• State TI Thinks Resolved
  • Locked Locked
  • Replies 1 reply
  • Answers 1 answer
  • Subscribers 33 subscribers
  • Views 297 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.

SN65HVD75: Will putting different variations of this transceiver on the same 2-way comm line complicate things?

Kevin Lai
Intellectual 2916 points
Part Number: SN65HVD75

Hi,

My customer has a couple questions regarding the SN65HVD75/722/82 transceivers below:

1. Could there be any complications if different transceiver p/n’s are on the same 2-way communication line? Especially if there are 3.3V and 5V transceivers mixed together? Are there recommended practices for this type of scenario?

 2. Is the maximum stub length specified in the datasheets for the above parts referring to the distance that each node can be from the communication line? Could this be affected by the number of nodes on a line? And could this be affected if different transceiver p/n’s (with different rise times) are on the same line?

Thanks,
Kevin

  • 0
    TI__Guru 62920 points

    Kevin,

    There should not be issues with using different transceivers at different nodes, even if they are powered off of differing power supply voltages, as long as all the transceivers meet the requirements of the RS-485 standard (which defines things like the required transmitter output differential amplitude, the receiver input sensitivity, etc.).  In fact, interoperating between transceivers is very common - of the many thousands of RS-485 interfaces operating in the world, I've never seen ones that specify either a supply voltage or a part number.

    One thing to be careful of, though, is using pull-up resistances on the bus.  (This is sometimes done in order to set a differential bias for the idle state - you can read more here: https://e2e.ti.com/blogs_/b/industrial_strength/archive/2016/12/06/rs-485-basics-two-ways-to-fail-safe-bias-your-network.)  If there are pull-ups to 3.3 V and pull-ups to 5 V present, there could be current flow from the higher to the lower supply.  This can be prevented by removing one of the pull-ups or by placing a diode in series to block the reverse current flow.

    It sounds like your understanding of the stub length is correct.  A "stub" would be any wiring branch that is not terminated at the end.  A signal propagating down this line will see an interface between two different impedances when it hits the end of the line: the characteristic impedance of the line itself (which is typically 120 Ohms differential for RS-485 data links) and the high-impedance end of the line (which likely connects to a receiver input).  Signals incident on an interface between dissimilar impedances generate reflections, and these reflections can interfere with the signal integrity of the intended signals on the bus lines.  This is only a concern when the stub is long enough to act as a transmission line, which is basically when the delay through the line becomes significant with respect to the transmission time of the signal (so that significant differences in potential could exist simultaneously along the length of the line).  The stub length calculations in the datasheet give a rule of thumb for when to start considering these kinds of concerns.  They would apply to not just the stubs connecting to that transceiver directly, but to any stubs on a network that is used by the transceiver.

    The topic of higher-speed signal integrity can be a little difficult to understand, so please let me know if the above info isn't clear to you or if you have any further questions.

    Regards,
    Max