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Monday, 20 May 2013 07:46
The barometer on my weather station is a AAG TAI-8570. I choose this device because it is factory calibrated. Over the last few years it has work flawlessly, however a few months ago I noticed odd problems on my 1-Wire network. Like the lightning sensor picking up just about everything, I was getting hundreds of strikes a day!!! I don't think so, lightning is quite rare in the South East of England. Anyhow I just put it down to the sensor picking up mains spikes. I then added two new sensors a Wet Leaf ( hobby-boards) and Soil temperature sensor. The situation seemed to get worse. So I decided to put a Oscilloscope on the 1-wire network and see what it was up to. The output was terrible. The network is meant to be a digital 5v, what I had was more akin to 3v with spikes up to 4v, not good. I thought this was down to the new sensors, no, wrong, I tested them, all fine. After isolating each sensor I found it to be the solar sensor. Luckily I had a spare and replaced it, bingo fixed. Well not quite I had forgot to test the whole network! Never assume. Next day I got the same problem odd things going on with the 1-Wire network. So again I put the Oscilloscope on the network, this time I got 4v with spikes to 5v, so better, but not good. So back to isolating the sensors. Bingo, the barometer sensor was causing it. I thought it might have been damaged, so got another one from audon.co.uk, great service, it arrived next day. Put it on, this time with the Oscilloscope connected, and got exactly the same as before!! I then decided to look at the circuit diagram for it. This device ideally requires a supplemental 5v supplied on pin 1 of the RJ11 connector, which it does not get. otherwise it uses the parasitic supply.
Both the 1-Wire bus master I have, the serial DS9097U-A and the USB DS9490R-A do not supply +5v on pin 1 of the 1-Wire network connector. So as I have a hobby-boards power injector, all I had to do was modify the connectors on the barometer and connect it inline with the power injector, Bingo... it worked, now I have a 5v 1-Wire network. I do not get any lightning hits on the lightning sensor, which is correct.
I'm quite amazed that AAG do not warn about his issue.. or have I missed something. It seems If you have a very simple setup the barometer works fine, otherwise your in trouble. I'm also not sure how I would have found this problem without an Oscilloscope.
Following are couple pictures I have taken of the trace from the Oscilloscope, the first is of the barometer connected on it's own to the serial bus master DS9097U-A, the second is with the modification to add the extra 5v.
Following are a couple of pictures of my modified TAI-8570