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THVD1406: How to test and verify THVD1406 can support 32 unit load or up to 256 unit load?

Gary Mao
Prodigy 30 points
Part Number: THVD1406

Tool/software:

Hi, Sir

I has design in THVD1406DR IC in our product.

But I have no idea how to prove my product can support 32 unit load or up to 256 unit load.

I study " THVD1406 datasheet " and " The RS-485 unit load and maximum number of bus connections -SLYT086 document" and "The RS-485 Design Guide - SLLA272D document.

Question 1:

Base on THVD1406 datasheet description "THVD14x6 can support 256 unit load" as follow picture 1.

Does that mean THVD1406 receiver input resistance around 96K ohm? 

Picture 1:

Picture 1

Question 2:

If I want to simulate and verify whether the THV1406 can support a 256-unit load (ignoring cable variables for now), can I use the following model and test environment (as shown in Picture 2)? By measuring the positions of Probe 1 and Probe 2, and if the voltage difference (VOD) reaches ±200mV or more, does this prove that my product can support a 256-unit load?

Picture 2:

Picture 2

I look forward to your reply.

Thanks.

  • 0
    Guru 316780 points

    All transmitters support 32 unit loads.

    When a receiver has one unit load, then you can have 32 of these receivers on one bus.
    When a receiver has one eighth unit load, then you can have 256 of these receivers on one bus. (This does not increase the number of unit loads, because each receiver adds only 1/8.)

    A complete bus can be modelled as two 120 Ω resistors (for the two terminations) and one 12 kΩ resistor for each 1 UL receiver (or 96 kΩ for each 1/8 UL receiver), all in parallel. (12 kΩ is not an exact value.) The datasheet specifies the differential output voltage for a total load of 54 Ω.

  • 0
    TI__Guru 72901 points
    Gary Mao said:

    Question 1:

    Base on THVD1406 datasheet description "THVD14x6 can support 256 unit load" as follow picture 1.

    Does that mean THVD1406 receiver input resistance around 96K ohm? 

    You can check the unit loading of a device by looking at the max leakage current on the receiver.

    V=IxR --> R= V/I

    12V/100uA = 120k ohms

    Since 120k is larger than 96k it is a smaller unit load than 1/8 

    We normally use the 12V calculation since that one is usually has a higher restriction on the max load. 

    Gary Mao said:

    Question 2:

    If I want to simulate and verify whether the THV1406 can support a 256-unit load (ignoring cable variables for now), can I use the following model and test environment (as shown in Picture 2)? By measuring the positions of Probe 1 and Probe 2, and if the voltage difference (VOD) reaches ±200mV or more, does this prove that my product can support a 256-unit load?

    Picture 2:

    Picture 2

    The test circuit needs to have only one transmitter present. The test circuit looks like is provided in figure 7-1 of the datasheet.

    My understanding of this is the Vtest needs to be checked at 12V and -7V. You measure the VoD to be atleast 1.5V across the 60 ohm load. 

    The document "The RS-485 unit load and maximum number of bus connections" shows this is done using a DC 0 to 5V supply and a -7V to +7V noise generator. 

    -Bobby