inspecting the bit stream the code circulates once round the entire loop (being regenerated at the calling station) before the station is able to terminate itself on the idle channel and effect inversion of the second bit to signal the idle and ringing state. The connection of ringing or engaged tones at a calling station and detection of reply is effected by the circuit of Fig. 3. Engaged tone is a continuous 700 c/s. gated over NAND gates G3/13, G3/15, G3/16 to the PCM decoder associated with the station receiver. The eight bits of the decoder are normally all set to ones, bit 6 being set and reset to reproduce the tones. Ringing tone is the 700 c/s. interrupted over gates G3/11, G3/12 by a 2 c/s. waveform modulated by a ¢ c/s. waveform so as to give two “-second bursts of 700 c/s. spaced by a “sec. and followed by a silent interval of 1“ sees. With bi-stable FF2 set, engaged tone is connected. With a two-step bi-stable arrangement FF1 advanced to its second step, ringing tone is connected. With a wanted number keyed up and registered for forward transmission, the register position governing bit 2 stores a one and provides a signal QC which with a control pulse P1, from a sequential set P1 to P4, governs NOR gate G3/1 to sanction a comparison of the calling code sent and as set up in the register of the keyset, and the codes received back round the loop and set up on a stepping register. When the codes match, NAND gate G3/2 sanctions an inspection of bit 2 of the stepping register which bit gives rise to signal QS. Since bit 2 returns at least once uninverted, gate G3/4 invariably sets FF2 to initiate connection of engaged tone. Should bit 2 return inverted twice in succession gate G3/5 sanctions G3/6 for the appearances of bit 2 to step bi-stable arrangement FF1  twice, whereupon G3/3 is disabled to discontinue bit 2 inspection and FF2 is reset over G3/7, G3/8, to disconnect engaged tone. With FF1 set ringing tone is supplied. Should the wanted station be busy, bit 2 returns unchanged and engaged tone remains in force, an on-hook condition signalled over G3/7 resetting FF2 to disconnect the tone. Reply by the wanted station results in cessation of the reverted wanted station code so that G3/2 resets a bistable FF3 set by the match of sent and reverted codes. With FF3 reset gates G3/9, G3/10 reset FF1 to disconnect ringing tone and enable the speech encoder. Should no reply be forthcoming the on-hook condition over G3/10  operates to the same end. If a called station is busy but becomes free while engaged tone is connected at the calling station then, providing the calling station stays off-hook to camp on the busy condition, the engaged tone is automatically replaced by ringing tone.