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Better VGA DAC resistors - Page 2 — Parallax Forums

Better VGA DAC resistors



  • Cluso99Cluso99 Posts: 17,948
    edited 2011-10-26 10:39
    Drac: Don't forget the TV termination resistor to gnd. I would also change the values somewhat (see post #27 - plug values into spreadsheet #25 or #26)
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,049
    edited 2011-10-26 10:51
    I missed the TV connection. Dr_A, your situation is complicated by the fact that you're sharing the TV and VGA connections. This means that the voltage drop across the internal resistance of the Prop pin will derive from current through both circuits, even though you're only using one at a time. That will require a more complex analysis that what we've done here, since there's a codependence between the two circuits.

  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2011-10-27 01:37
    Many moons ago, on a Blade2 copy, I put an eight way resistor in reversed. Instead of getting P24 - 31 with 10K pullups I got a mind bending set of 20K interconnects.
  • Dr_AculaDr_Acula Posts: 5,484
    edited 2011-10-27 02:04
    Per the spreadsheet results shown in post #24, I'd change your 270's to 240's.

    Thanks Phil - have made the changes.

    Re the TV vs VGA, the *best* solution is to solder up one board for TV and another for VGA. With Gadget Gangster boards you can swap those in and out easily.

    Keep up the good work!
  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2011-10-27 06:46
    I seriously doubt that the vast magority of VGA cables have the characteristic impedance of 75 Ohms. They often just have "thin" co-axes or even single wires within an overall shield.

    Still it is good to get as close as possible.
  • Cluso99Cluso99 Posts: 17,948
    edited 2011-10-27 14:24
    Since Phil has done all the hard work here, may as well use the correct values. There is little cost in this.
    I just plug in VGA or TV modules into my baseboard - its easy as.
  • SapiehaSapieha Posts: 2,964
    edited 2011-10-29 04:35
    Sorry if it is of topic.

    BUT for any that need correct pin definitions and theirs Voltages on VGA.

    Look on attached picture

    VGA Connector Signals.jpg
    1006 x 616 - 57K
  • Oldbitcollector (Jeff)Oldbitcollector (Jeff) Posts: 8,091
    edited 2012-06-12 18:54

    Implemented your OP fix on my test rig tonight. Went from 48 countable colors to 64 colors with the driver I'm using.

    My hat's off to you. Nice fix!

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