What exactly is mdb?

[Coal River Vending]

What’s it do exactly?

[BTVFC]

See the 2nd paragraph.

https://en.wikipedia.org/wiki/Multidrop_bus

[AZVendor]

MDB stands for Multi-drop Bus which is the coinage and vending communication protocol adopted by NAMA in the mid to late 90's.  This is not DEX which is the Data Exchange protocol adopted by NAMA.  MDB is like USB on a computer; it interfaces all peripherals to a machine with a common 6 pin plug.  The protocol runs on 24 - 34vdc and allows all components to share the same computer connection.