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THVD1424: Is a pull-up needed for Line A? Will it cause problems if the VDD of the two devices is different?

Wilson Wang
Prodigy 20 points
Part Number: THVD1424

Tool/software:

Let's consider the half-duplex mode only.
Some people say the A line needs to be pulled up to VDD through a resistor, and the B line needs to be pulled down to GND through a resistor. However, I have a question:
If Device A’s VDD comes from a 3.3V common power plane, which also supplies power to other circuits and is made by OOO Company,
and Device B’s VDD comes from 12V and is made by QQQ Company,
once they are connected for communication, the 12V on Device B’s A line could cause issues for Device A, right?
Can a diode be added to the pull-up trace of the 3.3V line to avoid this issue in the design?

  • 0
    Guru 316600 points

    You would need only one set of fail-safe resistors on the bus. And the resulting voltages are inside the receivers' common-mode range.

  • 0
    TI__Guru 72521 points

    Hi Wilson,

    As Clemens hinted at, you don't need external fail safe resistors on the net/node (it's a nice to have). They are mainly there to help during idle bus states against noise to ensure an known state on the bus. (We don't want A-B pins to float to 0V). 

    It's pretty rare to see customers use a 12V Vcc on the external fail safe resistors but yes, you could place a diode on the 3.3V side to the A pin to try to prevent back biasing into the 3.3V rail.

    The RS485 transceivers can potentially handle a 12V bias if it's within the absolute max range for the input pins. (THVD1424 for example can handle a fault up to +/-16V so seeing 12V on the line isn't a problem from an absolute max perspective.)

    -Bobby

  • 0
    Prodigy 20 points

    Hi Clements and Bobby

    Thank you for your answers.

    I cannot be sure about the fail-safe resistors being pulled to 3.3V, 5V, or even 12V (if someone used discrete components to implement the RS485 circuit in older machines).
    This is why I asked the question.

    One more question: The pull-up resistor is used to avoid the fail-safe condition, so it is unnecessary to add a pull-down resistor to ground for Lines B and Z, correct?

  • 0 in reply to Wilson Wang
    Guru 316600 points

    When the bus is idle, the termination resistors will short both lines. The fail-safe resistors prevent this, but you need them on both lines to ensure a minimum voltage drop over the termination resistors. (The pull-up, termination, and pull-down form a voltage divider.)

    When the pull-up is to 12 V, then the bus lines will be near 6 V.

  • 0 in reply to Wilson Wang
    TI__Guru 72521 points

    Hi Wilson,

    As Clemens stated, it's basically pointless to have the pull up resistor and not have the pull down resistor. You should either include both or none at all. 

    With both, assuming the values are the same, then as clemens pointed out the bus voltage will be about half of the Vcc. So if Vcc is 12V then the A/B will be about 6V during the idle condition. 

    -Bobby