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SN65HVD1785

Bertie Deane
Prodigy 50 points
Other Parts Discussed in Thread: SN65HVD1785, SN65HVD485E

Hello,

I have been using SN65HVD485 on my control boards without issue, on my new control board I need to be able to hot plug the connectors, I tried to use the SN65HVD1785 as a direct replacement but I cannot get it to work. The master is an AB MV169e-MBS. I have 20 slave control boards per system.The received signal output is less than 2V in amplitude.

  • 0
    TI__Mastermind 20820 points
    Hi Bertie,

    Thanks for the question but I need more information to help you diagnose the problem. What is the status of the /Re and DE pins on the master and slave boards? I assume you toggle the masters enable pins through a few GPIO's on your microprocessor depending on whether or not you are driving or receiving a message? On the slave boards I would expect the same unless you are only receiving messages on them. In that situation you would simple tie /RE low and tie DE high permanently. One you have the devices configured properly what do the bus signals look like? Can you provide a screen capture of the driver input (D), and the bus outputs A/B? Also what data rate are you running? The SN65HVD485 supports up to 10Mbps while the SN65HVD1785 only supports up to 115kbps.
  • 0 in reply to Michael Peffers
    Prodigy 50 points
    Hello Michael,
    Thank you for replying, /Re and De are connected and are controlled from the micro.
    The R output, pin 1 lies at about 2V and if I fit a 390 ohm pull up resistor to 5V the comms work both read and write.
    The R output is feeding pin2 U1RX of a PIC24F and nothing else.
    I wonder why is the R output so low and is it ok to use a pull up of this value on this pin.
    Best wishes,
    Bertie.
  • 0 in reply to Bertie Deane
    TI__Mastermind 20820 points
    Hi Bertie,

    When the receiver is enabled the output should be in a known state like 5V, 3.3V for 3.3V devices or low. Since the SN65HVD1785 implement internal failsafe biasing I expect that the output should be close to Vcc if the receiver is enabled. When the receiver is disabled though the output is in a HiZ state and the voltage can be somewhere Vcc and GND like mid-rail. The external pullup resistor that you implemented forces the receiver output into a known state when the receiver is is disabled.
  • 0 in reply to Michael Peffers
    Prodigy 50 points

    Hello,

    I still have no resolution for this problem.

    I need to replace SN65HVD485E with a device that is  capable of surviving over voltage faults of up to 50V,  

    The SN65HVD485E are failing in the field with short circuits between A or B and Gnd.

    I tried the SN65HVD1785 but found problems as described in previous posts, namely that I had to pull up Rx (Digital out)  to Vcc with a 300 ohm resistor.

    Regards,

    Bertie.

  • 0 in reply to Bertie Deane
    TI__Mastermind 20820 points

    Hi Bertie,

    I am sorry you are still having this issue, lets see what we can figure out. You mention surviving voltage faults up to 50V which was not mentioned in your original post. These faults are bus faults (A & B) correct? I suspect that due to the inductance of the cable you are getting a lot of overshoot and ringing on the bus similar to this:

    This data was taken on a CAN transceiver with a 55V hot plug to show the effects of the overshoot. So it doesn't directly correlate to your situation but I suspect the same thing may be happening here. Can you send me an image of your schematic so I can review it and also so you think you can do a similar measurement to the one above on your board(s)?