Patent GB-827039-A (GB)
Title: Improvements in or relating to signal transmission systems for telegraphy
IPC: H04L25/22
Filed: 1957-09-20
Abstract: 827,039. Telegraph exchange systems. STANDARD TELEPHONES & CABLES Ltd. Sept. 20, 1957 [Sept. 21, 1956], No. 29631/57. Addition to 821,630. Class 40 (3). In a telegraph signal transmission network for connecting a single current 2-wire local line to the send and receive-line-pairs of a doublecurrent 4-wire circuit by a Wheatstone bridge circuit having the local line connected in series with one of the arms and with the send and receive lines of the 4-wire circuit connected to the two pairs of diagonally opposite terminals of the bridge network which is so dimensioned that the bridge is substantially balanced when the local line is in the marking condition, a delayedaction relay connected across the receive line is arranged to be operated only when the receive line is in the marking condition and, in operation, reverses the direction of current flowing in the local line and simultaneously interchanges the connections of the send line to the terminals of the corresponding or associated pair of diagonally-opposite terminals. In the rest or idle condition of the system, with contacts P1, c1 open, a very small current traverses the local loop LL, and the point S2 is more negative than S1 so that the modulator switch S5 does not transmit any voice-frequency signal to the outgoing pair of the four-wire line. To initiate a call the operation of push-button P1 shortcircuits resistor R so that S2 becomes more positive than S1 and the switch S5 transmits a voice-frequency signal to the exchange. When a free selector has been found a V.F. signal passed to the calling subscriber operates relay SA for a period too short to operate relay SB, but effective to reduce the loop current to almost zero and release indicator A which remains latched until dialling is effected. When the called line has been selected a continuous V.F. signal is transmitted to the called subscriber operating the relay SA and subsequently relay SB changing over contacts SB3, SB4 so that V.F. is now transmitted to the calling station and at contact SB2 reversing the current flow in the called subscriber's loop so that relay C operates, starts the motor at the called station and increases the current in the called loop. The V.F. transmitted to the calling station operates relay SA and subsequently relay SB so that the current in the calling subscriber's loop is reversed, contacts SB3, SB4 switched over so that with the teleprinter transmitter contacts closed, V.F. is transmitted to line and is interrupted to provide the code signal elements. To terminate transmission button P2 is operated to open the line circuit so that relay SA at the called station is de-energized followed by the release of relay SB so that relay C de-energizes to open the circuit of the motor, and the changeover of contacts SB3, SB4 cuts off the V.F. transmission. At the calling station relay SA releases, followed by SB which effects the release of relay C, the arrest of the motor and the discontinuance of the V.F. transmission by the change-over of contacts SB3, SB4, so that the whole circuit is restored to the rest position. The rectifier W1 and resistor R6 are provided to prevent the modulator S5 at the distant station from being placed in the transmission condition, e.g. passing V.F. over the line, until the local loop LL has been switched over to the condition for message transmission. When relay SA operates, and before SB operates, the rectifier W1 blocks current which would pass through R1 to make S2 more positive than S1 and therefore prevents signal transmission. When, however, contact SB1 is changed over so that W1 is short-circuited, the contacts SB3, SB4 are changed over so that signal transmission over the outgoing pair in response to operation of relay SA is prevented, and the modulator S5 can be operated to transmit signals over the outgoing line.