On Monday, 30 December 2019 16:17:18 GMT Graeme Gemmill wrote:
> 1. All my sensors are mounted on small circuit boards bought from (now
> defunct) American company Hobby Boards. I believe the temperature
> sensors are powered parasitically, though there is a 5v power injector
> to feed a humidity meter.
>
> 2. They are connected to the Pi by a DS2490 USB-RG45 bridge, and the
> software that drives them is based on the Dallas 1-wire software
> implemented by a third-party, which has been in use for 10 years.
This adds another device to the system and therefore another possible point of
failure. I presume that there is a Hobby circuit board for each of the
sensors, which in turn, connect to the DS2490 bridge at the 1-Wire end and the
data is transmitted to the Pi over USB?
> 3. The second net is completely independent though same set-up, feeding
> its data directly to a postgresql database.
Presumably via a similar setup to the Temperature sensors?
> 4. I see no reason for the values of electrical signals to have changed
> unless something has happened to the Pi.
In this case, unless I've completely misunderstood the function of the DS2490
bridge, the Pi only has to accept the DS2490 as a USB device; in other words,
all the voltages and clock processing for the sensors is performed by the
DS2490 chip inside the bridge. It is therefore entirely possible that a
component in the bridge has failed (or drifted out of spec) and that is
causing your problem. If the USB Port on the Pi works OK when you plug a
memory stick into it then the Pi is almost certainly OK.
A copy of the Data Sheet for the chip inside the bridge may be found at
https://pdfserv.maximintegrated.com/en/ds/DS2490.pdf.
> Anyway, this is all a bit moot, since I managed to break one of the
> DS1820 leads when examining the board!
;-(
> So I'll have to wait for a replacement to arrive before investigating
> further.
Take care with this in case the fault is taking out sensors. It might be
worth also getting a new DS2490 USB-RG45 bridge (although, I notice that they
are quite expensive).
Alternatively, if you have any breadboarding kit, try the circuit that I
originally posted at:
http://www.reuk.co.uk/wordpress/raspberry-pi/connect-multiple-temperature-sensors-with-raspberry-pi/
That will isolate the sensors because you won't be using the bridge.
--
Terry Coles
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