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Multipoint-lvds line driver and impedances

Other Parts Discussed in Thread: SN65MLVD206, SN65HVD21

Dear all,

I have a test application that interfaces with a multiple modules using the RS-485 transceiver.  I'm looking specifically putting the transceiver as the driver.  That being said, I was hoping to drive multipoints but only one module is powered on at any one time. 

The issue I have is the terminating resistors for the differential lines are actually located in the modules themselves and not on the test board so I cannot mount them at each end of the transmission lines.  One "fixed" module is always connected, producing a impedance of 100 ohm on the line.  However, during a different test, another module can also be installed which also has terminating resistors inside.  Would this mean that the RS-485 transceiver I have would see an impedance of 50 ohms total considering that would be installed in parallel with the "fixed" module?  If this is the case, I am specifically looking at the SN65MLVD206 which supposedly can support multipoint buses with loads as 30 ohm, would I be fine to keep the "fixed" module connected? 

If not, are there perhaps any suggestions to how I could go about making this application work?  Perhaps, switching the "fixed" module lines in with actually relays.  I appreciate any insight that can be given for multipoint use of the RS-485.  Thank you very much in advance

  • Hello Nicole,

    From what I understand, at any point in time you will have the fixed RS-485 driver and one other receiver connected to the bus? If the driver and the receiver both have 100 ohms termination built in, then you are correct that the transceivers will see a 50 ohm impedance on the line. Typically for RS-485 applications it is suggested that termination resistors are placed at each end of the bus and these termination resistors should match the characteristic impedance of the transmission line (commonly 120 ohm). Two 100 ohm resistors should be ok as well depending on the application. Here is a nice guide on RS-485 line termination: 

    RS-485_Design.pdf

    If the situation above is true, you should be able to use RS-485 transceivers with no problem, however if the system will have multiple modules installed on the bus at once, each with 100 ohms termination, the bus load will continue to drop and the driver will not be able to operate correctly. It is not recommended to include termination resistors on each node for this reason. 

    The SN65MLVD206 is an MLVDS transceiver which operates lower differential voltage levels than RS-485, so I wouldn't recommend this part if you are working with RS-485 modules. If you can give more details about the system requirements (signal rates, distance, etc) I would be able to better recommend a transceiver.

    Best Regards,

    Casey McCrea 

  • Thank you Casey McCrea. I apologize I wrote down the wrong part for some reason the part I am using to drive is the SN65HVD21 not the SN65MLVD. Which is the same part used in the module. However, I believe this has no effect on the impedance lines themselves.

    I'll need to find out about the speed since I don't know off the top of my head. However, basically the system communicates through this transceiver to for clock and control. The worse case would be one two receivers are in place and I'm thinking of a suitable solution for the system:

    1) REmove the terminating resistors from the "fixed" item and chance the unterminated condition in this case.
    2) Put in a mux to switch the RS485 signals accordingly to each device
    3) Can I add series resistors to offset the impedance...problem would probably be still the change in impedance so probably not

    This are the possibilities I have come up with but any other possible suggestions would be appreciated. Thanks again for your time.
  • Nicole,

    If the worst case scenario is only two receivers on the line you may be able to leave the configuration as is. To be clear, in your worst case scenario there would be one driver at one end of the bus with 100 ohm termination and two receivers on the bus with built in 100 ohm termination, correct? This is certainly not ideal, but if communication is happening over a relatively short distance this configuration should work. Excessive loading would really become an issue if you had a bus with 4 or more terminations. See section 6 in this guide:

    Bulletproofing RS-485.pdf

    The best thing you could do if possible would be to remove the 100 ohm terminations from the receivers and place a 100 ohm termination at the other end of the bus from the driver. This way the transceivers would see 50 ohms across the bus and signal reflections would be minimized. (I'm also assuming your transmission line has 100 ohm characteristic impedance). 

    If that's not doable, it could be possible to implement what I said in the first paragraph, but to give you a more firm answer I would need more details on the signaling rate, distance, and transmission media.

    Best Regards,

    Casey McCrea