onewire-Bus over a wire 75feet long - what can I do to improve signal-quality

StefanL38

Hi,

after a long time of abstinence I'm back on prop-coding.
I want to install a temperature control for room heating.

The sensors work in VDD-supply-mode ground Data and Vdd connected.

For sensing the temperature in the rooms I want to use onewire-sensors DS18S20 (9bit resolution) or DS18B20 (12bit resolution)
In my pretests on the workbench I connected some 18S20 and some 18B20 together and all worked well.
As long as I connect the sensors through short wires aprox 3feet long all works well.
In a test with a single DS18S20 over a wire 75feet long worked to.

But as I added a seconds one I got strange results.
instead of two sensors six sensors were scanned. two times "three" sensors with the same Sensor-ID.

Is this some kind of signal reflection on the long wire?

How can I improve signal-quality? Or is a cable 75feet long just too long for the specifications of a onewire-bus?

Any kinds of hints and tips are very much appreciated.

best regards

Stefan

Edit: by the way where have all the options about receive email or message if somebody answered gone?

Erlend

As the cable gets longer, the resonance frequency gets lower / time constants gets larger. At some point the frequencies will be close to the signalling frequencies/time constants, and you risk interference. One mitigation is to kill the antenna/resonator behaviour of the cable. Such as fitting an end-of-line terminator with a characteristic impedance similar to the cable. I don't know if that will work here - you need to find a combination that represents that impedance without killing your signal. Wild guess: 200ohm in series with 100pF. Along the cable the DS'es will also cause reflection point in the cable. This effect can be reduced by connecting them through a series resistor - again I guess 200ohm is close. Lastly, if the rising edges of the signal are to sharp - too high slevrate - it may be wise to soften them by means of an RC network. I have no idea what values to use.

A last minute idea: try to leave a long coil of cable at the end of the line - at least 100m.

Erlend

StefanL38

Hi Erlend,

thank you very much for your suggestions. At the moment I'm busy with redrawing the DXF-File of the
MeArm V1.0 robotarm. I want to mill them out of plywood to bring it tomorrow to my students at school.
If I have finished this and if I have mounted the ventilationducts in bathroom and toilet
I will test your suggestions.

best regards

Stefan

P.S. lol cool bottomline :-))

Mike Green

Maxim/Dallas used to have some application notes on large 1-wire networks since these devices are used in control systems for buildings. These included suggestions for drivers, termination, etc. They also have special driver ICs for large / long networks. Check out the 1-wire section of their website.

StefanL38

Hi Mike,

this is a good idea. Thank you.
Will do that.

best regards

Stefan