An automatic impedance matching front end interface circuit includes a processor and a plurality of variable resistors, the resistances of which are adjustable by the processor to achieve impedance matching such that the upstream and downstream signals flow in desired directions. A method of adjusting the resistances of the variable resistors includes measuring two voltages at different locations along a downstream transmission line, obtaining a measured voltage ratio, comparing the measured ratio to an optimal ratio, and adjusting the resistances of the variable resistors to cause the measured ratio to approach the optimal ratio.