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VGA resistor problem — Parallax Forums

VGA resistor problem

solarprismsolarprism Posts: 23
edited 2010-12-05 03:07 in Propeller 1
HI everybody,sorry for being annoying lately and thanks for all your help.
I make a Protoboard myself and when i solder the resister network for VGA port something came in my mind
2 bits with 2 resistor create 4 states:
bit 0
R
VGA pin
/
bit1
2R

The voltage V is voltage at VGA pin

00: V=0V
01: V=3,3*75/(R+75)
10: V=3,3*75/(2R+75)
11: V=3,3*75/(Re+75)

Re is the total resistor of R and 2R in serial or 2R/3

The maximum voltage at VGA pin can withstand about 1V (i m' not so sure) and the standard is 0.7V
So the maximum voltage happens at the last state,calculate back it give us the R is 417ohm
In the Protoboard i saw the value for R is 240ohm and some other FPGA board with 3,3V voltage output they use 330ohm /680ohm
So is there a problem when we use 240/470 or 330/680 pair?

Comments

  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2010-12-03 01:38
    Back when the Propeller was new, I was quite concerned about using 1% tolerance and getting this exactly right. The reply was that I was being too exact and that the ratios of the resistance are what produces the appropriate levels of voltage. 5% tolerances were quite adequate.

    Also the whole resistance ladder has to be looked at to decide if it will work well. It is the ratios that are important.
    Admittedly higher resistances will provide a bit less power, but that doesn't seem to be a critical issue either and the VGA inputs are rather high impedance.

    Since available resistors for exact ratios were not easy to get, Parallax accepted ballpark values, tested their use, and found them to work well.
  • solarprismsolarprism Posts: 23
    edited 2010-12-03 05:57
    uhm so that we only care about the ratio,can i use 60/120ohm resistor,i have some of them now :D
  • KaosKiddKaosKidd Posts: 296
    edited 2010-12-03 08:30
    I did the same thing when I made my development system. I was so worried about being precise that I got 20 turn 1% tolerance resistors to make my ladder. I tuned each until it read exactly what I needed, then hot glued the ends so it didn't stray. Yeah, it was just an exercise in being "overly cautious"; I really wanted the hardware to work the first time, thus if I made programming errors I didn't have to worry about determining if it was a mistake in my hardware connections or software... I learned...
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    edited 2010-12-03 09:10
    It's not just the ratio that's important: it's also the voltage output when driving a 75-ohm load. If your resistor values are too small, you can easily overdrive the VGA monitor's inputs. In the most benign cases, your colors will look washed out. In more extreme circumstances, you could damage the monitor.

    -Phil
  • solarprismsolarprism Posts: 23
    edited 2010-12-03 16:40
    Phil Pilgrim (PhiPi) wrote: »
    It's not just the ratio that's important: it's also the voltage output when driving a 75-ohm load. If your resistor values are too small, you can easily overdrive the VGA monitor's inputs. In the most benign cases, your colors will look washed out. In more extreme circumstances, you could damage the monitor.

    -Phil

    can you suggest some minimum value of resistor we can use?
  • solarprismsolarprism Posts: 23
    edited 2010-12-05 01:39
    :depressed: i guess i should try from 240/470ohm and above
  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2010-12-05 01:51
    I have never had any 240 Ohm resistors, I use 220s or 270s. We are not building calibrated test generators here.
  • Dr_AculaDr_Acula Posts: 5,484
    edited 2010-12-05 03:07
    I use 220 and 470 ohm too. Works fine. The eye is remarkably tolerant to color errors anyway - compare the blue on a screen to blue on a print - the print needs to be almost cyan to match the screen. I wouldn't worry about 220 vs 240 ohm.
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