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SN65HVD3082E: Power Consumption Questions

Part Number: SN65HVD3082E
Other Parts Discussed in Thread: SN65HVD3088E, SN65HVD3085E

Hi team,

I have a few questions regarding SN65HVD308xE. As a quick note, I've been referencing Section 6.7, Figure 2 and Figure 5 of the datasheet and this e2e thread.

  • If there are two termination resistors on each end of the bus, and each is 120Ohm, should I use RL = 120Ohm in Figure 5? Termination is 120Ohm, but with these in parallel and the effective resistance being 60Ohm, I just want to make sure I'm looking at the right numbers.
  • Can SN65HVD3085E & SN65HVD3088E run at 19.2kbps? I believe the answer is "yes," so would there be any drawbacks for using them at such low data rates?
  • I'm trying to understand the Bus Input Currents better. My understanding is that this is a current drawn per transceiver that you multiply by however many transceivers are on the line to get your total. This total is the amount of current needed to be driven from a transceiver onto the bus in order to have the signal reach the transceiver it is talking to considering the loads from all the other listening transceivers on that same bus. This total is added with others to find the total current draw on a slave transceiver that is in use. Do you have any more documentation on this subject?

Thanks,

Brian

  • Hi Brian,

    If there are two 120-Ohm termination resistors, they will combine in parallel and present an equivalent load resistance of 60 Ohms. (In actuality it may be slightly lower than this if you were to account for the input resistances of all of the transceiver devices sharing a bus. Because of this, RS-485 driver specifications are often evaluated with a worst-case load of 54 Ohms.)

    Yes, in general there are no minimum data rate restrictions on RS-485 transceivers. Lower-speed devices are typically chosen for lower-speed applications in order to mitigate EMI and signal integrity concerns that are associated with faster signal transition times. (The output transition time of the driver circuit is the main difference between lower-data-rate and higher-data-rate transceivers. Sometimes the receiver circuit is modified to support higher input bandwidths for the faster devices as well, although this typically results in a trade-off with respect to bus input currents.)

    It sounds like your understanding of bus input currents is correct. It's kind of another way of looking at input impedance. For further reading, though, I would recommend referring to this blog:

    e2e.ti.com/.../rs-485-basics-how-to-calculate-unit-loads-and-the-maximum-number-of-nodes-on-your-network

    Please let us know what further questions you have.

    Best regards,
    Max