1-wire resistor choice and wire size Topic is solved

[n1c0l45]

Hi

I plan to replace most of my temp RF sensors with wired ones. I would like to do this bus


The orange part is optional... i would like to do it (freezer & fridge sensors) but they may get things too complcated?

So the bus has a lenght of 14,1m and a weight of 22.9m (optional version) or 18.9m (short version). I also plan to use 3 wires (not a parasite setup)
My three questions:
1- What wire size should be best for this? (I can get nearly all type of cable for free at work, so if there is something better than network cable...)
2- Is there formula or else to calculate the good size of the resistors (& capactiors?) and where it should be best to put them?
3- Is there a test that i can do with an oscilloscope to test my setup to be sure to get things working nicely?

thank you guys for your help and sorry for my english

Nicolas

[n1c0l45]

No ideas?

[Marci]

Dunno about electronically correct but I've done the above on cat5 with 4.7k resistors... all works fine.

[n1c0l45]

Thanks for your answer!
How many sensors have you got? And what length?
Thanks

[cul8r]

I have 9 sensors, length appr.15 m and weigth appr. 35 m with 4.7 kOhm pull-up and in parasitic mode with no problems. (Using Raspberry GPIO and kernel support). I have a second installation close to a compressor and in that case it is only 4 m length and 2 sensor, was force to go to 1.2 kOhm pull-up.

Maxim recommends max length/weigth 200 m with pull-up of 1 kOhm, also if you have many stubs (branches) they could cause problem due to impedance mismatch and solution is to have 100-150 Ohm in serie where the data line where the branch begins.

For cables in the range 0-30 m it is not critical, but using a CAT5 cable is to be in the very safe side. I am using a mixture of of telephone cables (0.15 mm2) and even using 2 spare pair of the alarm installation! (20 pairs, 0.2 mm2 cable).

There is a very good application note at Maxim:
https://www.maximintegrated.com/en/app- ... mvp/id/148

/C

[n1c0l45]

Thanks for the answer!
I have already red the maxim note but there is no informations on resistors values

[n1c0l45]

I have 9 sensors, length appr.15 m and weigth appr. 35 m with 4.7 kOhm pull-up and in parasitic mode with no problems. (Using Raspberry GPIO and kernel support). I have a second installation close to a compressor and in that case it is only 4 m length and 2 sensor, was force to go to 1.2 kOhm pull-up.

Maxim recommends max length/weigth 200 m with pull-up of 1 kOhm, also if you have many stubs (branches) they could cause problem due to impedance mismatch and solution is to have 100-150 Ohm in serie where the data line where the branch begins.

For cables in the range 0-30 m it is not critical, but using a CAT5 cable is to be in the very safe side. I am using a mixture of of telephone cables (0.15 mm2) and even using 2 spare pair of the alarm installation! (20 pairs, 0.2 mm2 cable).

There is a very good application note at Maxim:
https://www.maximintegrated.com/en/app- ... mvp/id/148

/C

Do you have just one 4.7k resistor?
So if I understand well, I use a 4.7k between 3.3v and data line near the gpio, and a 100ohm resistors on the data line of each branch?

[cul8r]

Yes, only one 4.7 kOhm from 3.3 to GPIO/dataline at the Rasberry connector.

The 100 Ohm is only if you have problems with impedance mismatch, and in your case should not be a problem with that. With your topography but with much longer cables and having problem, I would try 100 Ohm at the start of the "3 m" branch.

The application note states that a FET driver and 1 kOhm pull-up is good for 200 m length/weigth, the 4.7 kOhm value is a standard value recommended in Raspberry 1-wire applications.

[cul8r]

By the way, this is the board I made for my application!

/C

[n1c0l45]

Thank you for all of that!