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ISO1176: communication issue will casue other VFDs disconnection

Gary Zeng
Intellectual 1105 points
Part Number: ISO1176

Tool/software:

Hi ti experts,

My customer uses ISO1176 in VFD(variable frequency drives) products.

Their customer use one Simens PLC to control 24x VFD. For former 23x VFDs. They use RS485+ISOLATION. For the 24th VFD of my customer, they use ISO1176. But when putting 24th VFD into the system, former 13x VFD will be offline. While my customer changed ISO1176 to competitor device, only the 23rd will fail to be online.Customer found adding 24th VFD will let RS485 voltage level drop. is it the reason why other VFDs are affected.

Could you please help review the schematic ASAP.

  • 0
    Guru 316440 points

    Please shoe the bus topology. An RS-485 bus should be linear with exactly two ends, and with exactly two termination resistors at these ends.

  • 0
    TI__Expert 7325 points

    Hi Gary,

    The ISO1176 schematics overall looks good except one thing Bus Line A/B:

    1. What is the Purpose of C31 in the application and how is A/B of ISO1176 getting terminated? 
      1. This Cap will load the Bus Pins from all the VFDs an can result in Level Drop of RS485 lines.

    Regards
    Varun