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ISOW7821: Check the isolator in sequence

Barry lu
Prodigy 30 points
Part Number: ISOW7821
Other Parts Discussed in Thread: ISOW7841, ISO7821, TPS54040, ISOW7843, ISOW7842

Hi All,

We want to check the isolator in sequence. We provide two version schematic for your check. The schematic structure design is as bellows. The power is provided by two option: 1. RS485 2. DC Jack  The power design will be affected by isolator in sequence.

Version 1:

Structure: MCU20190307_LUNA2_VERSION1.pdf20190312_luna2_version2.pdf20190312_luna2_version2.pdf20190307_LUNA2_VERSION1.pdf->RS485 transceiver->Isolator->RS485 connector

The isolator is powered by RS485

 

Version 2:

Structure: MCU->Isolator->RS485 transceiver->RS485 connector

The isolator is powered by DC-DC IC(3.3V).

  • 0
    TI__Mastermind 49435 points

    Hi Barry,

    Thank you for showing your interest in device ISOW7821.

    I have gone through both the schematic, Version1 and Version2.

    In Version1, I see that the RS-485 bus lines A/B are passed through the CMOS channels ChannelA/ChannelB of ISOW7821. Please do note that the RS-485 bus voltage levels are not compatible to CMOS digital I/Os. This is primarily the reason the RS-485 transceiver ST3485EIDT is used. ST3485EIDT converts the CMOS/TTL digital I/Os from MCU/MPU to RS-485 voltage levels.

    Version2 is the right way to use ISOW7821 where the digital I/Os of WSL309S are connected to the digital I/Os of ISOW7821. The digital I/Os on the isolated side are then connected to RS-485 transceiver ST3485EIDT which converts the digital I/Os to RS-485 compatible voltage levels.

    I noticed that TX and RX signals from WSL309S are isolated through ISOW7821 before they are connected to ST3485EIDT while the UART2_DE pin of WSL309S is directly connected to ST3485EIDT without any isolation. I think this needs to be isolated as well so that the RS-485 bus is isolated from WSL309S. Instead of ISOW7821, you can use ISOW7841 to support higher digital I/O channels.

    In addition, you have mentioned that power supply is available at the RS-485 bus. Please do let us know more about your application and where is the supply on RS-485 coming from. Thanks.

    P.S.: I have edited your post to indicate the correct part number ISOW7821 which was earlier written as ISO7821.


    Regards,
    Koteshwar Rao

  • 0 in reply to Koteshwar Rao
    Prodigy 30 points

    20190312_LUNA2_VERSION2_power.pdf

    Hi Koteshwar,

    Thanks for your detailed explanation. Our device(RS485) is connected to solar system. The solar system can provide power through USB or RS485 connector that why we have DC jack or RS485 connector for powering our device.

    For power circuit design, the input of DC jack and RS485 voltage is converted to 3.3V through TPS54040ADRCR. We hope that DC jack & RS485 voltage can operating 5V-30V so we choose TPS54040ADRCR. The primary side of Isolator is powered 3.3V by TPS54040ADRCR. The secondary side (viso) power other IC. If you have any suggestion, please let me know. Thanks

    B.R.

    Barry

  • 0 in reply to Barry lu
    TI__Mastermind 49435 points
    Hi Barry,

    Thank you for your inputs and sharing TPS54040 schematic, glad to know that you indeed have power supply available on RS-485 side.

    Please do not forget to isolate the UART2_DE pin from WSL309S to ST3485EIDT. Like I have mentioned earlier, you can use the 4-channel version ISOW7841 for isolating UART2_DE as well. Thanks.


    Regards,
    Koteshwar Rao
  • 0 in reply to Koteshwar Rao
    Prodigy 30 points

    Hi Koteshwar,

    Thanks for your remind. I've modified schematic from your suggestion. The pin UART2_DE from MCU(WSL309S) is connected to OUTB, and INB is connected to RE/DE. But the RE/DE of transceiver is input pin. The OUTB should be connected to RE/DE and vice versa. The IC may not have enough pin for this design because only 1 channel is different from other 3 channels by signal direction (4 channel unidirectional).

    I have checked your TI online schematic for ISOW7841. The primary side of isolator(vin) is powered by 3.3V or 5V and secondary side (viso) provide 3.3V to RS485 transceiver.

    Do you have any suggestion for this issue? Thanks a lot

    20190321 _ Page2 WSL309S.pdftidrq99.pdf

    B.R.

    Barry

  • 0 in reply to Barry lu
    TI__Mastermind 49435 points
    Hi Barry,

    Thank you for trying to implement the isolation for the UART2_DE pin as well. ISOW784x is available with various input/output channel configuration. Usually the primary of ISOW gets connected to MCU and secondary to RS-485 transceiver, for this use-case ISOW7841 would provide the right channel options.

    Since your application has primary of ISOW at RS-485 transceiver and secondary at the MCU, the suitable device to use for your application would be ISOW7842 or ISOW7843. Please check the datasheet for more information on channels directions of these devices. Any of these two devices should help you with your requirement. Thank you.


    Regards,
    Koteshwar Rao