a Problem with the max232 level shifter

Discussion in 'Embedded Systems and Microcontrollers' started by a.salah, Apr 10, 2009.

  1. a.salah

    Thread Starter Member

    Mar 24, 2008
    after supplying the max232 with Vcc(5v)(before connecting it to the circuit) i noted something strange

    pins #10 and #11 are input pins but when measuring the voltage on them the multimeter reads 5v(without connecting them to any nodes)!!!

    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?

    What's wrong with the IC?

    Thank you
    [​IMG] [​IMG]
  2. davebee

    Well-Known Member

    Oct 22, 2008
    As long as the pins present a high impedance to the external signal, their no-load voltage is really irrelevant. The internal chip circuitry must be pulling the pins high, but it's the impedance of the pins as compared to the impedance of the driving signal that matters.

    As long as the input signal is a low enough impedance, it will force the pins to the proper voltage in the circuit, regardless of the pins no-load voltage.
  3. a.salah

    Thread Starter Member

    Mar 24, 2008
    thanx davebee for the explanation