A few weeks ago I reported a problem with some DS18B20 sensors, then
found it was my mistake. I have just looked at all this again and it is
not that simple!
Having tidied my desk I found I had a lot of DS18B20 sensors and I have
been testing them -
11 work as expected
21 work fine when powered but do not work in parasitic mode. These were
bought from at least 2 different suppliers, but all have code 1827C4
+051AG marked on the body.
Test results are at the end if anyone is interested.
In summary the 'new' DS18B20 sensors that I have not only do not support
parasitic mode, but they actually pull the whole network down when
connected that way. I looked on the Maxim site and didn't see any
reference to problems like this, so I contacted their support. They
asked for the id's and body codes of some problem sensors, which they
then checked and confirmed that these are actually counterfeit. They
can do nothing about it as I did not purchase they from an authorised
supplier.
It amazes me that anyone would bother to counterfeit a device which
sells for just over £1.00, but it seems that they do!
I have never tested the accuracy of sensors, just assumed that they
would be sufficiently accurate for my needs, but I now I think I will
try to test that as well. I also wonder if these devices my contribute
to other problems, no reads, 85 reads, etc.
I have just ordered a bunch of sensors from Farnell, one of their
authorised suppliers in the UK. It will probably take me a while but I
will replace all of my temperature sensors and see if I notice any
difference. I will also keep records of body markings and id's so I
have traceability.
Results ---
Testing with Sheepwalk RPi2 adapter, good (old) DS18B20
Connect all 3 pins (powered) Vdd = 5.01V, DQ = 4.53V, working OK,
displays Power = Yes
Connect Vdd to Gnd DQ = 4.53V, working OK, displays Power = No
Testing with Sheepwalk RPi2 adapter, suspect (new) DS18B20
Connect all 3 pins (powered) Vdd = 5.01V, DQ = 4.53V, working OK,
displays Power = Yes
Connect Vdd to Gnd DQ = 0.73V, Not working & no other sensors working,
also simultaneous & alarm are not there
Testing with DS9490 USB adapter, good (old) DS18B20
Connect all 3 pins Vdd =335mV, DQ = 5.04V, Working but 85 on temp 9 &
10,displays power = No
Connect Vdd to Gnd DQ = 5.04V, Working OK, displays power = No
Testing with DS9490 USB adapter, suspect (new) DS18B20
Connect all 3 pins Vdd = 4.32V, DQ = 4.99V, not visible on network but
other sensors still work
Connect Vdd to Gnd DQ = 20mV, Not working & no other sensors working,
also simultaneous & alarm are not there
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