How to connect M-BUS converter to heat meter?


Apologies in advance for the newbie question. I’m reasonably confident with software, but I’m not a hardware person.

I recently moved into a house with an LG Therma V ASHP - my first heat pump. I have a Sontex Supercal 531 heat meter.

To monitor my ASHP, I’ve bought an EmonTx to sit in our electrical box and monitor both the main meter and the ASHP AC, and an EmonBase to monitor the ASHP itself. To collect data from the heat meter, I’ve bought the M-Bus to UART converter.

The documentation on how to set up all this brilliant and really easy to follow, but I’m stuck on what is probably a really basic point: what kind of cable or wire do I use to connect the M-Bus to UART converter to the heat meter?

The guide here just shows a single wire going from the converter to the heat meter, but doesn’t specify what wire/cable? Is this sort of thing sufficient?

The terminal board in the heat meter has two terminals for M-Bus - so do I connect both?

And finally, the guide doesn’t specify which of the three terminals on the convert to connect?

Any support gratefully received! Thank you

Welcome, Greg, to the OEM forum.

That’s what we call a “single line diagram”. It uses a single line to show the path a signal, or the power, takes. It doesn’t necessarily show details of the connection.

The clue to what cable you’re looking for is in the name “MBus”, if you look that up, you’ll find lots of data. Essentially, it’s a data bus, so although that wire will probably work fine for a short distance, I’d normally want to use a telephone-type of two-core cable with twisted cores (to cancel out interference). You could also use one pair in a Ethernet cable (CAT anything!).
You do connect both, because it’s a balanced signal. 3 terminals - I’d guess the centre is the earth/common; the outer two will be the bus. The bus should not be polarity-sensitive, i.e. connected either way round, it should still work.
This might help:

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Hi Robert,

Thank you so much for such a comprehensive answer. I’ve ordered some telephone-type two core cable and will look forward to connecting them up.

Thanks again,


Hi Greg,

Did you get your connection working?
We have the same heat meter.
So far I have not got the connection working and it doesn’t help that the Sontex doc does not make it clear what the polarity of the 2 m-bus terminals might be (24 and 25).
I’ve tried various permutations.
I noticed that the m-bus address in the Sontex config was set to 0. So far I have not managed to change that.
Have written to Sontex to ask about changing the config as I do not see how to do that.
All suggestions welcome.
See also modbus to USB adaptor installation · Issue #172 · openenergymonitor/emonhub · GitHub


Sontex sent a reply via a distributor which referred to a different model of meter. The I contacted Stockshed and got some better information. You can change the meter address via the control panel. Did so. However, it’s still not working as I write.

Mbus is polarity independent, so either polarity should be fine.

Here’s our guide to reading from Sontex Mbus using emonHub: EmonHub Interfacers - Guide | OpenEnergyMonitor