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SN65HVD77: Series Resistor,Termination Resistor and Pull up and pull Down for Fail Safe.

HARSH PALIWAL
Prodigy 190 points
Part Number: SN65HVD77
Other Parts Discussed in Thread: STRIKE

Tool/software:

Hi,
I have few questions design related

My Application would be using around 40mA meaning 40mA RS-485 IC would have to pass through it with around 1.8m Cable between Two devices 
Communication mode will be UART between just  devices a Master and Slave.

1.Do i need Pull up and pull down for extra protection saying IC have Failsafe operation and how should i decide the value of pull up and pull down.

2.As RS-485 Connector would be exposed to outside world there is always risk of ESD so my standard says 8kV air and 4kv contact so i need to design in such a way that it can sustain ESD,so should i go with just TVS diodes or zener with a series Resistor?.

3.Is there a Need of series resistor and what should be its value? and postions of series resistor before protection or after.

4.Need of Termination resistor as i have just one device connected to my master is there still  a need for termination resistor? and what value should i choose and how will it influence my current consumption saying my product is battery powererd.

  • 0
    TI__Guru* 76991 points
    HARSH PALIWAL said:
    1.Do i need Pull up and pull down for extra protection saying IC have Failsafe operation and how should i decide the value of pull up and pull down.

    The device itself has an internal fail safe feature so you technically do not need the external resistors but you can populate them if you'd like as this will add additional noise immunity when the bus is idle and there isn't an active driver or if the driver is not connected.

    You're essentially forming a resistor divider with the pull up and pull down resistors where there will be a voltage drop across the termination resistor. (With your short distance and UART speeds you may not need a termination resistor). So you could potentially just put pull up and pull down resistors without termination at something like 4.7k.

    If you actually want to calculate the values then this document can help:https://www.ti.com/lit/an/slyt324/slyt324.pdf?ts=1755807729109&ref_url=https%253A%252F%252Fwww.google.com%252F

    HARSH PALIWAL said:
    2.As RS-485 Connector would be exposed to outside world there is always risk of ESD so my standard says 8kV air and 4kv contact so i need to design in such a way that it can sustain ESD,so should i go with just TVS diodes or zener with a series Resistor?.

    I normally see TVS diodes, you will want to use a bi-directional TVS if you expect GND shifts that could be negative.

    HARSH PALIWAL said:
    3.Is there a Need of series resistor and what should be its value? and postions of series resistor before protection or after.

    Generally series resistors are discouraged because it weakens the drive strength if it's infront of the termination resistor or the driver. If you do use a series resistor, I would recommend keeping it to 10 ohms or so and making it a pulse proof resistor.

    HARSH PALIWAL said:
    4.Need of Termination resistor as i have just one device connected to my master is there still  a need for termination resistor? and what value should i choose and how will it influence my current consumption saying my product is battery powererd.

    1.8m and UART speeds usually mean you don't need termination. If you care about battery life, then not using one would be better if you don't need it. If you wanted to though, you could use AC termination. If the communication is one direction, place the termination on the receiver node (not the driver node). My guess is you don't need the termination since the distance is so short and the speed is going to be slower.

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 190 points

    Thankyou bobby for your response,i have few more
    1.As you said Pull up and pull down would be voltage divider with termination resistor so what package should i go with for 120 ohm considering VOD is 2V and i am pulling up by 3.3V
    2.Same Question for Series resistor what package should i go with ?

  • 0 in reply to HARSH PALIWAL
    TI__Guru* 76991 points
    HARSH PALIWAL said:
    1.As you said Pull up and pull down would be voltage divider with termination resistor so what package should i go with for 120 ohm considering VOD is 2V and i am pulling up by 3.3V

    Harsh, I don't think the resistor package size matters for this. Some prefer smaller while others want larger size for easier for soldering rework.

    Same goes for the second question.

    -Bobby

  • 0 in reply to BOBBY
    Prodigy 190 points

    1.I wanted to know how 120 ohm would impact my battery budget 
    2.My application needs ESD protection of about 10kV i would be using 3 5V TVs diode to protect it also i would add series resistor of around 10 ohm
    what package should i go with considering all the given conditions as i dont want my series resistor to be blown by esd most will be taken care by TVS but still i need to take care of it

  • 0 in reply to HARSH PALIWAL
    TI__Guru* 76991 points
    HARSH PALIWAL said:
    1.I wanted to know how 120 ohm would impact my battery budget 

    You can estimate the power usage by using Vcc/Rterm where your case will probably be 3.3V and the Rterm if you are using two termination resistors at 120 ohms each would be 60 ohms. If you assume 10% tolerance (depends on the resistor tolerance you choose) then worst case is 54 ohms. So the power used when driving from a DC point of view is 3.3V/54 ohms = 61 mA. DC power would be 61mA x 3.3V = 201mW

    The AC power depends on the capacitive loading and the frequency of switching. 

    HARSH PALIWAL said:
    2.My application needs ESD protection of about 10kV i would be using 3 5V TVs diode to protect it also i would add series resistor of around 10 ohm
    what package should i go with considering all the given conditions as i dont want my series resistor to be blown by esd most will be taken care by TVS but still i need to take care of it

    You need to calculate how much power is going to be present during the ESD strike. Just find the clamping voltage and the max current spec'd by the ESD protection you're using or find the ESD standard and multiple current x voltage. It will depend on the max strike you expect will occur.

    -Bobby