Patent GB-773030-A (GB)
Title: Improvements in or relating to local loop connectors to trunk telegraph circuits
Inventor: FOULKES EDWARD ARCHIBALD
IPC: H04L25/22
Filed: 1955-09-13
Abstract: 773,030. Code telegraphy; telegraph exchange systems. STANDARD TELEPHONES & CABLES, Ltd. Sept. 13, 1955, No. 26138/55. Class 40 (3). An arrangement for connecting a singlecurrent local loop circuit to a telegraph circuit having separate transmitting and receiving channels comprises a transmitting device having its output circuit connected to the transmitting channel, a receiving device controlled by marking and spacing signals arriving over the receiving channel and arranged to supply respectively marking and spacing potentials to the local loop circuit in such manner that part of the local loop current flows through the input circuit of the transmitting device, and a direct-current source of potential, separate from the first-mentioned potentials, connected to supply to the input circuit a marking current component of which the magnitude and sign are independent of the open or closed condition of the local loop circuit. The arrangement is such that the local operator, by opening the loop circuit, can signal to the operator at the transmitting station where the signals which are being transmitted are fed back in reversed form to the teleprinter at that station. The modulator 7 of the transmitting channel 9 forms one arm of a balanced Wheatstone bridge 18 having its other arms constituted by resistors 19, 20, 21 and a source 27 of voltage is connected across terminals 25, 26 to provide the fixed marking component 11. The armature 15 of the receiving relay 11, operated by the demodulator 8 of the receiver channel 10, is connected to the local line 3 through resistors 16, 17. Resistor 28 is adjusted so that with contact member 15 in the marking position the potential at corners 23 is greater than that at corner 22 so that an additional marking component I2 flows through the modulator 7. When the member 15 changes over to spacing, negative potential from the source 14, grounded at its centre, is applied to terminals 22, 23 and this is selected to be equal to that from the local source 6. As a result, there is no current in the line 3 or in the arms 19, 20 of the bridge 18 so that the local teleprinter is started and the modulator 7 is unaffected. With the relay 11 in the mark condition the opening of the transmitting contacts 5 reduces the local line current to zero and the point 22 becomes positive relative to the point 23 so that a spacing component I3 is produced. If I3 is greater than I1 the modulator 7 is changed to the spacing condition. Preferably the values of the components are chosen so that I1+I2=I3-I1, or I3=I2+2I1. If the contact member 15 changes over to its opening contact 13 whilst the contact 5 is open, the corners 22, 23 are connected to the same terminal of the source 14 and there is no spacing component so that the marking component from the source 27 changes the modulator 7 to the mark condition. If, therefore, the key 29 is opened whilst signals are being received from the transmitting operator, the arrangement functions so that the signals received by the relay 11 are passed back in reversed form to the channel 9 and provide an indication on the operator's teleprinter that the local receiving operator has interrupted the circuit.