Bus topology and max cable length

I note that recent Velbus documentation says bus layout is pretty flexible and can be star or loop or line or combinations of these but I am wondering if this is really OK regardless of cable length.

I have approx 60 dimmer/relay modules (VMBDMI-R and VMB4RYNO) on my DIN rail. These are a simple line bus over about 20m length (actually as 3 nodes, 10m apart).
I will also have approx 35 switch modules (each is a VMB8PBU) and I would like to connect these to the bus above as a single star (35 separate cable runs) where each cable may be anything from 5m to 30m.

Is this combination of cable lengths and bus topology likely to work OK? Or should I rearrange the switch modules as one long chain? or 3 chains (one attached per node above)?


In short, Yes, you’ll be absolutely fine with almost any kind of structure.

You might have to play around with multiple terminators until you kill all the data packet echoes, but it will work.

Personally & Professionally, I’d recommend a better / thicker cored cable than Cat6, something more like a Belden 9354 or Belden 3082A (basically an EIB grade) to help reduce any volt loss.

2 x pair of AWG18 (where the data pair is twisted)

Likewise, if you can create a couple of loops or long bus runs, it’ll be much easier to install and manage.

Good luck and please know we’re all here to help you.

Best wishes,



In addition to Stuart’s excellent answer;

Yes, the star configuration will work just fine granted the bus termination happens correctly. However this will be trial and error to find out how many need to be terminated. The VMB8PBU has a jumper to easily set the termination.

As for cable length, we’ve heard of installations that run up to 1 km in length…

Kind regards

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