• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Subscribers 33 subscribers
  • Views 697 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Sharing RS485 (SN65HVD1780) pins with generic I/O

Lada Lostak
Intellectual 360 points
Other Parts Discussed in Thread: SN65HVD1780

Hello,

we would like to share RS485 pins (A/B) with generic I/O pins. So, pins would be software selectable I/O pair or RS485. I/O pin is basically 0-20V input connected to amplifier and comparator. Output is open collector dirrectly connected to I/O pin. So, while RS485 will be 'disabled', A/B could be continuously under 0-20V potential. When RS485 will be enabled, there will be added some parasitic impedance (~100K) and RC filter (~10MHz) to GND to A+B separately. Ofcourse, it will degrade some parametres of RS485 tranceiver. But it isn't problem, since communication speed is low and cables doesn't excess ~50 meters inside building.

I am not sure about datasheed for SN65HVD1780. Is it possible, to have A continuously connected to 20V while B will be GND potential ? PDF just say, it is protected up to 70V bus fault. But there is nothing about continous 'fault'. Also, common mode votage will be outside datasheet specifies. In this case, device driver/receiver will be *disabled*. I am afraid about some possible fault currents and/or damaging device.

Thank you for hints :)

Ray

  • 0
    TI__Expert 3915 points

    Ray,

    This should not cause damage to the SN65HVD1780.  The bus pin breakdown voltage is well above 20V, so there will only be a small amount of leakage current.  As long as the HVD1780 driver is disabled, the bus pin current will be about V(BUS) divided by 120 [k]Ohms, so at 20V you can expect less than 200 uA.

    If the RS-485 driver is enabled at the same time as the generic I/O is enabled, there will be contention, with possibly high currents on the bus pins.  This also should not damage the device for short periods, but will cause internal heating and will activate the thermal shutdown circuit if allowed to continue.

  • 0 in reply to Clark Kinnaird
    Intellectual 360 points

    It is up to user, to keep attention for corrent wiring and do not mismatch RS485/Generic I/O. For this reason, there is INA device on HVD's VCC keeping eye on current consumption. So, even user mismatch wiring, system should recognize it by high current flow and report something like "broken BUS".

    Thank you :)

    Ray

  • 0 in reply to Lada Lostak
    TI__Expert 3915 points

    That seems okay.  By the way, the current will be V(BUS) divided by about 120 kOhms; in my previous post I wrote 120 Ohms.

  • 0 in reply to Clark Kinnaird
    Intellectual 360 points

    I didn't notice that that Kilo typo. I looked to the current value and see there is ~uA and didn't think over it, even I had bad feel about 120ohm and uAmp. And because "Humans see what they want to see" it seems I am human :o)

    Thx

    Ray