Am 18.03.2014 21:12, schrieb Jan Sennesael:
>
> Sadly it seems that the DS2409 is no longer available,
>
It's discontinued, so usual distributors don't list it anymore. You have
to ask around who has still stock. I bet there are millions somewhere
out there waiting to be used. Don't know where at the moment however.
That's a problem of my parts buyer.
> so i have to make it
> more like a *bus*. But by doing this (i think) i will lose the "easy"
> connection though the rj45 hub ports: every port needs to be connected,
> otherwhise the signal ends.
>
I suggest to use the 4-5 pair as forward direction and the 3-6 pair as
return. That way you can still use single RJ45 ports and cables and it's
compatible with the usual connector scheme.
1W-------. .--------------. .-------
GND-----. | | .----------. | | .----- inside each "hub"
| | | | | | | |
============== ==============
5| |4 6| |3 5| |4 6| |3 RJ45 connectors
============== ==============
| | | | | | | |
| | | | | | | |
| | | | | | | |
~ ~ ~ ~ ~ ~ ~ ~ <---- very long lobes
| | | | | | | |
| | | | | | | |
| | | | | | | |
| '-+--' | | '-+--' |
'---)+---' '---)+---'
|| || <----- Short tap (<1m)
CHIPS CHIPS
That way it's a bus electrically while you can still have a mechanical
tree layout. The only disadvantage is you have to connect a dummy lobe
to unused ports. (Or use jumpers inside the "hub")
> Is there any known number of DS18.20 (or length of cat5e cable) that will
> work without any problems in a tree like network?
>
There isn't a limit. Trees are bad from the beginning. It may work or
may not. It may stop working from one day to another. It's just not
designed to work in a tree topology.
Read this on the topic:
http://www.maximintegrated.com/app-notes/index.mvp/id/148
A note on parasite and host-powered devices:
When you are suppling a large network with parasite power or even power
on separate lines supplied from the host side, think about the ground
lift it creates! When you have a lot of sensors powered through the bus,
there are some mA for each sensor flowing through the GND line back to
the host adaptor. This may create a ground lift of some 100mV there,
making the communication to devices on the far end less reliable.
(communications in Onewire is relying on a common ground level.)
It depends on the wire gauge and the number of connectors how much the
ground is lifted. To avoid problems, it's best to have THICK ground
lines and/or local power. (Though the latter may create other problems.)
> I have "a feeling" that the counter modules generate the most problems, is
> this true?
>
I haven't had problems when I've used the DS2423 chips until a while ago.
Kind regards
Jan
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