Could someone explain attached M-bus master circuit?
This is an RS232 to M-Bus Converter/Repeater.  M-Bus format is used in Europe for automated meter reading.  Basically it converts an RS232 signal into a 2-wire +-15v signal.

Here is a similar schematic with some additional explanation:
https://code.google.com/p/iotsys/wiki/MBusConnector