Am 25.09.2013 22:10, schrieb Klaus Weglehner:
>
> There are about 15 DS18B20 mostly in a star topology.
>
This is near worst-case. You could make it only worse by having some of
these bus-powered. There is no way to have this working reliable.
If you can't help building a "star" because the sensors are laid out
that way, use lobes instead of single wires so the actual topology is a
bus. Length doesn't matter with onewire, bus topology does.
>
> But after a few minutes the Sensors starts disapearing.
>
First check if your kernel has support for the "w1" sensor bus. That is
onewire the kernel way, mostly for Li-battery control chips found in
some laptops. There is also a "ds2490" module which is accessing a USB
onewire host. The kernel and OWFS get in the way of each other when they
both try do access the adapter.
You may disable the "ds2490" kernel module by removing/blacklisting it
and keep all the internal onewire hosts (I2C and bitbanging) working
with the w1 kernel module.
>
> I would not be surprised if there are transmission errors on the 1-wire
> bus. But why can't the server recover from this? As soon as i restart
> the server, everything is fine again for a few minutes.
>
That's because the USB adapter is re-initialised when you start
owserver. Disconnect and reconnect it from usb while owserver is
running. Do the onewire devices reappear? (try scanning "uncached" twice
or three times in a row)
> Can this be a specific problem of the usb hostadapter which may go away
> by changing to an i2c interface? (I've considered this anyway)
> Or is it a pure 1-Wire bus problem?
>
The DS9490 usually makes no problems in my networks - dozens at various
locations.
If you are going for I²C, use two DS2482-800 so you get away from the
star topology. Beware that scanning for chips is very slow with the I²C
adapters, as the host CPU has to control it. The USB adapter has some
algorithm inside to do that independendly of the host CPU.
In general, I²C is only a bit better than bit-banging onewire with most
host boards, as the I²C host is often realized with bit-banging, too.
Some embedded boards may have a I²C shift register, but that still means
one interrupt each millisecond for just one I²C/Onewire byte.
Kind regards
Jan
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