A communication system includes master and slave controllers, a local device connected to the slave controller, and a communication cable having a pair of wires and connected between the master and slave controllers. The master controller feeds a first DC voltage to the slave controller via the communication cable and communicates with the slave controller by changing the first DC voltage such that voltages on the wires of the communication cable are opposite in phase. The slave controller generates a second DC voltage from the first DC voltage and feeds the second DC voltage to the local device. When the master and slave controllers communicate with each other, the slave controller changes the second DC voltage such that voltages on terminals of the local device are opposite in phase and vary synchronously with the voltages on the communication cable.