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SN65HVD1781: A and B outputs return exactly the same signal

Sebastiao Beirao
Prodigy 175 points
Part Number: SN65HVD1781


Hello everyone,

I'm trying to use the SN65HVD1781 transceiver to communicate between my microcontroller (Teensy 3.5) and a boat throttle with a RS485 interface. I'm trying to send a command to the throttle asking for some information. This command is composed of 5bytes which are correct at the microcontroller output (see second image), but I'm getting both RS485 signals (A and B) equal and not differential (see first image).

I've followed the guidelines in the datasheet, powering the transceiver with 3v3 and having the RE and DE connected to a single pin in the microcontroller, which correctly activates the transmission with a high logic signal. I have the throttle common ground connected with microcontroller. I have also the termination resistor (120ohms) and I've also tried biasing resistors in the RS485 lines with no luck.

Please give me some ideas on why I'm having both RS485 signals equal. I've tried with a USB transceiver pen which worked, but I really need to use the microcontroller.

Thanks in advance.

  • 0
    TI__Expert 6955 points
    Sebastiao,

    Would you be able to produce a schematic that shows the configuration of the MCU, transceiver, and throttle? Also, could you provide the model of the throttle?

    -Lucas
  • 0 in reply to Lucas Schulte9
    Prodigy 175 points

    Hello Lucas,

    Thanks for your reply on this. I'm leaving the schematic of the configuration of the MCU and the transceiver. I can't leave anything about the configuration of the throttle due to non-disclosure agreements, but the model is a Torqeedo 1918-00. In the schematic there are no biasing resistor but I've tried those also.

    Sebastião

  • 0 in reply to Sebastiao Beirao
    TI__Expert 6955 points
    Sebastião,

    Measure the A and B bus lines first without the throttle connected and see if that makes a difference. If there's no difference, then remove the termination resistor and measure again still with the throttle disconnected. Try to measure the Bus lines as close to the IC as possible. If none of this makes a difference, replace the IC with a new SN65HVD1781 and repeat measurements. Testing with a new IC helps rule out ESD damage which could internally short the A and B lines.

    -Lucas
  • 0 in reply to Lucas Schulte9
    Prodigy 175 points

    Lucas,

    I'm leaving some screenshots of the tests without throttle you've said me to do. I've tried with two different SN65HVD1781 chips, giving me the same result.

    With termination resistor (new chip):

    Without termination resistor (new chip):

    With termination resistor (old chip):

    Without termination resistor (old chip):

    Another thing: a friend of mine lent me a Maxim MAX485 just for me to confirm if the problem was not the transceiver and maybe something else, but I've got the good results from below, meaning that the problem may be transceivers...

    Is it possible that I have two faulty transceiver chips due to ESD problems?

    Again, thank you for your replies and guidelines,

    Sebastião

  • 0 in reply to Sebastiao Beirao
    TI__Expert 6955 points
    Sebastião,

    The new waveforms do not look good. Can you measure the current that the device is drawing for these conditions? We will see if there is an over-current condition happening.

    -Lucas
  • 0 in reply to Lucas Schulte9
    TI__Guru 62920 points
    Sebastião,

    Adding on to Lucas's comment, since it looks like there may be high leakage to ground in these new waveforms (they are not able to reach a full "high" level output) it would be good to try to understand both the power supply input current as well as the current flowing into the device from the bus pins. When the output driver is disabled or the chip is unpowered, the leakage current into the A/B pins should be low. If you measure a lower resistance to ground (or if you see higher currents flowing for a given applied test voltage), then it would most likely point towards some type of damage to the chips.

    Max
  • 0 in reply to Miguel Robertson
    TI__Guru 62920 points
    Hi Sebastião,

    I just wanted to check back in. Were you able to investigate this any further? Have you found whether the ICs were damaged somehow?

    Regards,
    Max