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THVD1420: Multi-load Question

Francis Jiang
Intellectual 2690 points
Part Number: THVD1420

Tool/software:

Hi Team,

Customer uses this device and meet multi-load question: Whether this device can connect with other 32 485 receivers and driver them? How to evaluate such condition? What's spec should they consider? Thanks in advance.

BRs,

Francis

  • +1
    Guru 303830 points

    The THVD1420 conforms to the RS-485 specification, so it (like any other RS-485 transceiver) can drive 32 unit loads.

    How many unit loads the bus has depends one how large the load of each receiver is. The THVD1420's receiver has one eighth unit load, so it is possible to use 256 THVD1420 devices on one bus (256 × 1/8 = 32). Most other modern transceivers have one eighth unit load, but if you are using older chips with one unit load, then you can have fewer of them.

  • +1
    TI__Genius 9497 points

    Hi Francis,

    Clemens is right here. THVD1420 can technically drive up to 256 nodes if the RS-485 bus is setup correctly according to the standard. When looking at datasheet specs, the VOD and receiver bus input current (II) should be considered between devices. Higher VOD and lower II generally implies improved node support.

    Please follow our design guide to ensure the best design practices: https://www.ti.com/lit/an/slla272d/slla272d.pdf

    Regards,

    Ethan

  • 0 in reply to Ethan Sempsrott
    TI__Intellectual 2690 points

    Hi Ethan,

    Customer total line length is ~50 meters, how to calculate whether the driving capacity is sufficient? Thanks.

    BRs,

    Francis

  • 0 in reply to Francis Jiang
    Guru 303830 points

    32 unit loads are supported with a 1200 m bus.

  • 0 in reply to Francis Jiang
    TI__Genius 9497 points

    Francis, 

    50 meters is well within the RS-485 standard of 1200m. 

    Another consideration is the network topology:

    Please ensure that the daisy chain (best implementation) or backbone with junction boxes method is used. This helps to minimize reflections and improve signal integrity. Another note is bus termination. This is described well in the design guide I linked above. 

    Best,

    Ethan