6This legacy technology is called Power Line Carrier, or PLC which is unfortunately confusing because it has nothing to do with Programmable Logic Controllers (also abbreviated PLC). The concept is not unlike the HART analog-digital hybrid system used to communicate digital information to process transmitters over 4-20 mA analog signal lines, except in the case of power-line carrier systems the signal frequencies are much higher and the challenge of safely coupling these signals to high-voltage power line conductors is much greater.