regarding the max232(rs232 level converter)

Discussion in 'Embedded Systems and Microcontrollers' started by a.salah, Apr 5, 2009.

  1. a.salah

    Thread Starter Member

    Mar 24, 2008
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    i connected the max232 with its components(4 1uf capacitors).

    in the datasheet of the max it says that if the pin T1in or T2in received low voltage the o/p will be high if the i/p is high then the o/p is low.

    in my case the i/p voltage is 1.5v and the o/p is 3.7v !!!

    should the o/p be 12v to transmit the signal to serial port?

    i am supplying the max232 with Vcc=4.6v so what's wrong with it?
     
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    The Max232 is an inverting voltage doubler.

    1.5V is below the minimum threshold (2V Guaranteed, 1.5V typical) for a high signal. Check the datasheet
     
  3. a.salah

    Thread Starter Member

    Mar 24, 2008
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    i tried to input 2 and 3.5 voltages but the output still very low?
     
  4. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Post a schematic of the entire circuit, the problem may be somewhere else, or it could be connected wrong in wiring.
     
  5. a.salah

    Thread Starter Member

    Mar 24, 2008
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    now the first problem is solved but there is another problem

    after supplying the max232 with Vcc(5v) i noted something strange
    pins #10 and #11 are input pins but when measuring the voltage on them the multimeter reads 5v!!!

    it's supposed to be 0 because i want to input TTL signal and max232 invert it to Rs232 level.

    Note: i tried two different max232 ic's but the two have the same problem?

    Thank you
     
  6. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
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    Said who? :mad:

    The internal schematic clearly shows two 400K pullup resistors on pin#10 and pin #11. This will cause your DVM to measure about +5V even when nothing is connected to these pins.

    [​IMG]
     
  7. a.salah

    Thread Starter Member

    Mar 24, 2008
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    0
    Pin 10,11 voltage being +5v makes the T1out and T2out -12 v so the serial device receives -12v all the time which may interrupt the communication and causes unexpected behavior from the device
     
  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    When connected to logic, it will be held at 0 or +5. Are you not using it for this purpose? The pullup is to help open collector outputs of some logic ICs.
     
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