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Compitable RS485 Transceviers

Discussion in 'General Electronics Discussion' started by New122, Feb 11, 2013.

  1. New122

    New122

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    Jan 29, 2013
    Hello All,

    I am not sure how basic my question. But seeking Experts help on this.
    Currently in one project I am using MAX13487EESA+ (Half Duplex RS485 Transceiver)> Right now It allowing me to use upto 3 Terminating Resistor(100 Ohm each), But I am not able to use more than that. What Information should I have to keep mind in datasheet of compitable Transceiver, if I want to use more Terminating Resistor(Upto 8).

    Thanks in Advance
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Using more than one termination resistor is outside the RS485 spec. and is not necessary. Use one single termination resistor at the end of the line. Read this for some detail.
     
  3. New122

    New122

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    Jan 29, 2013
    Re

    Hello Haralad !!

    THanks for your prompt reply. Actually Currently We can able to terminate only three sensors, So my company looking for more compitable Transciver which can help to terminate 8 sensors. SO thats why I am looking for compitable transciver which Can handle upto 8 Sensors. Also How is it needs to wire? We have two transciever in our board right now who can terminte three sensor t time.
     
  4. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    Read the tet I linked to. It shows how to connect more than one transceiver to a single RS485 bus. With only 1 termination resistor!
    Otherwise I do not understand your problem.
     
  5. New122

    New122

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    Jan 29, 2013
    Re

    Hello Again,

    Please find attached sample layout which I am planning to do. Currently Through RS 485 Receiver we are controlleignJ21, J22 and J23. Now We are planning to connect J24 through rest with exisiting Connection as shown In fig. I believe(I am not sure here) that we are connection 110 Ohm Resistor at somewhere between node 2 and 3 of Connector Pin from J21 onwards.But I am not able to operate here more than 3 Sensor with existing RS 485 Transciever. Your Opinion on this please.

    THanks In Advance !!
     

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  6. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    If each of those connections is to a device (not a cable) then they should be unterminated (except for the last one). i.e. a SINGLE termination at the end of the cable.

    If they go to a cable run, then you should have an RS485 tranceiver for each one.

    You clearly haven't read what Harald has suggested you read (twice!) because what you're doing is shown in figure 1 of the article on the second page! Even if you just look at the pictures you can see that.
     
  7. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    The datasheet for the MAX13587 states:
    • 1/4-Unit Load, Allowing Up to 128 Transceivers on the Bus
    Note that this is non-standard (1/4 load), but should easily accomodate your handful of transmitters.

    Note also that the stubs from the main bus to the individual devices shall be rather short. If possible, feed the bus directly to the first device, from there to the second device and so on. Pu a termination resistor only at the terminals of the last device.
     
    Last edited: Feb 12, 2013
  8. New122

    New122

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    Jan 29, 2013
    Re

    Hello All,

    THnaks for your explanations.
    Currenlty we have Transciver installed in board and receiver installed on field(We do not have control to change wiring) that having 110 Ohm termination resistor on three differnet receiver end. And as we have have board which having transceiver. Now we have plan to add another device which also having 110 ohm terminating resistor. So do you want to do entire rewiring RS485 bus. Cann't we have option like multichannnel, which can support multiple termination resistor. Please advice!!
     
  9. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    You don't have to rewire the bus. Just remove the termination resistors from all but the last device on the bus. Many devices have a jumper or switch to allow insertiion or removal of a built-in resistor.
     
  10. New122

    New122

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    Jan 29, 2013
    Re

    Hello Harald,

    Unfortunately We dont have access at sensors side. So I am thinking of using Repeater which can help me to build star network and allow me of using multiple terminations. What do you suggest? We have 115kbps baud rate.
    Can I connect RO pin to repater's(same Device) DI pin. And DI pin to RO pin. Also how could i avoid two differnent state of RO(1 and 0) at same time. (Can I use multivibrator)
    Please advice. Also It will be great if have schematic for that.
    Thanks in Advance !!!
     
  11. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
  12. New122

    New122

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    Jan 29, 2013
    Re

    Thanks Harald,

    I will review it.
     
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