M-Bus (short for Meter-Bus), is a European standard (EN 13757-2 for physical and link layer, EN 13757-3 for application layer) developed for the remote reading of utility meters. M-Bus is designed to facilitate the monitoring and measurement of energy consumption to assist with energy conservation. The M-Bus interface is designed to use two wires, though there is also a wireless version (EN 13757-4). 

M-Bus, uses a hierarchical system controlled by a master (e.g. a PC) connected to a number of slaves (meters), using M-Bus two-wire cable. 

The transfer of information from master to slave uses voltage level shifts, whereas that from slave to master is by modulation of current.