error reading multiple ds18b20 sensor in start network

Hi guys,

I have problems with reading of my ds18b20 network.
The setting is as in attached image. I’ve got a good reading when i connect one star, but a wrong reading when i connect both stars. I use the 4.7k transistor and the combined length of the cables is around 22m and the most distant device is 3m.

Somebody an idea what goes wrong?

Thank you in advance.

arduino ds18b20Knipsel.JPG

. I use the 4.7k transistor

The what?

There is a maxim application note here which descibes the problems (and some solutions) for larger networks. The longer the cabling that is connected increases the difficulty of communications.

AWOL:
The what?

Sorry, resistor.

rw950431:
There is a maxim application note here which descibes the problems (and some solutions) for larger networks. The longer the cabling that is connected increases the difficulty of communications.

Thanks, i see the star configuration is the least reliable.
I’ll try changing the cables.

I changed the cable connection to the image “arduino ds18b20 network” as attached.
At first i didn’t get a reading of the sensors, but when i changed the resistor from 4,7k to 2,2k, i did get a reading but not that stable. Sometimes 8, sometimes 6, sometimes 10 sensors readings.

I use 4 x 0,8 mm² cable (only use 3 wires) and connect the sensors with crown stones (don’t know if this is right name, we call them ‘kroonsteen’ in dutch). See attachement “ds18b20”.

Somebody advice in how to stabilize the connection?

The fact you are getting unstable readings and different numbers of sensors working, my first test would be to lower the data rate I was reading the sensors at. I would also consider using shielded cable for the connections. No idea what "crown stones" are. The issue of long cabling is going to be one that ties right into the data rate - often a small capacitor across the data lines to slow the rise and fall times can significantly improve the reliability. Also make sure each sensor has a bypass cap across the power and ground at the sensor.