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THVD1400: Communication error occurred with 50 nodes

Go
Genius 4190 points
Part Number: THVD1400

Hi,

my customer trying to use 200 of THVD1400 with daisy chain.

The device can support 256 nodes but an communication error occurred with 50 nodes under their evaluation. 

What point should we check first for solving a problem？

Any advice would be very appreciated.

Thanks,

Go

  • 0
    Guru 281350 points

    Please specify the exact number and values of all resistors that are connected to the A/B bus lines.

    Check the signals with an oscilloscope (compute A−B).

  • 0
    TI__Mastermind 46777 points

    Hi Go,

    Do you have a schematic to show how the nodes are constructed? I want to verify all bus connections to see if the bus could be overloaded from other sources. 

    Also how long is the bus between start and end nodes? 

    While this device can support up to 256 nodes - what it really means is that the minimum impedance*  is 96K (it may not be the "true" minimum but for sake of simplicity we use 96K for 1/8th unit load devices).  The RS-485 bus can have a common mode loading of 375 Ohms are greater -  so 256 nodes in parallel has 96k/256 = 375 Ohms. However other passive elements such as resistors and system parameters such as bus length and cabling can also impact loading and potentially degrade signal strength.

    Please let me know!

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Genius 4190 points

    Hi, Clemens, and Parker,

    Thank you for your comment.

    Here is the schematic outline.

    Are there some concerns we should point out? 

    THVD1400.pdf

    Thanks,

    Go

  • 0 in reply to Go
    Guru 281350 points

    There are no immediately obvious problems in the schematic.

    But it is possible that either the resistive or the capacitive load is too high. This would be visible in an oscilloscope trace as either small peak-to-peak voltage or slow edges.

  • +1 in reply to Go
    TI__Mastermind 46777 points

    Hi Go,

    I do have a few questions:

    1. How long is the stub from CN1/CN2 to THVD1400? If you have too long of stubs it could cause so SI issues.

    2. Do you know what cable they are using - a part number would be helpful so I can look at its characteristics to see if it could be negatively impacting the board

    3. Could you confirm there is only 1 termination resistor in the system? It seems its only at the End Node not the start.

    4. Is the total bus length 5m ? Am I correct or am I missing something? 

    5. Are the diodes at every node - and do you have the part number for the diodes? 

    6. If they could show the bus when they are attempting communication it may shed some light on the exact issue they are facing. 

    That being said there are a couple notes that could potentially improve performance.

    1. The 10 Ohm resistors should be between the IC and the 120 Ohm termination - not before the 120 Ohm termination. Customer is changing the effective termination - this could lead to signal reflections and attenuation of the main signal. Is customer expecting surges on the line at all - or larger transients? I ask because if you aren't expecting that to be an issue I wouldn't suggest adding 10 Ohm resistors at all because they are just going to degrade signal. If they are to be used put them between termination and IC. 

    2. If only the end node is terminated then the start node also needs to be terminated for best results. 

    Right now my  guess is that there is some heavier than expected capacitive loading and/or there is SI issues on the bus.

    Please let me know.

    Best,

    Parker Dodson