I have a vga that can be jumpered to greyscale , I don't think that putting resistors on the RGB line will remove color alltogether... maybe your card has a utility to force MONO mode?

keropi wrote:

I have a vga that can be jumpered to greyscale , I don't think that putting resistors on the RGB line will remove color alltogether... maybe your card has a utility to force MONO mode?

sure it will work: you connect them all in parallel. You just need to find the correct voltages as the monitor expects 0.7 V peak to peak 75 Ohms

This thread reminds me of my self-made VGA cable for the Atari ST.
It works pretty much in reverse to what you people said.
http://atariage.com/forums/topic/170385-st-to … e-only-adapter/

retrocanada76 wrote:

sure it will work: you connect them all in parallel. You just need to find the correct voltages as
the monitor expects 0.7 V peak to peak 75 Ohms

I wonder,s ince the VGA card is essentially the transmitter, are not there any caps required ?
To avoid a short-circuit or reverse running energy (yes, the resistors are there and avoid a full short, but still)?
For example, schematics of S-Video to Composite adapters also have a capacitor inside,
to combine two signals, luminace and chrominance. And I recall that the SNES cable had caps inside, too.
(In some flavors of the SNES, they were part of the console.)

Jo22 wrote:

This thread reminds me of my self-made VGA cable for the Atari ST. It works pretty much in reverse to what you people said. http […]
Show full quote

This thread reminds me of my self-made VGA cable for the Atari ST.
It works pretty much in reverse to what you people said.
http://atariage.com/forums/topic/170385-st-to … e-only-adapter/

retrocanada76 wrote:

sure it will work: you connect them all in parallel. You just need to find the correct voltages as
the monitor expects 0.7 V peak to peak 75 Ohms

I wonder,s ince the VGA card is essentially the transmitter, are not there any caps required ?
To avoid a short-circuit or reverse running energy (yes, the resistors are there and avoid a full short, but still)?
For example, schematics of S-Video to Composite adapters also have a capacitor inside,
to combine two signals, luminace and chrominance. And I recall that the SNES cable had caps inside, too.
(In some flavors of the SNES, they were part of the console.)

The caps are needed for the AC coupled signals (like chrominance). I think 0.7v is a low voltage and the 1000K resistor will be enough...

If you look at the VGA output side is a bunch of resistors in parallel for the DAC.

And to force a Green monitor you only input the green channel. To make a amber you will need to connect red and half-green. To make half green add another 75 ohm in series.

From adapter >>>>> to monitor.

R -----Resistor-| |---- R
G -----Resistor-|-|----- G
B -----Resistor-| |---- B

I wil try with 100ohm resistors first. Should not shot circuit. Maybe the picture is too bright. Will see.

Hey, i have had even a short circuit in the RGB lines, because on faulty VGA cable, and then even nothing happened, only colors got weird.

I want to play DooM with mono monitor and pc speaker, like in my school, in 1994.

Cyberdyne wrote:

From adapter >>>>> to monitor. […]
Show full quote

From adapter >>>>> to monitor.

R -----Resistor-| |---- R
G -----Resistor-|-|----- G
B -----Resistor-| |---- B

I wil try with 100ohm resistors first. Should not shot circuit. Maybe the picture is too bright. Will see.

Hey, i have had even a short circuit in the RGB lines, because on faulty VGA cable, and then even nothing happened, only colors got weird.

I want to play DooM with mono monitor and pc speaker, like in my school, in 1994.

NO ! I can't just ignore the color information on this. You need to extract the LUMINANCE! If you don't know what is is check it here:

https://en.wikipedia.org/wiki/Relative_luminance

You must respect this rule: 0.3 RED + 0.59 GREEN + 0.11 BLUE

Use variable resistors 1K and get for example the green: if you set it to 500Ohms, then Red should be 0.3 * (0.5 / .59) = 0.25 (250 Ohm) and blue = 0.11 * (0.5/0.59) = 0.09 (90 ohms), try different values, if you need lower and higher precision switch to 500 ohms resistors multiturn

Yeah, if you have a VGA to YPbPr adapter, plug in just the Y.

Well in this case overthinking is a bad thing, from my observation and testing, just shorting RGB is the best way to go.
Only some 16 and 4 color games need to only emitt Green to all colors, then color separation is better.
But for 256 color games, best way to go is brute shorting.

And i have made a universal two switch adapter. One switch shorts RGB. And one switch disconects R and B.
And you get a nice litle gadget, that lets you switch between monochrome or color and green monitor emulation.
It works great. I needed only 2 4pin power switches and 2 VGA sockets.

Hey even with some video cards, if you short RGB before turning on the computer, then switch back to color, it still stays mono, because the card thinks you have e mono monitor 🤣