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Vga dac

Lee MarshallLee Marshall Posts: 106
edited 2007-10-03 22:09 in Propeller 1
I have noticed that the resistor ladder Composite/VGA DACs in the prop manual and in the protoboard seem to be able to output 3.3v assuming that all 3 bits are set to 1.
from what i have learnt so far, the max voltage into a vga monitor signal is supposed to be 0.7v peak to peak.
would, accidentally pulling a signal to over 0.7v be damaging, or is there some kind of protection circuitry in modern monitors?

also, what are the voltage levels for TV composite?

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Holy parallel processing, Batman!

Post Edited (Mr Crowley) : 10/2/2007 9:04:06 PM GMT

Comments

  • Harrison.Harrison. Posts: 484
    edited 2007-10-02 20:03
    There is generally a 75 ohm termination resistor in the monitor, which forms a voltage divider with the 2 bit DAC resistors.

    From my best guess, the following happens when connected to the monitor:
    1/(1/470 + 1/420) = 221.8 ohms (both DAC resistors in parallel)

    (75 / (221.8 + 75)) * 3.3 = 0.834 V across the 75 ohm resistor

    So, the monitor would be *seeing* a 0.834 V signal.
  • Mike GreenMike Green Posts: 23,101
    edited 2007-10-02 20:12
    The Propeller outputs don't really reach 3.3V. The best you'll get is maybe 3.0V. If you plug that into your formula, you'll see the result is 0.75V which is as it should be.
  • Harrison.Harrison. Posts: 484
    edited 2007-10-02 20:17
    Thanks for the explanation Mike. Now it makes sense why Parallax chose those values.
  • Lee MarshallLee Marshall Posts: 106
    edited 2007-10-02 20:38
    thanks for the quick replies, this has been bugging me for a while.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Holy parallel processing, Batman!

    Post Edited (Mr Crowley) : 10/2/2007 9:12:55 PM GMT
  • Paul BakerPaul Baker Posts: 6,351
    edited 2007-10-02 20:41
    Hey Mr Crowley, we generally discourage people from placing graphics in thier sig line. The way the forum works, each is considered a seperate graphic and it can cause headaches for our server. I suggest cropping it down to the die itself and using it for your avatar pic.

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    Paul Baker
    Propeller Applications Engineer

    Parallax, Inc.
  • Lee MarshallLee Marshall Posts: 106
    edited 2007-10-02 21:10
    Paul Baker (Parallax) said...
    Hey Mr Crowley, we generally discourage people from placing graphics in thier sig line. The way the forum works, each is considered a seperate graphic and it can cause headaches for our server. I suggest cropping it down to the die itself and using it for your avatar pic.

    Done, sry bout that.

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Holy parallel processing, Batman!

    Post Edited (Mr Crowley) : 10/3/2007 1:42:27 PM GMT
  • deSilvadeSilva Posts: 2,967
    edited 2007-10-02 21:47
    Harrison. said...
    Thanks for the explanation Mike. Now it makes sense why Parallax chose those values.
    And you always thought they did it just for fun smile.gif
    In fact TVs are by no way such touchy. I soldered some video interfaces during the last weeks for my class and took the values out of my standard box: 1k, 470R, 220R
    This is absolutely fine!

    Edit: I even used a 7 resistor SIL package @1k once, shorting 2 and 4 of the resistors for the bit1 and bit2 output. This looks very professional, but is by no means practical and takes more space on a small perf board.

    Post Edited (deSilva) : 10/2/2007 9:56:46 PM GMT
  • Lee MarshallLee Marshall Posts: 106
    edited 2007-10-03 19:45
    I just want to be sure here, TVs also have this 75-ohm resistor in the composite input?

    ▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔▔
    Holy parallel processing, Batman!
  • deSilvadeSilva Posts: 2,967
    edited 2007-10-03 22:09
    Absolutely! Half the world would stop spinning if they hadn't smile.gif
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