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TIDA-00333: Modbus architecture and synchronisation

ömer yaman
Intellectual 390 points
Part Number: TIDA-00333
Other Parts Discussed in Thread: ISO3082

In my project i'll use modbus protocol for serial communication. There are more than 320 slaves which seperated equally in 2 groups(i attached a pic to make it clear). Every 16 slaves are powered from the same supply and isolated from others galvanically(Master'll be isolated from all the slaves) to make it robust. Isolation'll be achieved with TIDA-00333. My first question is if there is a problem in this architecture?

Secondly I want to synchronise all the slaves via 10ms period pulses that are derived from master microcontroller. How can i achieve a robust synchronisation(what type of line, single or differential signal, where to isolate)?

  • 0
    TI__Intellectual 2855 points
    Hi Omer Yaman,

    Thanks for the post.
    We are looking into this query, we will get back to you by Tuesday Morning IST with more details.

    Regards
    Tejas
  • 0 in reply to Tejas Hommaradi
    Intellectual 390 points

    Hello Tejas,

    This is an alternative one:

    Suppose each RS485 bus is 150 meters long, baud rate is 115200 and 200 slaves are connected. Can you consider this too. Which one is better in terms of EMC. Or do you have a better recommendation.

    Regards;

    Ömer

  • 0 in reply to ömer yaman
    TI__Guru 62920 points
    Hi Ömer,

    Either topology would be OK - it would just depend on whether you felt isolation were needed at every slave node or not. If isolation is only needed for an entire bus segment, you are right that TIDA-00333 could be used. Otherwise, if you use ISO3082 (i.e., in your second diagram) it should be able to support the 200-node bus segments and 150-m cable lengths at 115.2 kbps (assuming relatively standard twisted-pair cabling).

    It seems easiest to send the synchronization pulses via RS-485 as well as long as the slaves are able to tell the difference between synchronization pulses and normal data frames.

    Regards,
    Max
  • 0 in reply to Miguel Robertson
    Intellectual 390 points
    Hi Max,

    Thanks for your clear answer. I have one last question. If i want to use ISO3082(second diagram) how should i power all ISO3082's isolated parts.
    - Can i power them all from only one power source that is placed near the master(a parallel power line that follows RS485 bus). May it cause problems at last slaves because of long supply line? or
    - Can should i use more than one power sources that supply groups(16 or 32 forexample)? or
    - should i power them individually?

    Best regards;
    Ömer
  • 0 in reply to ömer yaman
    TI__Guru 62920 points
    Ömer,

    It looks like the isolated power supply would only need to power the RS-485 interface, and so there shouldn't be much current consumption (especially considering that only one RS-485 driver would be active on a bus segment at a given time). Therefore, I would not expect to see issues with using a parallel power cable. You could double-check this, though, if you knew the resistance per unit length of the wiring used. You should be sure to place a decoupling cap at the power input to each node and at the isolated power input of the ISO3082, though, to compensate for the inductance of the power cabling.

    It would be OK to power the nodes individually if you would like (e.g., in case you want to simplify the cabling connections or save cost on the wiring). In this case, though, you may still need to connect the isolated grounds of each node together, though, in order to ensure that the signals output by one node are not dramatically shifted in voltage with respect to another receiving node. This would depend on how much of a difference in potential you would expect between each isolated ground. (RS-485 interfaces can tolerate several volts of ground shift, but beyond a certain point communication errors may arise.)

    Regards,
    Max
  • 0 in reply to Miguel Robertson
    Intellectual 390 points
    Thanks Max for your clear answer.