Am 08.10.20 um 00:31 schrieb Mick Sulley:
> On my test system I have 20 DS18B20 temperature sensors (all powered) to
> test out various things. I am setting /simultaneous/temperature waiting
> 2 seconds then reading latesttemp from all of them.
>
> I have just noticed that several are giving 85 reads. If I look with
> owhttpd this is what I see -
>
> owhttpd screen
>
> Also owread /uncached/DHW_Mid_Btm/latesttemp returns 85. This is after
> many cycles of simultaneous/voltage and latesttemp reads.
>
Reading /uncached/<id>/latesttemp reads the contents of the temperature
register from the chip.
It never triggers a temperature conversion.
If you read 85°C from latesttemp, it means the latest conversion either
85°C (unlikely) or the chip has been power-cycled since the latest
conversion (likely). As 85°C is the power-on value of that register.
>
> It seems to me that it is a useful safeguard to run a single
> temperature12 read on each sensor during the initialisation of the
> system. I cannot see any downside to this but just thought I would ask
> for your thoughts.
>
That doesn't make any sense as you will read back the value from that
initial conversion again and again if your simultaneous trigger isn't
working. You mask the error that way, which is likely not what you want.
If a sensor reports a spurious 85°C, treat that as a power-cycle on that
sensor – check your hardware!
Kind regards
Jan
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