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ISO3088: Is there any need to have pull up register?

Part Number: ISO3088
Other Parts Discussed in Thread: ISO1430, ISO1450

Hi,

I'm going to work on ISO3088 to design Half-duplex RS485 circuit which is already developed but not tested. I want to implement Modbus protocol on the top of RS485.

When I compared my circuit with that of reference design which I found under Technical Support section of ISO3088, I saw that there is no pullup register being used in our design on input line A.

Please find the attached circuit. I also attached reference design. The image with black background is my design.

I have limited knowledge of hardware therefore I have very basic question - 

Will this circuit work?  Will I be able to use Modbus protocol on it? 

Thanks,

Abhay

 

  • Hi Abhay,

    Welcome to TI E2E forum!

    The pull-up and pull-down resistor network on the RS-485 bus is for failsafe biasing, i.e., to leave a valid state on the bus when the device is in idle condition. These are generally not required for most present day RS-485 devices as failsafe biasing is usually integrated in the device which you can also see mentioned in ISO3088 datasheet. Hence, this is optional and not having it shouldn't cause any issues.

    Please find below some useful material on this topic.

    www.ti.com/lit/slyt514

    www.ti.com/lit/slyt080

    ISO3088 is one of our older isolated RS-485 transceivers, I would recommend you to use our latest generation devices ISO1430 or ISO1450, based on your speed requirements, which offer significantly better performance from older devices from both isolation and RS-485 point of view. ISO14xx family of devices are also pin-compatible with the ISO308x family.

    Let me know if you have any other questions, thanks.

    Regards,
    Koteshwar Rao

  • Hi Koteshwar,

    I really appreciate such a quick reply!

    Thank you for such a descriptive reply along with relevant references. 

    This is going to resolve my issue.

    Regards,

    Abhay