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TIDM-02006: FSI Isolation in a Distributed Multi-Axis Servo Drive

Jejomar Ildefonso
Expert 8600 points
Part Number: TIDM-02006
Other Parts Discussed in Thread: ISO7820LL, TMDSFSIADAPEVM, ISO7763

Hi Team,

Taking a cue from the TIDM-02006 design guide, our customer has these questions:

- In daisy chain FSI, are digital isolators (example: ISO7820LL) like signal repeaters?
- Would using them lead me to increase the number of axes?
- Would the use of digital isolators lead to delays in the FSI daisy chain?
- Is it something that I can know a priori in the design phase or can I be understood by myself in the lab?


Thanks in advance!


Kind Regards,

Jejomar

  • +1
    TI__Mastermind 38295 points

    Hi Jejomar,

    Jejomar Ildefonso said:
    - In daisy chain FSI, are digital isolators (example: ISO7820LL) like signal repeaters?

    Digital Isolators will act as buffers for the single-ended signals.

    Jejomar Ildefonso said:
    - Would using them lead me to increase the number of axes?

    If your FSI communication is over a cable and there's a decent amount of distance between the axes (20cm or more maybe), I'd suggest using differential driver / receiver devices and not just digital isolators. Differential devices allow for robust communications over a long distance (10m or more).

    See TMDSFSIADAPEVM board and User's Guide for more info, which is used in the TIDM-02006 design.

    Jejomar Ildefonso said:
    - Would the use of digital isolators lead to delays in the FSI daisy chain?
    - Is it something that I can know a priori in the design phase or can I be understood by myself in the lab?

    Yes, it will add some propagation delay. The digital isolator device should have some specifications for this in their datasheet, same goes for differential devices.

    Best,

    Kevin

  • 0 in reply to Kevin Allen18
    TI__Expert 8600 points

    Hi Kevin,

    Many thanks for you response. As I understand it, in general single-end digital isolators (between tranceivers and microcontrollers / processors) are used when:
    1. there are large potential differences on GNDs in a network, (for unimportant distances, for noisy environments ...);
    2. the communicating boards have transceivers that have different commercial characteristics and that tolerate these potential differences on GNDs in a different way.

    While the differential digital isolators (between transceivers and cable) are used, in addition to the 1. and 2. situation, above all to obtain data integrity over greater distances (therefore not for Board-to-Board communication in a backplane, but backplane-to-backplane communication). Am I right? Please correct me if I'm wrong.


    Kind Regards,

    Jejomar

  • 0 in reply to Jejomar Ildefonso
    TI__Mastermind 38295 points

    Hi Jejomar,

    Jejomar Ildefonso said:
    While the differential digital isolators (between transceivers and cable) are used, in addition to the 1. and 2. situation, above all to obtain data integrity over greater distances (therefore not for Board-to-Board communication in a backplane, but backplane-to-backplane communication). Am I right? Please correct me if I'm wrong.

    Generally yes. Differential devices are used for communicating over longer distances (over a cable or PCB) and/or for increased noise immunity. You may not need differential devices for your system, it all depends on the physical communication medium and environment.

    Best,

    Kevin

  • 0 in reply to Kevin Allen18
    TI__Expert 8600 points

    Hi Kevin,

    My apologies for the delay. Got a feedback from our customer:

    a) why in TMDSFSIADAPEVM there are:
    -DSLVDS1047PWR and DSLVDS1048PWR (Designed for Signal Rates up to 400 Mbps) are better exploitable with RJ45 connectors for CAT 5 applications (100Mbps);
    - ISO7763 supports up to 100Mbps.
    Since the board is equipped with J18, J19, J20, J21 (Wurth Electronics-61500818512) which are useful for Cat 3 applications (10Mbps)?

    b) By fitting RJ45 connectors for CAT5 what would change in the network (number of nodes, distance between nodes, better synchronization..)?

    c) for the TIDM-02006 application, is CAT3 more than enough?"


    Kind Regards,

    Jejomar

  • +1 in reply to Jejomar Ildefonso
    TI__Mastermind 38295 points

    Hi Jejomar,

    No problem.

    Jejomar Ildefonso said:
    Since the board is equipped with J18, J19, J20, J21 (Wurth Electronics-61500818512) which are useful for Cat 3 applications (10Mbps)?

    I see they list CAT3 applications on the Wurth site. We did not notice this when selecting the part, but does this connector type listing CAT3 make a difference if you use CAT5/6 cable with it? We have tested with 100Mbps speed FSI and had no issues.

    Jejomar Ildefonso said:
    b) By fitting RJ45 connectors for CAT5 what would change in the network (number of nodes, distance between nodes, better synchronization..)?

    I'm not sure. We tested with Wurth Electronics-61500818512 and pushed MAX FSI speed at 10m cable distance between nodes with no issues.

    Jejomar Ildefonso said:
    c) for the TIDM-02006 application, is CAT3 more than enough?"

    TIDM-02006 uses Half-speed of FSI (i.e. 25 MHz clock, which is 50Mbps throughput). The speed requirement of FSI will depend on your application needs.

    Best,

    Kevin