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Defeating MAX232 and MAX 233.....

Discussion in 'Electronic Design' started by Jan Panteltje, May 19, 2007.

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  1. Well, I have this embedded system, and only 3.3V level out on serial.
    Asked for a MAX233 in the shop.. needed to be ordered, was expensive...
    Told them: Forget it, I will use transistors.

    But the second thing, No supply should be used, not even the 3.3V

    My solution? I control the terminal (Linux minicom) and the RS232 port too (outb() ).
    So here it is folks:

    Embedded connector

    9 pol D connector --------0 >
    TXD 3 -------------------------- 5k6 ------------------------ b NPN
    | e
    k |
    Diode 1N1448 |
    a |
    | |
    GND 5 -------------------------------------------------------------------0 GND
    +12V |
    DTR 4 ---------------------- |
    | | |
    | 5k6 |
    c | |
    b ------| |
    e NPN | |
    | | |
    RXD 2 ------------| | |
    | | |
    e | b
    b ------| ----- c e----- 3k3 --------------0 <
    c | |
    | PNP c | NPN
    | b------|
    | e NPN |
    | | 3k3
    - 12V | | |
    RTS 7 ------------------------------

    All NPN transistors BC547, all PNP transistors BC 557.

    As you can see the top is classical, but the reverse path uses a common base for level shifter :)
    I start minicom, hardware flow control off, and then after that a small program that sets
    DTR to +12 V and RTs to -12 V, there is my supply :)

    Maxim defeated :)
  2. Corrected diagram, the common base is a PNP of course :)
  3. Andrew Holme

    Andrew Holme Guest

    Nice level shifter :)

    What about using MOSFETs in the output circuit to eliminate base drive?
    Some RS232 ports have high source impedances, and the voltage can droop
    quite a bit when you start drawing milliamps.

  4. I think you should have a BE resistor on the input so that touching
    one's finger on the TXD line won't drive it nuts with line frequency
    signals. You could use all 10K resistors.

    You could also use the ST3232.

    Embedded connector

    Best regards,
    Spehro Pefhany
  5. Yes, right, I should perhaps have kept the 5k6 higher, but I am using flashing
    fast baudrates... and it was the second resistor I grabbed from the box...
    Are MOSFETS more expensive then junction stuff? I bought a load
    of junction PNP / NPN and they go into everything.
  6. Yes, true, I looked up that data sheet, but it needs caps I think.
    The MAX233 does not need caps on the charge pump I think.
    Maybe just a few resistors and junction transistors is even cheaper then
    a ST3232?

    This one could be integrated, if only everybody had full control of DTR and
  7. Jamie

    Jamie Guest

    that's cheating! :)
    Just hope the PC/Flap top supports the full voltages..
  8. Si Ballenger

    Si Ballenger Guest

  9. Yea that one is not to spec I think.
    I had considered the 3.3V zener method, but that
    1) pins you down to 3.3V (mine also works on 5V).
    2) those zeners are less stable, more dificult to get.
    3) an other capacitor.
    4) the output impedance driving low (towards negative) is only 4k7 in that circuit.
    5) it has 11 components, mine only 10.
    6) 4 different types of resistors, in my circuit all can be 5k6 if must be.
    7) less transistors, more looks good in the flyer.
    8) was not designed by me, so no fun.
    9) likely patented, mine is public domain now.
    10) Will think of some more over time, yes, mine is already build...
  10. Si Ballenger

    Si Ballenger Guest

    Are your assumptions of having power available from the DTR pin
    supported by all serial programs?
  11. If you re-read the thread (google?) you will find that I started out
    by saying that in this case I had full control over the terminal.
    That second circuit you referred to has the same issue.

    Anyways it makes sense to supply system inclusive cable (that has actually
    DTR and RTS wired), and software for the PC.
    Won't work with a null modem cable ;-)

    And it won't work in Vista... HAHHAHAHAHHAHAHAHA, I only support Linux.
  12. Joop

    Joop Guest

    I have never come around to build this, but found this years ago and
    kept it in my ideas book.
    Should be independent of embedded VCC level. A shunt resistor for
    input static protection might be wise. This is the version with the
    least amount of discrete components that I have seen.

    | |2K2
    | |
    RXD ||<-
    --------||-+ BS170

    BS250 +-||---
    TXD |
    | |2K2
    | |
    CTS |
  13. Si Ballenger

    Si Ballenger Guest

    Then I guess "Defeating MAX232 and MAX 233....." was an over
    stated bit of "fantasy thinking". ;)
  14. Would not a BS170 die if you connected the cable and there was some charge on
    Maybe some zener would be needed?
  15. Nope, it is working here.
    Vista is an insult to computing, more hardware and power needed to to less
    (because of constant encryption/decryption, why the thing eats power), and
    can no longer be considered sane electronics / programming, so it is irrelevant
    to computing and electronics.

    In fact this circuit is working so very good I am amazed.
  16. SioL

    SioL Guest

    Unfortunately it is far from irrelevant, but there's nothing me or you can
    do about it.
    You sure are easily impressed.

  17. Well i have been programming for Linux for >10 years now, many applications,
    look how MS starts to screek.
    So I *did* something about it.
    that proves it is possible to do somethin gabout it, start by installing Linux,
    not Vista.
    Not by you posts.
  18. SioL

    SioL Guest

    Well, Vista is still far from irrelevant so whatever you were doing those
    10 years... did not work, it seems Linux is loosing the desktop war.

    I appreciate the effort, though.

  19. Si Ballenger

    Si Ballenger Guest

    sour grapes



    affected scorn: the scornful denial that something is attractive
    or desirable because it is unobtainable

    [In allusion to Aesop's fable The Fox and the Grapes where the
    fox disparages some grapes as sour when he cannot reach them]
  20. I am familiar with teh expression.
    But it does not apply to a disgraceful attempt to show the world bloat
    and misuse of processor power, distrust of people, cheap go at commercial gain,
    Not to forget forced incompatibility with everything else.

    No, the label 'OS' does not apply to the Vista failure.
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