SDI-12 Soil Moisture Probe Help



  • @robert-hh This could be an issue, only having 12v and 3.3v sources, I thought the RS-485 breakout could do 5v to 3.3v step down but not the other way. I may have to build a 12v to 5v step down circuit.



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  • @robert-hh said in SDI-12 Soil Moisture Probe Help:

    @jimpower connect the 1.8V source to A and the moisture sensor to B. RS485 is a current loop signal, so swapping the cables inverts the polarity.

    This is how they are connected in the current state. A goes to 1.8V (which isn't a source just a Vref not sure if this should be 3.3v however) and B goes to the probe.



  • @jimpower connect the 1.8V source to A and the moisture sensor to B. RS485 is a differential signal, so swapping the cables inverts the polarity.



  • @robert-hh said in SDI-12 Soil Moisture Probe Help:

    @jimpower The nopte about swapping A & B was with the RS485 breakout board, if the quiescent level at the xxPy RX is 0, when it should be 3.3V.

    Could you help me to understand how i should do this?



  • @jimpower The nopte about swapping A & B was with the RS485 breakout board, if the quiescent level at the xxPy RX is 0, when it should be 3.3V.



  • @robert-hh "And some ground lines seem to be missing in your pictures." If you're referring to the A - 1.8V this is purely used as a reference from battery charge controller, I could run this back to the LoPy vref I guess. I didn't include the 12v and earth lines of the sensor but they are connected to a separate solar mppt controller.

    As SDI-12 uses the one wire for TX & RX (B) I didn't think i could use a simple UART TTL I could try this but I'm not sure how to go about swapping between them. A is used as a reference only. I do think I may need to add a resistor in series between the RX and 3.3V, and should probably add a cap in series between RO and ground.



  • @jimpower Looking at the protocol, a simple 3.3 to 5 V level shifer should be enough to connect to the SDI-12 data line. You should connect it with a series resistor in TX, to copy with the half duplex property. That might be the reason for the RS485 adapter, which would allow an easier coupling. xxPy's do not support half duplex operation at the moment. Maybe it is also possible to switch the TX line after UART init to open drain. But that is to be tested.
    For the actual device, you may maybe only have to exchange A and B at the breakout board. That should be required if the "silent" state at the RX side of the the xxPY is 0. And some ground lines seem to be missing in your pictures.


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