Conversion of unbalanced to balanced signals by means of offset voltage.

Unbalanced signals use Signal Ground as reference, balanced signals use potential difference between 2 wires - the potential of one wire going high means the other goes low (the difference is at least 200mV, in the range 0 to +5V). 

The voltage level of unbalanced V.28 signals (over +/- 3V) is big enough to drive the difference of +/- 200mV but the reference must be brought on a between 0 and +5V. Connecting the return wire of the balanced circuitry to an offset voltage of plus 1 to 2V makes a unbalanced signal look like a balanced one (in the range 0 to 5V). As unbalanced signals use voltage levels higher then specified for balanced circuitry  protection diodes/resistors might be needed. 

The 1.5 Volt source build-in our WAN Breakout Boxes can be used as offset voltage. The LED indicators on the breakout box show proper balanced signal levels.