Patent GB-1173981-A (GB)
Title: A Party Line Interconnection System
Inventor: MANUS MAX
IPC: H04M9/00
Filed: 1967-01-03
Abstract: 1,173,981. Automatic exchange systems. M. MANUS, and G. MANUS. 3 Jan., 1967 [6 Jan., 1966], No. 352167. Heading H4K. A party line system has a number of speech buses and each substation can seize a free speech bus to set up a call to another substation, connection of a calling substation to a speech bus busying the bus to calls from all other substations and locking the calling substation out against calls over all other buses, all substations being coupled to a pulse bus that repeatedly drives counters at the substations from 1 to 10 and zeros them in synchronism, each substation being identified by particular ones of such pulses and being connected to the speech bus seized by a calling substation by the appearance of the identifying pulses on the speech bus. As shown in Fig. 1, there are two speech buses S1 and S2, each with a supervisory circuit as shown at the left of the drawing. Only one substation is shown and that is at the right of the drawing. A pulse generator GP supplies the cyclic groups of decade pulses to the substations over the pulse bus. At the substations, counters follow each decade group to successively energize terminals of a key set P. Where 100 substations are served by the speech buses two digits are required to connect a called substation, and these appear in each substation as connections from the correspondingly numbered terminals of keyset P to a gate G1 for the first digit and to a gate G2 for the second digit. The speech buses are coupled to each substation by means of secondary windings on a transformer, the secondary windings bridging the line, having a centre tap to transmit control signals, and being normally disconnected by diodes D6, D7, biased off by a controlling thyristor T. The primary of the transformer is connected to a loud-speaking subset over a two-way amplifier. Seizure of an idle speech bus.-The cyclic groups of decade pulses are spaced apart and between these groups an " eleventh " pulse is put on the pulse bus to define an interval in which bus seizure can take place at the substation. Also during the gap between the decade groups, and following the " eleventh " pulse, the speech buses are pulsed one after the other to provide correspondingly distinct intervals during which each bus may be seized These speech bus free pulses are produced by circuits Ps1 and Ps2 in the supervisory circuit of each bus and as shown for bus S1 otherwise normally enabled AND gates N1 and P1 are fully enabled by the speech bus pulse so that the speech bus voltage drops, from the 17 volts of its idle state, to 10 volts. A calling subscriber operates the key of his key set corresponding to the first digit of the wanted subscriber and, whatever such digit might be, this primes a trigger circuit which operates in response to the next occurring " eleventh " pulse. The output of the trigger circuit persists for the gap between decade pulse trains and during this time it connects priming potential over OR gate G3 to the control electrodes of the thyristors T. Idle speech buses exhibit the speech bus pulse in a time position characteristic of each bus, as described above, and the first such one to appear drops the potential of the cathode of the associated thyristor in the calling substation. The thyristor consequently fires and draws current over an 820# anode resistor common to all the thyristors in the substation so as to lock all other thyristors out and prevent connection to any other speech bus. This lock-out potential also lights a station engaged lamp and, by back biasing diodes D2, blocks the digit selection circuitry involving gates G1 and G2 to prevent the substation being connected to any speech bus as a called party. Conduction of the thyristor removes the blocking bias on diodes D6, D7 to provide a speech path from the subset to the seized bus and also increases the potential of the bus to 20 volts. In the supervisory circuit for the seized bus a diode D4 in the output of AND gate P1 becomes back biased to block the application of further speech bus free pulses. A blocking circuit P3 also removes enabling potential from P1 for a period to allow the speech bus to be used for digit signalling during which its potential falls momentarily to 10 volts. Connecting wanted substation.-The digit key pressed down at the calling substation connects the corresponding decade pulse via a diode D5 to the speech bus. At all substations having this as a first digit the corresponding decade impulse will be relayed over the key set and by a " first digit " circuit to an AND gate G1 where it coincides with the pulse on the speech bus to open the gate and trigger a monostable circuit Om. The circuits Om prime the gates G2 in the substations having the same first digit for a period (said to be 2 seconds) pending the keying of the second digit. The next pulse in the pulse position of the second digit that is keyed on to the speech bus coincides with that pulse appearing at the called station key set and passed to the AND gate G2 so that G2 in the called substation alone opens and over OR gate G3 fires the thyristor T associated with the calling speech bus and thereby effects connection of the called substation to that bus. The additional current drawn by the second thyristor T connected to the speech bus increases the voltage on the bus from 20 to 23 volts and this activates a ringing generator in the supervisory circuit so that a fixed period calling signal is applied to both substation loudspeakers. Release.-Either substation may release by operation of a key A which raises the speech bus to 28 volts. A release circuit responds in the supervisory circuit and over OR gate G5 primes AND gate G6 so that at the next " eleventh " pulse G6 opens to pulse the speech bus with 30 volts to cut off all thyristors T conducting to the bus. Called substation busy.-Since a busy substation derives access to its digit selection circuits based on gates G1 and G2, diodes D2 effecting the blocking, the second digit pulse sent by a calling substation will not fire the correspondingly indicated thyratron. In consequence the speech bus will not rise to 23 volts and after an interval a monostable circuit 02 is triggered to supply a fixed period of busy tone from generator G0. Circuit O2 triggers so long as ringing generator GL is not activated and after a monostable circuit O1 restores, O1 timing a fixed interval of two seconds during which the second digit must appear. When monostable 02 restores to end busy tone, it triggers a monostable 03 which over OR gate G5 primes G6 so that on the next occurrence of an " eleventh " pulse G6 effects release as described above. To prevent possible misconnections that might occur if the calling substation clears down before being connected, the monostable circuits 01 and 02 when active both inhibit the application of speech bus-free pulses over gate N1.