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ISO1540: Isolation SDI-12 Bus

Michael Radermacher
Prodigy 40 points
Part Number: ISO1540
Other Parts Discussed in Thread: ISO1541

Hello,

I need to isolation the SDI-12 Bus.

The Bus is 1 Channel bidirektional. I want to use the ISO1540 like the follow circuit.

The signal at REF1 is OK at 4,8 V.

The signal at REF2 is less than 0,5V.

I also tryit with Pullups, but it dosn´t works.

Can anybody help me, were is my mistake.

thanks

Mike

  • 0
    TI__Mastermind 48515 points

    Hi Mike,

    Welcome to E2E. ISO1540 will not be able to sink enough current to drive a signal line low through a 1.5k-ohm series resistance. Typically in I2C applications, if series resistance is used, a very small value is recommended (1 - 10 ohms) to reduce the voltage drop between driving and receiving devices. Excess series resistance will also slow the rising edge of the signal, potentially lowering the system's maximum data rate. For what purpose are the series resistors being included in this design?

    ISO1540 also requires that pull-up resistors be present on both sides of an operating channel as the outputs are open-drain. 

    I would recommend testing the circuit again with the pull-up resistors included and series resistors R3 and R5 shorted. This should allow ISO1540 to drive the SDA1 line to a valid low voltage. 

    Please let me know what you find out and if you have any more questions.

    Regards,
    Eric

  • 0 in reply to Eric Schott1
    Prodigy 40 points

    Hi Eric,

    thanks for your answere.

    Eric: For what purpose are the series resistors being included in this design?

    Mike: That is thespefication from the SDI-12 bus from the SDI-12.org   Page 3 + 4 www.sdi-12.org/.../SDI-12_version-1_4-Jan-10-2019.pdf

    I tested now with shorted R3 and R5 an PullUp R2 + R6 1k5

    I give a report later.

    Regards,

    Mike

  • 0 in reply to Michael Radermacher
    Prodigy 40 points

    Hi Eric,

    I´m a little step forwards.

    I pull up the SCL Pins (R100 + R101)so that they are not in the air, only for saftey.

    1. When I shorted the R3 and R5 or use pull-up R2 the SDI system says that the SDI bus is shorted. I can´t use the pull-up on the left side SDI-12_BUS.

    2. When I swap the ISO1541 S1 and S2 the signal goes true the ISO and the µC answere.

    Thats my present circuit:

    The SDI bus signal to the µC is 5 V high-level thats OK, but the signal from the µC to the ISO will be pushed down to 1 V (high level).

    Thats the signal on the side to/from the µC-Pin:

    Regards,

    Mike

  • 0 in reply to Michael Radermacher
    TI__Mastermind 48515 points

    Hi Mike,

    Sorry for the confusion on this, I am not too familiar with the SDI-12 standard. 

    I don't believe that ISO1540 will be able to support SDI-12 communication. This device has open-drain outputs designed to be compliant with the I2C standard and it depends on external pull-up resistors to drive voltage-high signals. From what I understand, this is not sufficient for SDI-12's electrical interface which requires a push-pull output to be capable of driving through the higher source resistances of the equivalent circuit shown in the document you shared. 

    As our portfolio doesn't currently include an integrated SDI-12 isolator, you may consider a design which can convert SDI-12 to digital logic to be isolated using a digital isolator. If you would like more recommendations for solutions, please share more information about your system: what devices are being interfaced with? What are the system's isolation requirements? If you would like to share any schematics offline, please find my email by clicking on my E2E name.

    Regards,
    Eric

  • 0 in reply to Eric Schott1
    Prodigy 40 points

    Hi Eric,

    thanks for your help. I switch bach to the roots.

    I split the bus in RX and TX so I can use a ISO with forward and backward channel. Thats some parts more but its works.

    Regars,

    Mike