Thanks Jan,
that is an excellent explanation!
This is the reason why my plain telephone cable works so well for my
network.
Martin
On 11/07/2015 12:33 AM, Jan Kandziora wrote:
Am 06.11.2015 um 23:35 schrieb Colin Law:
What twisted pair does do is to reduce electromagnetic pickup.
Not for onewire. This only works for symetric drive, when both wires are
a data+/data- line separate from ground.
NOISE
Symetric Receiver
1V ___
---> | \
data+ )--------------->-----|- \
| | / \
| 0.2V 1V | \ >------
v ---> | / /
data- )---------------<-----|- /
|___/
ground)-----------------------------------
The symetric receiver measures the voltage drop over a defined load
resistor Let it be 100Ω, then the current for a forward voltage of 0.2V
is 2mA. So without noise, the receiver measures 2mA and knows "Ah, I
have 100Ω load, so the sender has given me 0.2V. Let's show that on my
output."
Because the noise on the line affects both data+ and data- in the same
direction, but the data wires are a current loop, the current isn't
affected. It's still (0.2V+1V-1V)/100Ω == 2mA.
This is how it works for symteric lines as ethernet and many others. And
that is why TP is good for ethernet.
But it is the also the reason it doesn't work this way for onewire.
Because onewire devices don't have symetric transmitters and receivers
but single-ended ones. Not two wires plus ground but only one plus ground.
When you connect the ground line to data-, you have a different current
on the common ground/data- line because the ground line also carries the
supply current. So there's no way the receiver can successfully
eliminate line noise. Instead, it picks up supply current noise. It just
doesn't work.
Kind regards
Jan
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