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SN65HVD1785: SN65HVD1785

Bertie Deane
Prodigy 50 points
Part Number: SN65HVD1785

I have been using SN65HVD485 on my slave boards, on my new  board I need to change to  SN65HVD1785 . The master is an ILX34_MBS485 from Prosoft, running at 19200 .I have 20 slaves on the bus and I am getting some SN65HVD485 failures, always short circuit between V+ and A or B. I have no ringing on  supply or comms cables. There may be issues of mis-wiring by operators in the field so I need a device with protection.

Unfortunately the SN65HVD1785 is not working as a direct replacement and I can only get it to work if I pull the output to 5V via a 390 ohm resistor.

I posted this problem previously, but it was nor resolved.

Thanks,

Bertie.

SN65HVD1785
  • 0
    TI__Guru 62920 points
    Hi Bertie,

    I assume based on your other thread that you need to pull the R output up in order to get communication to work - is this correct? If so my guess is that the issue is related to the voltage at the R output when the receiver is disabled (/RE pin high). Your system may be expecting a high level on this pin to indicate an idle state when the receiver is not being used. Without an external pull-up resistor, though, the R pin of HVD1785 will float (meaning the voltage will not be driven by the part and will not be well-defined) when disabled and may be interpreted as a logic low, resulting in issues.

    We could confirm this theory if you could measure the R output as well as the /RE input of the device on an oscilloscope when this issue is occurring. I would expect to see R toggling with normal amplitude during data reception (with /RE low) but drift to a lower voltage when the bus is idle (and /RE is high).

    If this is indeed what is happening, you should not need a pull-up resistance as low as 390 Ohms to get the communication working. Something in the 1 kOhm to 10 kOhm range would be OK, and would help minimize the current consumption required for low-level signals. If you see voltages that are too low with these weaker pull-up resistors, then we should investigate further to see if there is a potential source of pull-down leakage current on the R output node.

    Regards,
    Max
  • 0 in reply to Miguel Robertson
    Prodigy 50 points

    Hello Max,

    I measured the R output on the original SN65HVD485 device and it is exactly as you describe in Paragraph 2.

    When I fit the SN65HVD1785 to the same pcb, no other changes,  the R output stays at about 2V max amplitude.

    If I pull R output to +5 via a 600 ohm resistor its still the same however if I use a pull up resistor of 450 ohm it behaves exactly as the original SN65HVD485.

    I have oscillograms but I am unable to post them. The odd resistor values are because I had 1.8K resistors to hand!

    I wonder is it ok to use this low value pull up. My next build will be 500 units.

    Thanks for your help,

    Bertie.

  • 0 in reply to Bertie Deane
    TI__Guru 62920 points
    Bertie,

    The stronger pull-up resistor should not cause damage to the device, but the output "low" level from the receiver would be higher than our specification shows (the spec was written with a maximum output current of 8 mA in mind). If your system can still interpret the higher level as a logic "zero," then it will work. However, it seems to me like something else is going on with this board that isn't fully understood. The SN65HVD1785 receiver output will only sink or source 1 uA (maximum) while disabled, so it should not require such a strong pull-up. You may want to double-check whether there is anything else on the receiver output that may be pulling the voltage down.

    Max