DS18B20 intermittent "85" reading

[premelec]

I have an 18B20 on a fairly long lead on an 08M2 that intermittently reads "85" incorrectly and persistently... what causes this? Thanks [I did a search with no result found]

 

[hippy]

It usually indicates a problem with the wiring or supply to the BS18B20.

The 85 is the reset / default of the DS18B20 so probably indicates that an actual temperature reading has not been undertaken or did not complete, even though the comms side of things is working.

As to why that is the case would requires further information, particularly how it is wired, how you are powering it, what mode is it operating in ( powered or parasitic ) and how long is the 'fairly long' lead ? Is it an actual DS18B20, DS18B20-PAR or something else ?

If it's a voltage or current issue due to length of cable, things might improve by including an electrolytic reservoir capacitor near the DS18B20.

 

[premelec]

Thanks hippy - it's powered but has a longer lead than necessary [coiled up] so I think I'll shorten that - the fact that it's intermittent mostly reading OK is
pretty annoying... also on a 3 AA cell pack not up to 5v... anyhow I'll hunt it down. I thought I'd seen this mentioned before but forum search didn't turn up any *85 ... hippy New year!

 

[westaust55]

Hi Premelec,

You have posted in a past thread on the "85" topic:
http://www.picaxeforum.co.uk/archive/index.php/t-22383.html

and here is another thread about the 85 deg topic:
http://www.picaxeforum.co.uk/archive/index.php/t-20929.html

 

[premelec]

Hi Westy... thanks for that - I thought it sounded familiar - I'm turning 80 shortly and brain not getting any better! :-0

 

[BeanieBots]

I've had similar issues with very long cables. Try reducing the pullup resistor. You can go quite low for very long cables but below a few kOhms will cause self heating, so be aware.

 

[premelec]

Hi BB -did you by any chance try the far end capacitor on power leads fix?

 

[BeanieBots]

Hi BB -did you by any chance try the far end capacitor on power leads fix?

Yes I have and yes it did work.
More specifically, I had a large distribution of DS18B20s all on a common bus. The main controller had several meters of cable going to a junction from which eight spurs went off to individual sensors. Each of the spurs were also several meters long. Initially, all that was returned were zeros. Reducing the cable lengths fixed the problem but the cables needed to be the length they needed to be. Applying stronger pull-ups fixed the problem and fitting a 470uF cap at the junction also fixed the problem. Naturally, I did both.

 

[Hemi345]

The star topology as BB describes is one of the hardest to make work. Take a look at this Maxim document on making 1-wire networks reliable:

https://www.maximintegrated.com/en/app-notes/index.mvp/id/148#

It was invaluable when a colleague and I designed an envirommental monitoring system for our datacenter using 1-wire devices.

 

[westaust55]

On the topic of pull-up resistors for 1-wire networks, worth looking at my tutorial and specially section 3.2.
Can be found at post 43 here:
http://www.picaxeforum.co.uk/showthread.php?15306-One-Wire-Devices-Networks/page5

For DS18B20 reducing the pull-up down to 2k or 2.2 kOhm is warranted.