M-Bus is not just a ASCII protocol based on RS232/485. It uses 2-wire cable and the meters can be powered via that cable. The data transfer is done via modulating the voltage level over the wire. A "1" is 36V and "0" is 24V.
To connect a M-Bus meter to a standard RS232 port, you need to use a level converter device. This device needs to be specified based on the number and kind of clients/meters in the M-Bus network, to provide enough power.
Here in Europe we have several applications that connect either Opto-EMU or SNAP-PAC systems to M-Bus. We use external Modbus to M-Bus converters and connect them to the RS232 or RS485 port of the Opto 22 controller, or just via Ethernet using Modbus-TCP. The devices are from Piigab, as Mary has mentioned already.
I am not aware of anyone that has implemented the M-Bus protocol in PAC Control and just uses a external M-Bus level converter.
I hope this helps,