Multilevel signalling from telephone to exchange - has resistor chain of reference voltages and IR comparison at exchange of incoming signal level

摘要

Circuitry for signalling from a telephone, typically, into an exchange using a keypad and different voltage levels on the two wire line. Various values of resistance are switched into the line circuit at the sending end and are measured for recognition using a battery powered dc resistance measurement circuit to verify the incoming dc signalling information. At the reception end (Z) a chain of resistances, n+1, (R) is used between battery (UB) and earth as the reference to produce reference voltages (Uu1,Uo1....Uu5,Uo5) which are connected into a chain of integrated circuits (IC1...IC5 etc). Each integrated circuit has outputs (k,i,g) relative to each other IC(g,e) so that each can provide an individual output (k,i) when the input voltage (Um) is greater than one reference (Uu1..Uu5 gives i output) and smaller than the other (Uo1..Uo5). The second output (k) corresponds to an input voltage (Um) smaller than one comparison (Uu1..Uu5 as appropriate).