Patent GB-736201-A (GB)
Title: Improvements in or relating to regenerative repeaters for use in telegraph systems
Inventor: DAIN GORDON TAYLOR
IPC: H04L25/24
Filed: 1952-03-25
Abstract: 736,201. Telegraph regenerative repeaters. BRITISH TELECOMMUNICATIONS RESEARCH, Ltd. March 5, 1953 [March 25, 1953; May 29, 1952], Nos. 7695/52 and 13712/52. Class 40 (3). Relates to a regenerative repeater for startstop signal combinations in which a pulse generator is employed to provide pulses for determining the instants at which samples are taken of the incoming signal elements, normally the middle points, and in which outgoing signals of correct length are regenerated in accordance with the conditions found. In accordance with the invention the pulse generator is only started into operation providing an incoming start space signal persists for a predetermined minimum period but then continues in operation even if the signal is no longer present when the sampling point is reached, means being provided for adjusting the period from the commencement of the signal to the sampling point independently of the value of said predetermined period. The pulse generator when set into operation by the start signal comes under the control of a locking unit which is normally reset after seven pulses by a counter and allows the pulse generator to be disabled by the incoming signal which should then be at stop. The resetting of the locking unit also automatically inserts a stop signal in the regenerated combination. In accordance with a feature of the invention a long space supervisory signal is arranged to be correctly regenerated by employing a character detection circuit which determines whether a space-mark transition occurs during the five permutable elements of a signal combination. If no such transition occurs the locking unit is biased so as not to be reset by the counter, and providing also a mark stop signal is not then received, the pulse generator is allowed to function for as many complete cycles as the supervisory signal continues. Provision is made to prevent a spurious mark occurring during a supervisory signal from stopping the regeneration. According to another feature of the invention the pulse generator is automatically stopped as the end of a normal character should the stop signal be dropped completely or suffer more than 50 per cent distortion. This is effected by a circuit which also delays the restarting of the oscillator by a period sufficient to prevent loss of synchronism. In Figs. 2 and 3 incoming signals are limited by valve V and applied to trigger back and forth a cathode coupled stage V2, V3 whereby line A has a high potential for mark signals and a low potential for space signals, while line B has the reverse conditions. Links LK1-LK4 permit the circuit to be set up to receive either single or double current signals with either positive or negative as mark. On receipt of a start signal, the reduction in the potential of the A line cuts off normally conducting valve V6 of a spurious start discriminating unit and also cuts off normally conducting valve V9 which functions as a hold unit for a pulse generator V10, V11. Providing the start signal persists for a sufficient period, the potential across condenser C3 rises to a level at which a locking unit V7, V8 is tripped and provides a cut-off holding bias for V9. The pulse generator operates at element frequency as determined by the setting of resistor RV2 and produces a first pulse after the cutting off of V9 by the start signal at a time determined by the charging circuit including resistor RV1. RV1 functions as a margin control and is set so that the pulses occur approximately at the middle points of correctly timed elements. The pulses are applied through differentiating circuit C23, R91 to an output drive valve V20 and allow the incoming signals applied from lines A and B over lines 1 and 2 to trigger output valves V12 and V13 controlling the retransmitting relay R. Valve 20 normally conducts to offer a low impedance at the grids of the valves, but at the middle points of the received elements as determined by the differentiated pulses is rendered non-conducting to allow the signals to be effective. The pulses also trigger a seven-point counter V14-V19. The first counter operation results in the automatic insertion of a start signal by a positive pulse sent from V14 through C11 to the grid of V13, and also results in a pulse being sent over lead 6 to trip the locking unit if not already tripped. Normally the locking unit will already have been tripped due to the action of V6 described above but it is possible, if the stop element of the previous characters has been dropped completely, for the pulse generator to be running with the locking unit in the reset condition. At the end of seven pulses the counter reverts to normal and a negative pulse is sent over lead 5 to reset the locking emit and remove the holding bias on V9. This allows V9 to conduct to the mark (stop) signal which should now exist on line A and stop the pulse generator. The resetting of the locking unit applies positive potential over lead 4 to the grid of V 12 and results in the automatic insertion of a stop signal. To permit the regenerator to transmit correctly a long space supervisory signal there is provided a character detecting circuit in the form of trigger pair V4, V5 connected to the B line. The circuit does not react to the start mark-space transition but any other transition, as will occur with ordinary characters, triggers the circuit causing V4 to become non-conducting and thereby produces a low potential at the grid of V7 which permits the locking unit to be reset by the negative pulse appearing on lead 5 at the seventh operation of the counter. During an all space supervisory signal, however, the character detecting circuit is not triggered and, since the potential on the A line is low cutting of V6, the grid of V7 is held at a high potential whereby the negative pulse on lead 5 fails to trigger the circuit. No stop signal is inserted therefore and the pulse generator continues to function for as many complete cycles as the supervisory signal continues. A spurious mark signal occurring during a long space signal will operate the character detecting circuit and interrupt the space regeneration by allowing the locking circuit to be reset, and introduces a stop (mark) signal at the end of each cycle. The effect of this fault is minimised by a connection extending from V7 to V5 via rectifier MR34. This causes the character detecting circuit to reset with the locking unit so that space regeneration which immediately restarts continues without further interruption providing no further spurious mark signal occurs. During the transmission of normal characters, the character detecting unit is restored if the potential of the B line is low when the locking unit is reset by the connection extending from the anode of V8 through rectifier MR7. If the incoming stop signal suffers from more than 50 per cent distortion or is dropped completely the pulse generator will not be stopped when the locking circuit is reset. To allow for this condition a single-shot multivibrator V21, V22 is provided which is triggered when the locking circuit is reset providing line A is not at mark potential. The rise in anode potential at V7 renders V9 conducting and stops the pulse generator for a period determined by the setting of resistor RV3.