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List:       owfs-developers
Subject:    Re: [Owfs-developers] DS2482-800 long wire problem
From:       Jan Kandziora <jjj () gmx ! de>
Date:       2015-01-05 1:14:25
Message-ID: 54A9E571.9050606 () gmx ! de
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Am 04.01.2015 um 17:33 schrieb Jan Sennesael:
> @Jan: in the document linked by Paul
> (http://www.maximintegrated.com/en/app-notes/index.mvp/id/148) they start by
> saying "The scope of this document is limited to 1-Wire networks that use
> Category 5, twisted-pair copper wire and have 5V bus power supplied by the
> master" 
>
I think this is the only place where they mention Cat5e cables.

Anywhere else they talk about limiting bus capacitance, and using a
twisted pair for 1W/GND is sure the worst thing you can do when your
goal is limiting capacitance.


> So i guessed that this was the preferred cable... (in addition, hobbyboards
> also talk about this type of cable and connectors)
>
Well, it's a poor choice. You don't gain any noise immunity by using a
twisted pair for 1W/GND, because the pair is driven unbalanced. But you
gain a lot of additional capacity, which is very bad when your bus is
open-drain instead of push-pull. Like onewire is. So don't do that.


> Normally when I crimp the rj45 connector, normaaly it is secured with the
> piece of plastic.
>
The plastic notch of a standard RJ45 plug is designed to cut into the
soft, flat mantle of a 8-wire phone cable. Nothing else will fit, even
6- or 4-wire cable mantles will not connect tightly. There even exist
variants of the RJ11/RJ12 and RJ45 connectors which have a smaller cable
opening, just for the purpose of having tight fit for 6- or 4-wire
cables. So this is crucial for the function of the connector.

For cables with round mantles, the mantle has to be fixed another way,
e.g. by melting it into additional plastic.


> But it's a great tip if i run into some errors.
> I am getting some improvement by using 7/8 as gnd (i can succesfully connect
> a sensor at the end of the long wire, but not yet the actual sensors (they
> have their own cat5e cable of 8m). So still trying ;-)
>
Another tip: Use as many ground wires as possible. Long cables produce a
ground-lift (especially when far devices are bus-powered), and keeping
the ground level on the far end as seen by the host below 0.7V is also
crucial for the bus function.

Kind regards

	Jan


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