GB662222A - Arrangement for signalling over a two wire line - Google Patents

Abstract

662,222. Electric control systems. TELEFONAKTIEBOLAGET, L. M. ERICSSON. Nov. 3, 1949. [Nov. 5, 1948] No. 28196/49. Class 40 (i). [Also in Group XXXIX] In a system in which different signals are transmitted by the insertion of different resistances in a two-wire line loop the signalling circuit includes at the receiver an adjustable impedance (e.g. a thermistor) which automatically regulates the current in the line to a fixed value and the receiving device detects the magnitude of the change in the voltage across the line loop resulting from the insertion of a resistance in the loop. In the embodiment of Fig. 4 the invention is applied to setting a register RM in accordance with signals transmitted from a telephone subscriber's substation T. The impedance X automatically changes its resistance to maintain the current in the line substantially constant during signalling. Each of the keys K1 .. Kn inserts a resistance r1 ... rn in the loop and to each key there corresponds a pair of tubes such as V11, V12. These pairs differ only by the grid bias batteries E1 ... En having different voltages. Normally V11 is biassed negatively by the voltage drop in r1 and V12 is biassed positively. Relays R1 ... Rn are therefore normally operated as is relay AR also. When a key is depressed the impedance X maintains the current constant and the voltage across condenser C and resistance M and therefore the grid potential of tubes V11 ... Vn1 increases in proportion to the value of the resistance inserted in the loop. The resulting increase in anode current in these tubes decreases the bias of tubes V12 ... Vn2 and some or all of the relays R1 ... Rn relapse. The arrangement is such that for a resistance rm the relays Rn, Rn-1, ... Rm relapse. Thus in all cases the relapse of Rn brings up BR and during the release period of AR the mth marking wire is marked over front contacts of R1, .... Rm-1 and a back contact of Rm. The adjustable impedance X may be a thermistor, alone or in series with a large inductance, or a saturated diode or a pentode. Where an inductance is included the voltage change across the inductance may be used to operate the receiving device.