Do i really need termination resistor in very short distance of rs485

Discussion in 'Embedded Systems and Microcontrollers' started by anhnha, Apr 23, 2012.

  1. anhnha

    Thread Starter Active Member

    Apr 19, 2012
    773
    47
    I have a problem when communicate between 2 computer use max232, max485 and 555.But the data that I have received is wrong.I have add termination resistor but it is not effect.Then I remove it and it work for a while then it is wrong again?
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,440
    3,361
    You have to show your circuit diagram.
    What computers are you using to send and receive the data?
    Are you using a crystal?
     
  3. panic mode

    Senior Member

    Oct 10, 2011
    1,320
    304
    and what is "very short"? provide actual distance.
    and what is 555 doing in RS232 or 485 connection?
     
  4. jimkeith

    Active Member

    Oct 26, 2011
    539
    99
    Non-standard RS232 drivers e.g. LM555 working against ground potential sometimes function without a negative supply--it depends upon the threshold level in your computer(s)--however, the signal zero level may be so close to the threshold that it may be flaky and not work all the time--I have seen this--make sure that the data voltage signal actually swings negative per the RS232 specification.
     
    Last edited: Apr 23, 2012
  5. anhnha

    Thread Starter Active Member

    Apr 19, 2012
    773
    47
    Thanks.
    Yes.I have just my circuit.
    I used a PC to send and received.I want to test it at home before used it in fact.
    No.My circuit is quite simple but I don't know why it is not work.I used the schematic in this:http://www.bb-elec.com/bb-elec/literature/485ptbr-3903ds.pdf
    and built it in breabboard.But It is not work.
    I even received data without power supply and GND of comm port.As for the GND of comm port is conneced and power is tied it is not work.I think my circuit is have no problem.
     
  6. anhnha

    Thread Starter Active Member

    Apr 19, 2012
    773
    47
    Yes.In this the it can be say have no distance.I test it at home with one PC and i connected 2 breadboard
    It is used to control DE and RE of max485
     
  7. MrChips

    Moderator

    Oct 2, 2009
    12,440
    3,361
    Generally speaking, yes, the A and B lines must be properly biased.
    I put a 1kΩ resistor from VCC to pin-6 (A), a 220Ω resistor from A to B, and a 1kΩ resistor from pin-7 (B) to GND.

    Do this at only one end of the RS-485 line.
     
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