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Hot!Communication RS485 design

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AMPS
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2020/10/23 22:17:14 (permalink)
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Communication RS485 design

Dear all
 
I have used PIC18F45k40 device for RS485 communication. I have 2 ckts one is isolated and another non isolated RS485 design.
I have schematic & placement for reference.
I have connected multiple slave device independently isolated and non isolated product , its working well. There is no issue with poll mismatch
 
Now i am combining isolated and non isolated module in parallel.during this stage, after hour isolated module stop communicating but isolated version product working well.
 
difference i found in both device i found is voltage level during operation isolated version if measure voltage across A& B w.r.t gnd is 7V-9V respectively i.e for ADM2483(Isolated ground)
 
where as SN75176 non isolated IC voltage level across A &B w.r.t ground is 2.5V-5V respectively.
 
i would like to why my communication of isolated version get disturbed when non isolated version is introduced.
 
which one is better to use to make solve this kind of problems.
 
 
post edited by AMPS - 2020/11/02 18:30:15

Amps
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#1

14 Replies Related Threads

    ric
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    Re: Communication RS485 design 2020/10/23 23:46:25 (permalink)
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    ajitnayak87
    ...
    Now i am combining isolated and non isolated module in parallel.during this stage, after hour isolated module stop communicating but isolated version product working well.

    Did you leave out a "non" somewhere in this sentence?
    As is, you are saying ALL the modules are "isolated".
     

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    #2
    Mysil
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    Re: Communication RS485 design 2020/10/24 00:28:52 (permalink)
    +1 (1)
    Hi,
     
    Even with non-isolated transciever,
    common mode voltage, that is between GND and A, or between GND and B,
    between -7V and +12 V should be acceptable.
     
    Is there anything other than DC voltage going on?
     
        Mysil
    #3
    AMPS
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    Re: Communication RS485 design 2020/10/24 00:29:51 (permalink)
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    ADM2483 module is Isolated ground
    SN75176 module is non -isolated ground.
    i have attached circuit for both device.  i have connected Non-isolated gnd module with 6 nos for 1000 meter cable with terminating resistor 100E and tested found ok
     
    similar test conducted for isolated module and results are ok.
     
    Now i am bringing 3 isolated and 3 non isolated module together with 100E terminator resistor with 1000 meter cable . communication with this combination found acceptable for 1 hour. Both device stopped working. But when i tested non isolated module it works normally without any issue. where as isolated module (ADM2483) completely stop communicating.

    Amps
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    #4
    Mysil
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    Re: Communication RS485 design 2020/10/24 01:20:12 (permalink)
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    Hi,
    Where is VCC2 coming from?  Is it the same power supply for all VCC2 on Isolated segment?
    Is GND2 connected together for all isolated devices?
    How do GND2 relate to "real earth" PE?  Is it connected to PE in one place? Is it connected to PE in many places,
    or is GND2 connected to PE by a resistor (1 MOhm).
     
        Mysil
    #5
    Antipodean
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    Re: Communication RS485 design 2020/10/24 05:51:14 (permalink)
    +2 (2)
    ajitnayak87
    ADM2483 module is Isolated ground
    SN75176 module is non -isolated ground.
    i have attached circuit for both device.  i have connected Non-isolated gnd module with 6 nos for 1000 meter cable with terminating resistor 100E and tested found ok
     
    similar test conducted for isolated module and results are ok.
     
    Now i am bringing 3 isolated and 3 non isolated module together with 100E terminator resistor with 1000 meter cable . communication with this combination found acceptable for 1 hour. Both device stopped working. But when i tested non isolated module it works normally without any issue. where as isolated module (ADM2483) completely stop communicating.




    Are all the non-isolated devices close together (i.e. same room or office block)? if spaced out on 1000m of cable there may well be ground problems causing common mode noise that is outside the common mode range for the transceivers, which may cause things to lock up. 
     
    What sort of environment are you using these in? Are you testing in a laboratory with a reel of wire, or is it wired throughout a building or industrial complex?
     

    Do not use my alias in your message body when replying, your message will disappear ...

    Alan
    #6
    ric
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    Re: Communication RS485 design 2020/10/25 04:56:17 (permalink)
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    It looks like Hen has quoted Anti-podean's message without mangling his username.
    He has that message in his signature for a reason...
     

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    #7
    Hen
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    Re: Communication RS485 design 2020/10/25 05:07:03 (permalink)
    +1 (1)
    Aha, ok, no clue, thank you for the hint Smile: Smile
     
    Just refered too Mysil and Negative-podean about RS485 and reliability.
     
    It's not always going to be reliable if not al systems draw power from the same point of the power grid, isolated or not.
     
    However with a separate isolated bus (including power i.e. four wires) you can be succesfull if you think about voltage drop, environmental factors suck location and weather.
     
    #8
    Hen
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    Re: Communication RS485 design 2020/10/26 01:47:33 (permalink)
    +1 (1)
    BTW, you can rather reduce wire count to three excluding the power but then you'll need a local DC/DC with adequate isolation. Alot of signal cables are twisted pair thus automagically give the power wire for free (two pairs).
    #9
    AMPS
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    Re: Communication RS485 design 2020/10/26 20:53:40 (permalink)
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    Are all the non-isolated devices close together (i.e. same room or office block)? if spaced out on 1000m of cable there may well be ground problems causing common mode noise that is outside the common mode range for the transceivers, which may cause things to lock up. 
     
    What sort of environment are you using these in? Are you testing in a laboratory with a reel of wire, or is it wired throughout a building or industrial complex?

     
    I have test setup in office itself . i am using CAT5 cable for evaluation. i have 2 nos of 1000 meter cable separate cable for testing . 1 no 1000 no connected for isolated and second cable for non Isolated version,.we are simulating field condition . Individually there is no issue. I have attached test setup here. first 6 device are very close to each other . My 6th device is connected to 1000 meter cable then to master . 6th device is having 100E terminating resistor. Individually  for isolated and non isolated working well. In both case i haven't connected GND point .
     
    For isolated RS485 i have isolated gnd from transformer. one gnd uses 12V to 5 v from 7805 . which i have used in primary side
     
    another gnd from transformer itself which gives seprate 5v and ground. 
     
     

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    Amps
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    #10
    Jim Nickerson
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    Re: Communication RS485 design 2020/10/27 08:23:39 (permalink)
    -1 (1)
    Are the 485 cables attached to the individual boards using plug in connectors ?
    #11
    NKurzman
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    Re: Communication RS485 design 2020/10/27 09:07:06 (permalink)
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    100 Ohm termination resistors, one at each end? none in the middle?
    What Baud Rate?
    what do the Signals look like when is is not working compared to working?
    How do you restore operation?
     
    #12
    AMPS
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    Re: Communication RS485 design 2020/10/28 01:28:02 (permalink)
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    i am working on 9600 baudrate . 100E i used @ end device. i haven't check waveform when all brought together . 
     
    But ADM2483 will dip in voltage when it brought in parallel with non- isolated design.

    Amps
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    #13
    AMPS
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    Re: Communication RS485 design 2020/11/02 18:34:15 (permalink)
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    Dear all,
     
    I have product which uses isolated and non isolated RS485 communication. I have issue with isolated design. I have used ADM2483 rs485 transceiver for communication. I have setup like this. by seeing this its obvious that unless i have 85-270vac aux supply i will have communication. since ground are different
    i don't have issue with above setup. problem occurred when i introduce battery charging unit here. 
    output of Battery charging unit 12V/GND are connected to same terminal of 12V/gnd 
     
    i found communication has stopped . if i remove battery charging unit it started working again.
     
    i have tried making communication ground of converter to secondary ground of adm2483 , but it didn't worked
    i have tried using 100E termination resistor but it didnt worked. 
     
    my question over here is
    1) if i wanted to make in all prospects means if i removed 85-270v ac primary connection also with 12V standby connected  how can i make it work. usually how isolated design will work in this scenarios.
     
    2) weather for isolated design, primary and secondary ground which are galvanized isolated  can be made common?? for ADM2483 . i didn't get any information on it. 
     
    3)Is there any article  where above case i can implement??.
     
    4) is there any alternate transceiver in non-isolated design which pin config is similar to adm2483 
     
     
     

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