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SN65HVD01: The output power from A, B was over 20v.

won seungyeon
Prodigy 70 points
Part Number: SN65HVD01

I measured A with an oscilloscope.

Setting

 * VCC, VL = 3.3V

 * nRE / DE = Pull up (3.3v / 6k Ohm)

 * Terminator Resistor = Side 120 Ohm

Ch 1 = SN65HVD01 R Digital Output (2pin)

Ch 2 = SN65HVD01 A  Bus I/O

The timing is correct, but it is strange that it outputs more than 20v.

Why is it outputting more than 20v????

  • 0
    Prodigy 70 points

    Do the A and B Output voltages change depending on the situation?

  • 0
    Guru 257020 points

    The transmitter cannot generate a voltage higher than its own supply. Is there anything else connected to the bus?

    Is the ×1/×10 mode of the oscilloscope probe set correctly?

  • 0 in reply to Clemens Ladisch
    Prodigy 70 points

    no.
    The supplied power is 24v,
    After converting the voltage to dcdc, I supply 3.3v to the RS485 transceiver.

  • 0 in reply to Clemens Ladisch
    Prodigy 70 points

    I measured the ground and output B. Is the measurement method wrong?
    Connect the probe's ground to ground,
    I connected the probe pointer to the b output.

  • 0 in reply to won seungyeon
    TI__Mastermind 45397 points

    Hi Won,

    I have a few questions:

    1. Are both devices connected to the same ground? They have different symbols - and that could affect readings.  

    2. Have you verified the supply voltage fed to the transceiver? Is it 3.3V?

    3. I know a lot of  scope probes have a "10x" feature on them - is the probe set to "10x" ? I ask because a value 10x lower would make more sense with what you are seeing. 

    So based on what you have shared I have a couple ideas of what could be going on. First Clemens is correct - that transceiver cannot generate 20V at all - so the circuitry inside is not outputting 20V. That means that there is 20V being supplies somewhere to the device (maybe via the power supply would be my guess if this is the case), it could be caused by a large ground potential difference between the devices that's why what grounds are connected to are very important, or it could be a measurement error due to probe settings or where you measured. If you could answer the above questions it will help narrow down this list or perhaps highlight another possibility.

    Please let me know!

    Best,

    Parker Dodson