Data communication and power supply on a two conductor system

Abstract: This master thesis aimed to develop a prototype of a hardware platform which allowed digital communication as well as DC power to be transmitted over one pair of wires. The prototype was used as a pre-study for Axis Communications AB that wanted a bit rate high enough to transmit digital audio, as well as enough power to supply a set of speakers. The prototype included one circuit to send power and data (master), as well as two receivers (slaves). With this configuration data rates of up to 17 Mbps were achieved as well as an output power of 90 W from master. The data was transmitted on a RS-485 bus type, which was AC coupled to a 48 V DC bias. Because of the AC coupling, the bit stream needed to alternate, therefore Manchester encoded data, or similar, was recommended. The receiver cards included a buck converter to supply the circuits with a stable 5 V DC. The converters needed at least 10 V supply voltage to function, which gave a theoretical max range in cabling when accounting for voltage drop in the cables. For a 0.75 mm² power cord at max allowed current, this range was 500 metres per wire.