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THVD8000: Solving for interference in Power over RS485 bus

sajul babu
Prodigy 85 points
Part Number: THVD8000

We are using THVD8000 for implementing power over RS485 in one of our products, during the beta testing it was observed that a customer connected two of the devices using a 4 core cable approximately 100ft long and there were errors in communication reported> His setup looks like below

  • The signal from device 2 seems to be causing issues in device 1.
  • Both Modbus master 1 and 2 are devices which source power and slave 1 and 2 power from their respective masters. each of these slaves consume 100mA from 28V power available on the RS485 bus.
  • The communication over the bus is Modbus and baud rate from devices are 9600 and the carrier frequency is set to 135Khz.

The issue disappears if Modbus Master 2 is disconnected from 24V

 and Modbus Master 1 and Slave 1 communicates without any error.

Attached are images of the signal and noise observed at A and B inputs to the THVD8000 driver on Modbus slave1

The center purple trace is A-B, Blue and yellow are B and A respectively

As the signal and noise are  having same frequency , there are no effective ways I can think about to get rid of the noise.

Any suggestions to improve the situation?

  • 0
    TI__Guru 52172 points

    Hi Sajul,

    Is it possible to share the schematic of how the power network is configured? I just want to ensure there are no schematic level issues before we move on with further troubleshooting. 

    I also do have a few confirmation questions as well:

    1. Is it correct that you have two different differential buses that each are point to point and share the same cable. I.e. Master1 goes to slave1 and Master2 goes to slave2 and master1 is not connected directly to slave2 and master2 is not connected to slave1 -please let me know if this is not the case. 

    2. Is it correct that both busses are connected to the same 24V source 

    Right now my best guess is that THVD8000 signal is working its way back through the power coupling network as the coupling networks will load on one another - the THVD8000 design assumes that behind the inductor is the power supply or power load that is AC ground (from the point of view of THVD8000 - so there needs to be some higher frequency AC grounding cap on the power supplies to prevent noise from one bus coupling onto the next.  Please ensure design follows guidelines here as well: https://www.ti.com/lit/an/slla600/slla600.pdf?ts=169896040226 as I think part of the problem at least is the power supply/power loads are not AC grounded with respect to the modulation frequency - since your source is also AC - its not a true AC ground - but effectively a high frequency ground - which is achieved with a filtering cap to ground on power nodes - when sizing the capacitor you want to pick the minimum value that will get desired result to prevent too much power loss from AC source. 

    Please let me know on the questions I had - as I am bit worried with the overall bus setup as the idea is one source to one bus - not one source to 2 so there could be some additional issues due to that (I won't say not possible at this point as this device is generally very flexible0 

    Best,

    Parker Dodson