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MAX232 testing

Discussion in 'Electronic Design' 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 can 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

    The reason I ask is, I checked the PIN2 -> pin 15 & get 8.5V and pin15 & pin
    6 gives -8.2v
    Isn't it supposed to be +10 -10v ?? or is that due to my cap choice?


    TIA For any input / help
     
  2. Tim Dicus

    Tim Dicus Guest

    Hi FredBear,

    Troubleshooting this IC is not difficult.

    Power up the MAX232 and check voltage across supply pins to insure correct voltage input.

    The following two tests 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
     
  3. budgie

    budgie Guest

    The 'extra diode' improves things, but there are a number of issues with all
    these designs.

    1. The ability of serial port handshake lines to source enough current to
    feed a MAX232 is questionable in a lot of modern com port implementations.
    Several years ago I made up a few of these interfaces for my high school kids,
    and the results varied from great to nogo as they were moved from one computer
    to the next.

    2. Even if the port can supply the current as above, the 78L05 has a
    reasonably unfriendly Vin-Vout differential. Again, we used a Seiko S81250 LDO
    regulator and that improved things. For the same reason, using Schottky diodes
    cut the v9olt drop across the diode(s).

    An even better solution can be found at - bugger, it's not there anymore.
    Anyway, although I use heaps of MAX232 chips in other interfaces, I DON'T use
    them in port-powered types. For the Casio I use a circuit which has a 74HC14 as
    the interface device. As the current required is dramatically lower, the above
    issues with MAX232 designs cease to be issues.
     
  4. Leon Heller

    Leon Heller Guest


    [deleted]

    Apart from the other suggestions, you could try checking the voltages on
    the capacitors. They will show if the charge pump is working. I think
    the data sheet tells you what to expect.

    Leon
     
  5. dont know

    dont know Guest

    hi
    there are acually two versions of the max232
    one is the max232 that requires 1uF caps for opertion and the other
    is a max202 the requires .1uF.
    Our purchasing manager gets us both of them, (disti tells him they are
    the same) but in the end you got to watch what is on the assembled pcb
    End result: use the right caps for the IC's
    rw
     
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