Patent GB-807500-A (GB)
Title: Improvements in or relating to telegraph systems
Inventor: HINTON RAYMOND CECIL PRICE
IPC: H04L9/18
Filed: 1953-01-16
Abstract: 807,500. Type-printing telegraphy. STANDARD TELEPHONES & CABLES Ltd. Jan. 16, 1953, No. 1411/53. Class 40 (3). A half-duplex telegraph system in which ciphering and deciphering keys are provided at each of two interconnected stations, which keys are advanced correspondingly for each signal exchanged between the two stations, is provided with means, at one or both stations, for sending a break-in signal independently of the ciphering apparatus at that station and means responsive automatically to the recognition of the signal at the other station as a break-in signal for correcting any out-of-step condition in the ciphering keys at the two stations. Fig. 1 shows the connections of a teleprinter TEL at one station to a two-wire line L1, L2 connected to a distant station, the send-receive switch sr1, sr2 being shown in the "send" position. Transmitted signals pass through modulator MOD1, line l4, regenerator REG to line L1 and received signals arriving from line L2 pass through MOD2, line l5 and regenerator REG to the teleprinter, the modulator and regenerator serving to cipher and decipher the signals. A break-in key BK, in unactuated position, maintains negative or masking potential on the teleprinter when transmitting and a line L1 when receiving. If the key BK is operated when transmitting the only effect is to replace the masking potential applied to TEL by spacing potential; this causes the receiving cam shaft to be kept rotating and serves to draw the attention of the operator to the fact that his break-in key has been operated. Operation of key BK when the station is receiving replaces the masking potential applied to line L1 by a positive or spacing potential. Referring now to Fig. 2, received or transmitted pulses on line l5 or l4 operate either gate G2 or gate G1 and through gate G21 operate two-condition switch so that tube ST is operated if not already operated. Operation of tube ST starts the time scale TS; T followed by a number indicates that a pulse is applied at the number of milliseconds after the start of the time scale. Thus at T1 potentials are applied to gates G201, G202, Fig. 3, and according to whether the pulses which started the time scale appeared on l4 or l5 gate G202 or G201 is operated. If an incoming signal has started the time scale the stop element should commence at T120, and thus at T130 there should be 20 pulses on conductor l5. If a break-in signal should start the time scale, or arrive during the sending of a signal that has started the time scale, at T130 there will be pulses on conductor l5 and a potential will be applied to gate G205, Fig. 3, simultaneously with a potential from the time scale; gate G205 is then opened and the two condition switch F11 is operated in the condition in which BIS is fired and relay BR operated to give a signal that the other station desires to break in. The firing of BIS inhibits gates G1, G2 and G200 so that the ciphering key K cannot be advanced and is thus in step with that at the other station. If the link between the two stations is a long one the break-in signal may have been sent from a station receiving at the time of sending the break-in signal and the sending may have ceased at the other station by the time the break-in signal is received. This state of affairs is automatically recognized by the coincident states of relay F10 in condition of SDT operated and relay F11 in condition of BIS operated. At the end of the time scale G207 is opened to operate F2 and start the time scale again; at T1 G204 is opened and causes relay F10 ot operate to change the send-receive switch with the " send position, and the station then sends a starting element to the distant station to start up the time scale at the distant station and bring the two ciphering keys K into step with each other. Specification 692,411 is referred to.