M-Bus Master Design for more than 100 Slaves

[Flamador]

Hi. I need to read M-bus slave devices. There is 100 or maybe 250 slave devices. I want to convert this m-bus protocol to rs232 to read on MCU. How can I design this circuit ?

[Renate]

M-Bus a/k/a Meter-Bus is a European standard for energy consumption monitoring that they'll tell you is EN12345678, but not what it is.

TL/DR

It's a two wire system.
The master transmits by modulating between 36 V and 24 V across the wires.
The slaves respond by consuming 11 mA to 20 mA for a zero and less for a one.
Data runs up to 9600 baud, default 2400 baud.

Presumable you would need a driver supply for master and a current sense for the slaves.
The interface would be either 3.3 V or 5 V UART signal (not particularly "RS-232").

250 units on a wire drawing less than 10 mA would have to be doing < 40 µA each.
I guess that they'd accept a higher baseline current and simply look for an increase of 10 mA?

[Renate]

Does this installation already have a master driving it?
Or is it new and it's not even powered?

You're going to have to do some experimenting.
The first thing I'd do is hook a current-limited 24 V supply (with the right polarity) to these sensors and see what the quiescent (baseline) current is.

[MarkL]

Take a look here:

  https://github.com/rscada/libmbus

In the "hardware" directory you will find a schematic for an m-bus master.  It incorporates a PL2303 USB to serial converter but you can get rid of that if you want to drive it directly from your micocontroller.

In looking at the circuit it may not have the capability to drive 250 devices, but maybe it can give you a start.  You can try it in a circuit simulator.

The library works fine; I've used it.

More documentation on the physical and higher level is at https://m-bus.com if you haven't already found it.