App note from Holtek on power line transceiver found on their BA45F55xx MCU. Link here (PDF)
A Power Line Transceiver is a fire protection dual-line bus which is used as a data transceiver. The master gives a command to the slave using a voltage signal modulation method while the slave responds to the master using a current modulation method. In this way a complete communication system between a master and slave only requires two lines. This communication method is very common in fire protection related products.