Am 01.06.2016 um 22:51 schrieb Loren Amelang:
>> Date: Tue, 31 May 2016 23:04:52 +0200 From: Jan Kandziora
>> <j...@gmx.de>
>
>> That's okay, but keep in mind for later installations twisted pair
>> is BAD for onewire. Twisted pair is only good if the drive is
>> symetrical. For asymetrical drive as with onewire, it only adds
>> cable length and capacity.
>
> Wouldn't parasitic power onewire be symmetrical?
>
Colin Law asked the same in
<CAL=0glsp6xab4p-1vsyso-kehbwc9-fkxgr29hyaiaodudy...@mail.gmail.com>.
I drew a circuit with noise sources in response.
----------------------------------------------------------------------
noise
------>
__ +---------+
1.5k pullup / \ | |
+-----/\/\--------(----------------+--+ |
| | \__/ | |
/|\ | / |
| | | |5v ------> \ load |
\|/ | __ / |
| v / \ | |
+---------+-------(----------------+--+ |
| \__/ | |
| === ===
| | |
+------------------------+---------+
The cable capacitance on the ground wire directly shortens the noise
source while the cable capacitance on the DQ wire only shortens the
noise source through the pullup resistor.
So for the same influx, the noise voltage on the ground line is much
smaller than on the DQ line.
[...]
The noise has a certain power which flows into these voltage sources. So
it's either high voltage at low current or high current at low voltage.
If and how much ground capacitance is relevant depends on the frequency
of the noise. R_c = 1/(2*pi*f*C), calculate for yourself. Given f=1MHz,
C=1nF, it's 160 Ohms. That's smaller than the 1.5kOhms of the pullup
resistor. The load resistance of the open circuit of a slave
(master,too) is very high for onewire, megaohms, so it doesn't matter
for the above calculation.
----------------------------------------------------------------------
So, no. Parasite powered Onewire isn't equivalent to symetrical.
> Seems to work for me...
>
Your cables may work, though. That depends on the R', C', L' properties.
> I have probably 150 feet of CAT5 chained around my house. Sometimes
> out on one pair and back on another to continue the chain. At the
> far end of the chain, a 1N5711 Schottky diode and a 1.5KE20A-T TVS
> reduced errors a bit. What really cut my errors to zero was adding a
> 0.02 uF capacitor across the start of the chain.
>
Uh, 20nF is really MUCH.
> It drastically
> slows the signal rise times, but doesn't seem to hurt readings. It
> completely stopped the seemingly random storms of errors that would
> appear for a few hours and then go away for days. Military radar?
> Spooks?
>
Mobile phone is the most likely culprit.
>
> But that is with an ancient dedicated microprocessor at 5V - not
> with owfs. It reads and sets up a different single DS18B20 of the
> ten connected, every ten seconds. Often goes for months without a
> single missed reading now.
>
Ah! That way you can extend the expected rise times easily. The
bit-to-bit timing is not fixed in Onewire slaves, it's only the host
adapter chips which put an upper limit on this.
Kind regards
Jan
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