1-wire more than 20 sensors

[RSin]

I've read you can only have 20 1-wire sensors. Can you have more than 20 temp sensors on 1-wire in total? Is it just a case of 20 per extension so buy another extension? Can't find it written anywhere.

Thanks.

[Duncan]

The 20 limit is only loxones limit because they suggest you star wire using the same cat7 that you use for light switches. This is bad advice. THe correct way to wire 1-wire is to use a single separate linear strand of shielded cat5/6/7 and then you can get very long chains with loads of sensors to work reliably, potentially 100s of sensors over 100s of metres. I've got 35 temp sensors and 2 RFID sensors working 100% reliable with zero CRC errors.

[Deac99]

I had a problem with 29 sensors and ended up buying a second extension - all are working fine now (3 weeks anyhow).

@ Duncan  I would agree probably not a good idea to share the cables with lighting etc.  I am using double twisted pair shielded (4X0.8) cable for my 1 wire sensors.  I have long chains of sensors daisy chained together but I do have branches with additional linear chains so I suppose it is a star topography.  I was surprised to see you are getting away with 35 sensors.  Are all yours on one long linear chain?  How long?  Of course my longest run is probably 50 meters and each branch is 10 - 30 meters (it's a big building) so maybe that is why I hit the maximum so quickly.

Also, do you have an opinion of the optimal wiring on twisted pairs?  I have 2 twisted pairs, say blue/blue white and yellow/yellow white.  I connected the signal to the yellow, the -5V to the Blue and the +5V to the 2 whites twisted together.  Do you think it makes any difference how to connect to the twisted pair?

[Duncan]

yes mine are all in a single long chain a per specification wriing by the 1-wire chip designer.

1-wire can manage upto 500m in a linear chain, but each snub or branch or connector adds a larger notional value to the chain so it can rapidly add up if you use any kind

https://www.maximintegrated.com/en/app-notes/index.mvp/id/148 gives you all the details

1-wire isnt a differential bus - there is a 0v, 5v, and signal, so twisted pair choice doesnt matter, however i think using screened rj45 with the screen connected to the 0v does make a difference for a larger install.

[Deac99]

Very interesting - thanks for sharing that link.  My topography is essentially like the switched network example with a long stub on a couple of the runs.  Out of curiosity and for future reference do you think the output relays of a miniserver would be sufficient for the "Low Impedance Single Supply Analog Switches"?  That would be an elegant solution for my future expansion to simply program the miniserver to cycle through the various branches.

[Duncan]

i think that solution is too complicated and you would get lots of errors - if a temperature sensor is switched out of circuit at the time of reading you might get a 0degree for instance and this would fire off your heating when not required.

a single 1-wire extension with correct wiring should be able to service all your temperature sensor requirements but it has limitations as to the range of devices supported.

if you want a solution with multiple 1-wire branches, wider device support and cheaper cost, a pokeys57e would be a better solution, search the forums for info, but a £70 device can do 99 1-wire buses, with a wider range of device support including non-1-wire devices such as dht22 humidity sensors, and also digital inputs i.e. switches, if using wired ethernet and UDP

[Mark Deacon]

The Pokeys57E looks like a really interesting device.  Funny it seems as if I'm coming full circle as I started out with monitoring about 15 DS18B20 sensors using a raspberry pi but didn't want to control them via the Pi  because I didn't want a "Homemade" solution.  I'm now controlling temperatures for both frost protection when I'm not in my home (for 6 months over the winter in the mountains of Austria) as well as for comfort during the rest of the time so I opted for the Loxone 1 wire extensions thinking they would be optimal.   

I read through the Pokeys 57E manual and have another question - my interpretation in the manual is that the 57E only has one 1-wire bus on pin 55 and can handle up to 10 DS18B20 devices.  You mentioned it can have up to 99 buses - I'm not seeing the documentation on that?  I may buy one of those this winter just to experiment with - it looks way more advanced and easier than the Pi's.

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[Duncan]

later firmware can enable 1-wire on any of the input lanes, not just the pin55 when the manual was written.

it works very well, ive bought one to test it with loxone - and as it can also work with other sensors and provide extra digital inputs , it seems a bit of a no-brainer really.

im not using mine because my loxone 1-wire is reliable as is, and the only place i need a few extra inputs is for a the garage doors, so i use a espeasy wifi board which provides 11 digital inputs for $4, which again a bought just to try really.

[Mark Deacon]

Thanks for that link - I can see where a standard would be in order if you were connecting 1-wire boards using RJ45.  So far I'm only using the waterproof temperatures themselves - no boards involved but I've been reading up on the sensors that are attached to boards, may make sense for lightning / surge protection.  I'm on top of a small mountain and have lots of problems with surges.  This past summer a lightning storm took out my miniserver for the 3rd time - I sent it back to loxone for diagnosis and this time is was not through the 24V which is heavily protected, but it came through the ethernet connection and blew out the ethernet chip - uggh.  Any of you have experience protecting from DSL / ethernet surges?  (yes it took out my DSL modem too.)

On Sun, Oct 29, 2017 at 8:54 PM, Andrew B <andrew.e....@gmail.com> wrote:

http://www.opencircuits.com/images/a/a3/A_Guide_to_the_1WRJ45_Standard.pdf

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