Arcus KNX M-bus gateway

[stianj]

Hi!

Does anyone here have a Arcus KNX M-bus gateway (https://www.arcus-eds.de/mbus.html)?

If so, could anyone be so kind to measure the voltage on the M-Bus? I measure between 19 and 22 volts (both with and without load connected...) and that's below the standard, and probably (hopefully) what is causing me grief...

Have been in contact with Arcus support. They give good answers, but it literally takes weeks between each reply...

Regards,
Stian

[Gast1961]

I measured 20.8V DC

[stianj]

Bugger... But thank you

I have two M-bus slaves. One is working just fine, the other is not. But I found out that the one working draws power from the meter, while the one not working is supposed to be powered from the bus, and the manufacturer says the voltage must be at least 24V, and that the standard does not allow anything less. Hmm.

[Gast1961]

I must admit I never thought about the voltage.

My only device is a power meter (Easymeter Q3BA) connected using yet another interface (optical to M-Bus) which is also bus powered. That interface was initially configured for the wrong meter, and took us quite a while to figure out what went wrong. The communication concept of the Arcus interface, which only triggers for known sequences, isn't ideal for debugging purposes.

[stianj]

I never thought about voltage either, but the M-bus slave I'm trying to connect has a LED. It flashes/pulsates when M-bus is connected. I thought that was to indicate M-bus traffic, but when I explained this to technical support of the manufacturer, they said that this LED should have constant light, and if it flashes/pulsates it's either because of drop in bus voltage, or bad connection. After this I measured the voltage to something between 21,22V and 19,72V. I expect this variation to happen when transferring data over the bus.

I don't have the standard available, but Google tells me this about M-bus: "A logical 1 corresponds to a signal level of 36 V DC, a logical 0 is represented by 24 V DC. This ensures that the power supply for the slaves can take place via the bus."

As I don't have the standard available, I just have to believe the technical support of the M-bus slave when they say that bus voltage must be 24V. Since both yours and my Arcus interface have voltage way below that, Arcus is either not adhering to the standard, or the manufacturer of my M-bus slave have misunderstood it. Either way I'm f*cked...

I guess my only alternative would be the Intesis M-bus KNX gateway (I already have the Modbus gateway from Intesis...), but that's a totally different price than the Arcus, way to steep for my usage... I see from the datasheet that the M-bus voltage is 36V on that one, though.

Else, I really agree that it's close to impossible to debug with the Arcus interface.

[stianj]

Got a reply from Arcus today;

"most of the MBUS-Slaves work with the devices from TI TSS721(A) or similar. These devices are much more tolerant than the standard says.
since we only support up to 3 devices and demand <1m length we can deal with much lower voltage. Without problems ever!
The only problems that occure are too much energy drawn for too long telegrams, which happens on low baudrates and long REQRESP telegrams.
This problem can be avoided by selecting higher baudrate.

This problem would increase if we increase the voltage and such the power."

So it seems they are admitting to not following the standard, even though they claim so in the datasheet.

Very unfortunate.