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MAX232 testing

Discussion in 'Electronic Basics' started by FredBear, Jan 29, 2004.

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  1. FredBear

    FredBear Guest

    I stuffed up while building a cable for my casio calculator.
    The circuit http://hw.cz/english/constrc/casio/casio_en.html uses a
    MAX232.

    The prob is I initially soldered up the 78L05 around the wrong way. & when I
    tested I couldn't get comms
    so I went back & rechecked etc...

    So now I have the 7805 around the right way and I get 4.58v measured on pin
    16 pin 15 of the MAX232
    But still no comms.
    My breakout box shows pin 20 going high & it has 12V.

    Since this is such a basic circuit, I'm starting to think that I've 'blown'
    the Max232....

    Any ideas / help on how I ca test the MAX & or the circuit would be
    appreciated.

    FWIW I'm using 1uF Caps for C1 & C2 & C3 and 2.2uF for C4 & C5 (these are
    what I had avail)

    BTW I'm fairly new @ electronics etc, so please don't make any assumptions
    about basic stuff! :)

    As an aside, the following page has a similar circuit except for the pinouts
    on the RS232 & an extra diode, as far as I can see...
    is this a 'better' design?
    http://www.gbnetwork.co.uk/circuits/boss/index.html

    TIA For any input / help
     
  2. Tim Dicus

    Tim Dicus Guest

    Hi FredBear,

    Check each "TTL in - RS232 out" converter.

    Put 0 volts into pins 10 and 11, then check pins 7 and 14. Should have about 10 volts output on each.

    Put 5 volts into pins 10 and 11, then check pins 7 and 14 again. Should have about -10 volts output on each.


    If that succeeded, then check each "RS232 in - TTL out" converter. Connect pin 7 to pin8. Connect pin 13 to pin 14. This will be a
    "loopback test".

    Put 0 volts into pins 10 and 11, then check pins 9 and 12. Should have about .6 volts output.

    Put 5 volts into pins 10 and 11, then check pins 9 and 12 again. Should have about 4.5 volts output.


    By this point, you should be able to tell if all is well, or where the damage is.

    Tim
     
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