# 1-wire : 4.7k resistor not working / how to compute optimal resistor topology for star layout

I've build a 1-wire network using a star topology (networking done trough my home's ethernet cables).

At the begining I didn't own a 4.7k resistor but some 1.2k resistor so I lined them to achieve 1.2k*3= 3.6k "virtual resistor".

This first version of my bus was working fine.

Today I tried to replace those 3 resistor by a single 4.7k and the bus stopped working.

==> Why ? :)

Following maxim's guide I think I understand that, with a star topology, one should rather use a switched network or a Distributed Impedance Matching.

I'm more keen to try this second option but how can I compute the optimal resistor to add to each branch given their length ?

Star topology metrics are : 4 branches diverging at the master

• 1st branch = 18.5m
• 2nd branch = 26.5m
• 3rd branch = 25m
• 4th branch = "1cm" (direct connection to my 3.6k resistor)

Thus the radius of the longest branch is ~27m and the radius of the shortest is 0cm and the total weigth is 71m

• how long are the cables? Did you try to use a smaller resistor back again? – Wesley Lee Nov 24 '15 at 8:35
• Are you able to look at the signal levels with an oscilloscope? – Icy Nov 24 '15 at 9:20
• @WesleyLee I don't know yet. I'll borrow a device on thursday to get the real value. Yet I'd bet on 15m + 20m + 25m + 0cm ;) – Cerber Nov 24 '15 at 9:47
• @Icy no, I don't own one, nor my friends. Yet I hope that my friend's device will show me resistive values of the cables – Cerber Nov 24 '15 at 9:48
• @Icy I've updated the question with the length metrics but, unfortunately, my friends device didin't show the resistive values (but I think it uses it to measure the length of the cable. Settings were on 49 PF.m) – Cerber Dec 4 '15 at 10:33