• State Resolved
  • Locked Locked
  • Replies 2 replies
  • Subscribers 33 subscribers
  • Views 375 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

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.

SN65HVD37 / the value of Termination

Toshiyuki
Mastermind 7075 points
Other Parts Discussed in Thread: SN65HVD37

Hi,

I understand SN65HVD37 meets TIA-485-A specificaitons.

So, should the value of Termination be 120Ω ?

In case of SN65HVD37, how much power is Termination(120Ω) consumed ?

Best Regards,

Kato

 

  • 0
    TI__Expert 5400 points

    Kato,

    Operating an RS-485 bus without termination causes standing waves to occur on the bus due to reflections at the unterminated ends.  This significantly reduces the distances and data rates at which the network may operate without communication errors.

    For this reason, parallel termination (2 x 120ohms) is generally recommended at the furthest most points of the bus.

    For a 3.3V transceiver, such as the HVD37, the RS-485 minimum driver output voltage of 1.5V, and the RS-485 maximum of 32 units loads, you will have a maximum sink current of 55mA over common-mode voltage (-7V to +12V) with parallel termination (60ohms).

    By removing the differential termination, you eliminate the differential current by 25mA (1.5V / 60ohms).  However you still have common-mode currents up to ~30mA.

    I recommend this article for more thorough discussion on bus termination techniques and power consumption.

  • 0
    TI__Mastermind 28265 points

    Depending on your driver and the common-mode voltage, the differential voltage across the 120 ohms termination resistors can be up to 2.5V. Calculate the resistor power via P = V2 / R = 2.52 / 120 = 52mW. Make your resistors 100mW resistors.

    Regards, Thomas