strange problem in breadboard

Discussion in 'Embedded Systems and Microcontrollers' started by anhnha, Apr 22, 2012.

  1. anhnha

    Thread Starter Well-Known Member

    Apr 19, 2012
    Hi all,
    I have wire up two circuit that are same to communication between 2 computer:
    + Rs232 to rs485
    + Rs485 to rs232
    Both circuit of 555 timer to control receiver and transmiter.And two circuit are identical and half dulex.I have program to trnsmit data to comm port but there is a strange that happen.It can transmit data from one breadboard_1 to breadboard_2 but I can not transmit it from the breadboard_2 to breadboard_1 despite they are identical.I have check all connection of them and they are good.I also check idle state of them and they are same.Would you show me what really happen in this? And I have a confusion about the 232 standard.I know that:
    + 1 logic <=> -12 to -3 V
    + 0 logic <=> 3V to 12V
    but when I transmit data and I used Voltmeter to measure the voltage of TX and I supprise that the voltage is always negative about -11V.I don't know this happen.I think the data have many 0 and 1 logics and the TX have to have all value from -12v to +12v :confused:
  2. MBVet05

    New Member

    Jul 21, 2011
    Check to make sure your TX and RX lines are not reversed. I have ran into this problem on occasion. You can also check this or resolve it with a null modem. There are also RS232 to 485 conversion units that might make the transfer work better.
  3. anhnha

    Thread Starter Well-Known Member

    Apr 19, 2012
    I checked all connection and they are good.I aslo check by run a program to sent and receive data with TX and RX is tied to each other.The result is good all data is exact what i sent.
    I have used this schematic in this:
    but I used max232 and max485 in place.
    I built 2 circuits like this and used it to test with to send and receive in a comm port.But It is not work.I even received data when the power is not connected.
    and GND of comm port is removed.But when power is available and GND of comm port is connnected I received nothing.Would you show me what is wrong in my circuit or suggest me how to debug it?