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Thread: A bit of 1-wire wire help please.

  1. #1
    Automated Home Jr Member
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    Default A bit of 1-wire wire help please.

    I will be wiring the house with many 1-wire points for monitoring various sensors. (Mainly heat) I hope to do this with using rj45 outlets with a plug-in lead to each of the sensors.
    Am i ok to just come from each rj45 outlet with a 1 or 2 meter lead & just using 3 cores of cable to monitor the sensor? Or would i be better making the leads up as a 6 core lead so i can still keep the sensor next to the main run of cable.
    Naturally if i used the 2nd method i would make up a bridge for any rj45 outlets if no sensors where used.

    Thanks

    Chris.

  2. #2
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    From what I read of one-wire you only need 2 cores, one for data and one as ground. I also think it works on a bus and not star topology.

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    Moderator toscal's Avatar
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    You actually need 3 wires one for ground, one for data and one for power. 4 core is normally used just to provide a return for the ground.
    More information here http://www.maxim-ic.com/products/1-wire/
    If you ask them nicely you can often get a sample of one or two temp sensors or similar, just click on the sample link on the left hand side.
    Another link is http://www.audon.co.uk/1-wire_index.html

    Also on the audon site http://www.audon.co.uk/1-wire_weather.html almost near the bottom is an RJ45 connection diagram. This may help you.
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    Quote Originally Posted by toscal View Post
    You actually need 3 wires one for ground, one for data and one for power. 4 core is normally used just to provide a return for the ground.
    More information here http://www.maxim-ic.com/products/1-wire/
    If you ask them nicely you can often get a sample of one or two temp sensors or similar, just click on the sample link on the left hand side.
    Another link is http://www.audon.co.uk/1-wire_index.html

    Also on the audon site http://www.audon.co.uk/1-wire_weather.html almost near the bottom is an RJ45 connection diagram. This may help you.
    No , they only use 2, the data and power use the same wire, data is sent by pulling down the voltage on that wire.

  5. #5
    Moderator toscal's Avatar
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    Your not quite correct there. As it all depends on what mode you want to run the sensor in. If you have only one sensor then parasitic mode will indeed use only the ground and data line, but you have to ground the power line (Vdd). If you are running them on a bus topology, so using more than one sensor it is recommended to use an external supply connected to the Vdd pin. Maxim also recommend the use of an external supply. When using parasitic power data transmission and timing become much more of an issue as does high temperatures if the sensor is a temperature one.
    So that is why I said three wires.
    Have a look here
    http://datasheets.maxim-ic.com/en/ds/DS1822.pdf
    Last edited by toscal; 20th November 2009 at 01:03 PM.
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    Automated Home Legend TimH's Avatar
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    Quote Originally Posted by toscal View Post
    Your not quite correct there. As it all depends on what mode you want to run the sensor in. If you have only one sensor then parasitic mode will indeed use only the ground and data line, but you have to ground the power line (Vdd). If you are running them on a bus topology, so using more than one sensor it is recommended to use an external supply connected to the Vdd pin. Maxim also recommend the use of an external supply. When using parasitic power data transmission and timing become much more of an issue as does high temperatures.
    So that is why I said three wires.
    Have a look here
    http://datasheets.maxim-ic.com/en/ds/DS1822.pdf
    Yup, two or three wires for "1-wire"
    IIRC it's around 80degC temps that the 3rd (power) wire is needed.

    I'm still figuring out the best way to wire mine up. There were some commerial enclosures with either RJ11 or RJ45 sockets to allow bus loop-through, but when the sensors are really tiny, a large matchbox-sized box look sreally out of place

    Cheers,

    Tim.

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    Automated Home Sr Member Quinten's Avatar
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    Just rehouse them?

    Mine look like these:







    More here

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    Thanks guys, but the room temp i should be able to sort in a similar way to Quinten has done. But there will be about 8 points that may have temperatures up to 100c & wont be able to run the cat 5 near this. This is where i wanted to make up some 2 meter fly leads with silicon heat proof cable with a DS1822 on the end. But wasn't sure if i should run 3 core only down the lead or 3 in & 3 out from the DS1822.
    Last edited by Collectors; 20th November 2009 at 06:40 PM.

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    Moderator toscal's Avatar
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    Always run more wires than you need, you never know when you will need the extra wires.
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  10. #10
    Automated Home Jr Member Simon300's Avatar
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    Quote Originally Posted by Collectors View Post
    Thanks guys, but the room temp i should be able to sort in a similar way to Quinten has done. But there will be about 8 points that may have temperatures up to 100c & wont be able to run the cat 5 near this. This is where i wanted to make up some 2 meter fly leads with silicon heat proof cable with a DS1822 on the end. But wasn't sure if i should run 3 core only down the lead or 3 in & 3 out from the DS1822.
    Sorry to dig-up an old thread but this is exactly the same question as I have now...
    I am using 1-wire for CH zone control so am looking to put temp sensors (DS18B20) in several rooms. An initial thought is to drop the sensor plus cable down an existing conduit (to get them to a more representative height in the room) and join it to the 1-wire bus above the ceiling.

    Looking at Maxim's Reliable Long 1-wire Networks doc (http://pdfserv.maxim-ic.com/en/an/AN148.pdf) they call these drops to the sensors "stubs". They call the network a "linear topology" (page 2) if the stubs are shorter than 3m or a "stubbed topology" (page 3) if longer.

    It would appear that the linear topology will give the most reliability, and possibly the 3m value is fairly arbitrary.

    The standard 1-wire RJ45 wiring seems to be:
    1=GND (power)
    2=Vdd 5V
    3=? GND or not connected?
    4=DQ
    5=GND (data)
    6=not connected
    7=unregulated 12-24V
    8=GND unreg

    So, like Collector, I was also thinking should I drop the DQ and GND down to the sensor and then "back up" again - in other words, passing the network via the sensor so that there is no stub?

    The advantage of this is that you reduce the number of stubs. The disadvantage is that you increase the length of the network ("radius" in Dallas/Maxim terminology) by, say 3-4m, per drop.

    Assuming I only want to drop one, not two, CAT5 cables to the sensor (I'm using the Clipsal pink cable which isn't too big) is this a good idea? If so, what pins/cores do people use? You'd want DQ/GND on another pair so "incoming" could be 4+5 and "outgoing" could be 3+6? Then the ceiling/void bus connector would have 3 RJ45s with 2 "bus" sockets and one "sensor" socket. All would have 2, 7 & 8 connected. "Bus" A would have 4 connected to "sensor" 4 and 5 connected to "sensor" 5. "Bus" B would have 4 connected to "sensor" 6 and 5 connected to "sensor" 3. The sensor end, or a bridge made from an RJ45 plug if no sensor fitted, would connect 3 to 5 (GND) and 4 to 6 (DQ).

    Of course if you put sensors in the ceiling, like Quinten has, this is a non-issue . However there may still be situations where you really want to branch off the 1-wire network only running one cable where this technique would be better.

    By the way, where did you get your little PCBs from Quinten?


    Simon

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