Loxone Modbus wiring

[Thorfun]

Greetings. this is, hopefully, a simple question that will have you facepalming at! I don't mind, I'm very special sometimes.

 

I'm looking at controlling AC units via Modbus with my yet to be installed Loxone system. According to the Loxone Modbus Extension web page Modbus devices need to be daisy chained.

 

Quote

"Modbus devices are daisy-chained, the last Modbus device must be terminated with a 120 Ohm resistor."

 

so, does this really mean that I have to run one cable from the Loxone cabinet to each device or can you 'virtual' daisy chain from within the cabinet so each device has it's own Cat5/6 cable running back to the cabinet and then using some form of magic with terminal blocks to make the daisy chaining happen? I'm hoping the later as it'd be a nightmare to know where any future modbus devices would be to include it in the single magic daisy chained cable run!

[jack]

I'm far from an expert, but my recollection is that implementing daisy chain wiring at the cabinet (basically star wiring but with terminations to convert to daisy chain) will cause issues due to noise caused by reflections.

You could look up the spec - sometimes there's guidance about what's allowable where a star wiring approach is taken. Search for "star wiring modbus".

[Thorfun]

30 minutes ago, jack said:

I'm far from an expert, but my recollection is that implementing daisy chain wiring at the cabinet (basically star wiring but with terminations to convert to daisy chain) will cause issues due to noise caused by reflections.

You could look up the spec - sometimes there's guidance about what's allowable where a star wiring approach is taken. Search for "star wiring modbus".

just done a basic search and quick read and there's mentions, as you say, of reflections and needing to terminate each point of the star with 120ohm resistors which causes issues etc. also some mention that if the length of cable is short enough you can get away without using terminator resistors so it 'could' work.

 

I wonder if you run a cat 6 cable which has 4 pairs of wires, can you connect 3 to the first device and then a different 3 to the same device and bring it back to the cabinet into a connection block, then run another cable of 3 wires 'joined' to the return 3 from the first device to the next device where you'd then attach another 3 from the same cable to the same device back to the cabinet and so on. that way it is daisy chained but the length of the cable run will get long as it's back and forth from the cabinet? but in a home environment and, for me with only 5 devices, I doubt the total length would be an issue.

 

I guess I could simply daisy chain it for the AC units but then I'm thinking about maybe adding the ASHP to the Modbus extension. and then maybe some other stuff. starts to do my head in thinking about daisy chaining a single CAT6 cable around the house to all possible locations of stuff! 🤯

[Thorfun]

something like this?

 

[joth]

3 hours ago, Thorfun said:

something like this?

 

Seems reasonable but the bus cable length could get very long and I'd be worried about cross talk, especially if not using S/FTP individually screened cat6.

 

DMX has similar daisy chain requirements (both are RS485 derived??) and there the preferred workaround is to use an signal regenerator/splitter.

Looks like something similar maybe available for modbus?

E.g. https://www.se.com/us/en/product/LU9GC3/modbus-splitter-block-10-rj45-and-1-screw-terminal-block/

 

[Thorfun]

8 hours ago, joth said:

Seems reasonable but the bus cable length could get very long and I'd be worried about cross talk, especially if not using S/FTP individually screened cat6.

 

DMX has similar daisy chain requirements (both are RS485 derived??) and there the preferred workaround is to use an signal regenerator/splitter.

Looks like something similar maybe available for modbus?

E.g. https://www.se.com/us/en/product/LU9GC3/modbus-splitter-block-10-rj45-and-1-screw-terminal-block/

 

But $300!! Great info though, thanks. 

[Thorfun]

34 minutes ago, Thorfun said:

But $300!! Great info though, thanks. 

actually, only £128 when setting the country to UK. that's a great find and definitely something to consider. thanks @joth