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SDI-12 level shifting

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    • #15387
      neilh
      Participant

      I was wondering does anybody have a suggestion for a hardware level shifter for the SDI-12 interface and if there are any software mods needed.

      I’m interfacing to the LT500 that requires at least +8V, and drives back +5V from the SDI-12 half duplex line.

      Right now I’m using a current limiting resistor, but it would be nice to figure out if I can use a level shifter.

      This is the circuit that I’m thinking of :

    • #15395
      Shannon Hicks
      Moderator

      We’ve encountered a few SDI12 sensors that are more picky about the data line voltage being actually 5v instead of 3.3v, and when sensor excitation line needs 9v or 12v, then we definitely need something between the Mayfly and the sensor.  I’ve hand-built a bunch of little adapter boards (see attached photo) for those sensors, using a BSS138, some resistors, and a really cheap 9v or 12v step-up module from Pololu.  This seems to work for every sensor we’ve tried it on, but it’s not ideal.   I’m working on an even better SDI12 level shifting module using a different technique, but I’ll write more about that in the next week or two.

       

    • #15398
      neilh
      Participant

      Hello @shicks thanks for the response. I was thinking partly of the SDI-12 specification modules driving 5V back into the 3.3V ports.  The Insitu LT500 that I’ve been using does respond to Mayfly driving it on 3.3V, but then drives back SDI12 specified 5V which has to be mitigated.

      Do you see any issues with the level shifter and the port interfacing software interface on the Mayfly.  https://github.com/EnviroDIY/Arduino-SDI-12

      I guess I’ve been trying the option above, and again the LT500 seems to then not respond. A slight difference is that when Mayfly port is in tri-state, which  can looks like 0V, the level shifter pulls up which then looks like a 5V.  So the level shifter flips it.

      Seems like that shouldn’t be a big issue – but then the LT500 didn’t respond, so became a headscratcher.

      So very interested to continue the conversation and I’m happy to proto type a trial circuit.  🙂

    • #15407
      neilh
      Participant

      Hello  @shicks  ~ I’m just following up on this as I’m respinning my  Modbus Board  MayflyWingShield schematic.pdf

      to have a SDI-12 interface, and your board got me thinking .

      The Mayfly socket has +5V and +3.3V and the board can have the boosted +12V. ( I had done it from the Seeed D6-7).

      So thinking of trading off the third 4pin “modbus socket” for a 3pin “SDI-12 socket”. That is 3pins screw connector and  “JST PH 3-pin”. The JST PH 3pin 2mm have Adafruit mating cables.

      The line interface following https://sdi-12.org/specification

      The benefits are one digital interface board, with options for simplicity like 3.3V pass through interface, or plain SDI-12 instrument +5V current limiting.

      So just putting it out there for a technical review if you have insights/comments  ~ always appreciated before committing to copper.  🙂

      SDI-12 interface

    • #15409
      Shannon Hicks
      Moderator

      I’m still finalizing testing of a circuit for this, parts are on order but due to supply and shipping delays, I don’t have them yet.  Hopefully I’ll have something to report next week.

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