• State TI Thinks Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 1 answer
  • Subscribers 33 subscribers
  • Views 466 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

SN65HVD1781A-Q1: Inquiry on setting pull-up and pull-down resistor values

Do-hyun Kim
Intellectual 305 points
Part Number: SN65HVD1781A-Q1

Tool/software:

I am using SN65HVD1781A-Q1.
However, how can I calculate the pull-up and pull-down resistance values ​​in the RS-485 A port and RS-485 B port that are connected to the outside?
Please provide the calculation formula.
And, please also explain the meaning of pull-up and pull-down resistance. Thank you.

  • 0
    TI__Guru 72181 points

    Do-Hyun,

    Do-hyun Kim said:
    how can I calculate the pull-up and pull-down resistance values ​​in the RS-485 A port and RS-485 B port that are connected to the outside?

    I assume you mean an external fail safe biasing circuit. Below is a document that walks through the calculation.

    https://www.ti.com/lit/an/slyt324/slyt324.pdf?ts=1724185761577&ref_url=https%253A%252F%252Fwww.google.com%252F

    Do-hyun Kim said:
    And, please also explain the meaning of pull-up and pull-down resistance.

    They provide a biasing current which generates a voltage on the bus when the driver isn't enabled. This ensures a known state on the idle bus during that situation. It can also help with some noise when the bus is idle and the driver isn't enabled. 

    An example calculation would be, you want 200mV across the termination resistors. Your Vcc is 5V.

    Your equivalent resistance is normally 60 ohms. so to get 200mV you need 3.3mA of current across the resistor. (V=IxR)

    If your Vcc is 5V then you need a total series resistance of about 1.5k ohms (5V/3.3mA = 1.5k)

    If we assume the termination resistor is 120 ohms (we will readjust it slightly later), then the pull up and pull down resistor should be around 700 ohms each (1.5k-120 = 1.4k then 1.4k/2= 700 ohms).

    The termination resistor will be modified up a bit since the external resistors will act in parallel from the AC sense. (termination resistor in parallel with the 1.4k ohm resistance)

    We want the termination to be about 120 ohms for this network since it will be in parallel with the receiver 120 ohm resistor. 

    Rt//Rbias=120 solve for Rt

    Rt = 120xR_bias/ (R_bias-120)

    R_bias is 1.4k

    Plug in Rbias as 1.4k then Rt=131.25 ohms

    So the pull up resistor on A would be 700 ohms, pull down resistor on B would be 700 ohms, and the termination resistor between the two would be 131 ohms.

    -Bobby

  • 0 in reply to BOBBY
    Intellectual 305 points

    VCC:3.3V

    Is it appropriate to set the RFS value and RT2 value to the values ​​below?

    RFS: 470Ω
    RT2: 120Ω

  • 0 in reply to Do-hyun Kim
    TI__Guru 72181 points

    RT2 is going to act in parallel with your 470 ohm resistors (which will be in series with each other so 470+470=940ohms // RT2 which you want to be 120 ohms) in the AC sense. I think your RT2 should be around 137~138 ohms if your RFS is 470 ohms. 

    If the distance isn't too long and/or the data rate is slow then the value of RT2 isn't as important. My guess is using 120 would probably be okay in most systems since it's not that much of a difference compared to the target of 138 ohms. 

    -Bobby