Am 29.09.2013 21:46, schrieb Markus Gaugusch:
> 
> I've now successfully used simultaneous command, but my bus has one 
> iButton DS18S20 (which does not have separate power pin)
>
Which package? Both the TO-92 and SO-8 packages have a Vdd power pin.


> while the other 
> ones are 3-pin TO-92 with separate power supply. Now, I sometimes get read 
> errors (85 degrees)
> 
In contrary to common rumors, 85°C isn't a kind of read error but the
power-on value. See the DS18S20 datasheet, Figure 7 (page 6).

Before doing a simultaneous conversion, owfs checks whether there is at
least one temperature sensor on the bus which is not powered. That's the
reason you have to tie Vdd to GND when using parasite power - if it's
floating, the detection may fail. If at least one temperature senosr
isn't powered, owfs automatically inserts a one-second delay after
placing the "Convert T" command. This is neccessary to avoid de-powering
the chips on the bus.

When you read 85°C after doing the initial simultaneous/temperature,
there can be two reasons:

Either you are reading the chip before the initial conversion after
power-up is completed. That shouldn't happen on a bus with a parasite
powered temperature sensor because of the delay. But it can happen on
any other bus.

Or, when you get it right in-between, it means your bus line has been
too long too low regardless of the delay and the chip with
power-on-value has done a reset when the power came back. This should
only happen when there are too many parasite-powered chips on the bus.


> I don't want to change my scripts to read one sensor apart from the 
> others. Or is there any other solution? While writing this, I thought I 
> could just re-read from uncached in case of 85 ... would that work?
> 
I'd say, fix that power problem and you get rid of the occasional reset.

Sure you can always retry sampling.

Kind regards

        Jan


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