Patent GB-1229149-A (GB)
IPC: H04L12/437
Filed: 1969-07-28
Abstract: 1,229,149. Party line arrangements. STANDARD TELEPHONES & CABLES Ltd. 28 July, 1969, No. 37764/69. Heading H4K. In a TDM, PCM transmission system utilizing a unidirectional looped highway, a second looped highway is parallel with the first and having a signal transmission direction opposite to that of the first is provided for use under fault conditions.  In Fig. 1, the main highway 2 is looped on a synchronizing and "free-channel" pulse generator 1 via the subset circuitry AB of a plurality of substations 3-8. As is usual with these systems, a caller e.g. 4 seizes a free channel and injects the identity of the wanted party thereinto. The identity passes along the highway in the arrowed direction until it is recognized by the wanted party e.g. 7 who thereupon extracts the identity from the channel if he is free. The empty channel continues its journey via generator 1 back to the caller who is thereby advised of the free state of the wanted party. (The identity would still be present if the party was busy so that a busy tone would be generated at 4 in these circumstances). Ringing, reply, conversation and hang-up occur as per normal. The secondary highway 9 normally carries a pulse train produced by generator 1 and this train is repeated via a normally open gate CD in each substation. If now substation 6 say or the primary highway between stations 5 and 6 becomes faulty, the absence of incoming primary signals is detected by station 7 which reacts by looping its terminals D and A and then sending an alarm signal along highway 2. The absence of primary signal will also be detected by station 8 but since the alarm signal follows immediately, its own AD looping action is countermanded. Similar action ensues at each station up to station 5. Here the absence of secondary signals (due to the AD looping in station 7) is effective to loop terminals BC whereby the alarm signal is passed to the secondary highway and makes its way back to the generator 1. The appearance of the alarm signal here causes the secondary pulse train to be stopped and a fault alarm to be operated. If the fault is in a secondary highway or if further faults develop a similar sequence of events occurs. When the fault is rectified the system is reset manually. Substation equipment (Fig. 3).-Normally, gates 31-35 are open so that incoming primary signals are passed to the output after passage through the subset's calling-signal detector, modulator &c. connected across terminals AB. Similarly, a pulse train having a p.r.f. of half that on the main highway is passed straight through gates 38-42. If a fault occurs such that incoming primary signals cease, detector 44 (transistor circuit and alarm lamp depicted in Fig. 4, not shown) responds by closing incoming primary gate 31 and outgoing secondary gate 41 and by opening gate 45 so that the incoming secondary is connected to the outgoing primary. If the incoming secondary signals cease, detector 46 inhibits gates 38, 34 and opens gates 47 so as to connect the incoming primary to the outgoing secondary. The above actions occur at stations 7 and 5 respectively in the example given above. At intermediate stations, an automatic reset (monostable circuit depicted in Fig. 5, not shown) 48, 49 is turned on by the appearance of an alarm signal CW forwarded from the adjacent stations so that a signal is transmitted over the outgoing primary and secondary. As soon as the longest loop possible has been restored (within the pulse time of the monostables) the detectors 30, 37 will become effective in the ordinary way to unblock gates 31, 41, 34, 38 and to block gates 45, 47. When the fault is corrected the two "end" stations 7, 5 are reset and the system restored to normal by operating switch 50. Logic and schematic (transistor) circuits for producing a secondary pulse train and for detecting the appearance of such a train on the primary highway and for inhibiting the secondary pulse train in these circumstances are described with reference to Figs. 6-8 (not shown).  Alternative system (Fig. 9).-Here the secondary signals are produced by feeding back the output of each station along the secondary highway 91 in addition to sending it forward along the primary highway 90. 'Each station includes a comparator for detecting the simultaneous presence of signals on both highways and for initiating switching akin to that described above in the event of a fault that causes cessation of signals on one highway end, in response thereto, for sending an alarm along a separate loop highway (schematic logic, Fig. 11, not shown).