SCART pin 16 RGB select

[john]

Hi there, I've signed up to your site just to ask this question, as you seem to be knowledgable about these things. All my consoles use RGB SCART leads, and I'm now trying to make up a lead to connect my PC VGA output to an RGB SCART input. Until recently I didn't even know such a thing is possible, but apparently it is, as long as you can modify the timings coming out of your video card and set it up to output interlaced.

I'm using the design at http://www.nexusuk.org/projects/vga2scart/ and I've got everything clear except what to do about SCART pin 16. This is the pin that is supposed to be between 1V and 3V to tell the TV to display RGB input instead of composite. I have determined that my TV does indeed need that pin high (apparently not all TVs do).

On the nexusuk page he links to a circuit (http://www.nexusuk.org/projects/vga2scart/...age_from_vsync/) which has four components in it. I think (although I'm no expert) that this circuit is called a 'voltage divider'. Strangely he takes the input from the Vsync pin, when there's a perfectly good +5V on VGA pin 9.

Alternatively, people making up the same cable on http://thegreenbutton.com/forums/1/168107/ShowThread.aspx are simply using a 68 Ohm resistor connected between VGA pin 9 and SCART pin 16 (see http://img142.imageshack.us/my.php?image=cable1dy6.gif). Similarly, all the games console leads shown at http://members.optusnet.com.au/eviltim/gam...t/gamescart.htm use just a 180 Ohm resistor.

Which is the way you would recommend to get 1 to 3V on SCART pin 16? I would rather not use the more complicated circuit on nexusuk if I can get away with just a single resistor.

[NeWmAn]

Hi,
Not all the VGA cards have the +5v on the 9th pin, so the circuit from nexusuk may be needed in those circumstances.
Since you have the voltage on yours, then you can go with the simpler (resistor or diode) solution.

[john]

Ah, thanks for the explanation. I didn't realise that not all cards had the +5V on VGA pin 9. I have to admit I haven't actually tested the voltage on my particular card yet, I just assumed it was always +5V going by http://en.wikipedia.org/wiki/VGA_connector.

When I make up this lead I'd quite like it to work with other PCs/video cards too. I'll only be trying to use it on modern cards, not 'retro PCs'. Does anyone know how likely it is that modern video cards will have +5V on VGA pin 9?

Also, what do you think is the best value resistor to use? Why does http://img142.imageshack.us/my.php?image=cable1dy6.gif use 68 Ohm, http://members.optusnet.com.au/eviltim/gam...t/gamescart.htm use 180 Ohm, and http://www.retroleum.co.uk/PALTVtimingandvoltages.html uses 100 Ohm, all from a 5V input? Is this due to the range of voltages (1 to 3) allowed at pin 16?

Interestingly, I measured the voltage output at pin 16 from various official SCART leads connected to my consoles, and they were 2.8V (Xbox), 4.5V (Wii), and 12V (Dreamcast)! All of them seem to work fine on my TV. Does that mean that maybe the actual voltage doesn't matter too much, as long as it's higher than 1V?

[viletim]

The voltage to SCART pin 16 is important - in practice it should be between 1 and 2 volts. The pin has an input resistance of 75 ohms (ie. there is a 75 ohm resistor connected between this pin and ground inside the TV) so by far the easiest way the correct voltage to this pin it to take a higher, regulated voltage and drop it to the correct value with a resistor divider.

Code Select Expand

.                 pin16
. +5v -----[Rs]-----o----[Rt]---GND
.          180            75

I like to use a 180 ohm resistor because it resaults in about 1.5 volts on the pin. Enough voltage to make the switch in all circumstances while not wasting too much power through the resistors. I don't reccomend using values as low as 68 ohms for Rs because it causes excessive heat dissapation in Rt (which in some cases it a tinly little SMD resistor). Use 5/(Rs+75) * 75 to see what voltages resault from different resistor values.

You can only get an accurate voltage measurement when the cable is plugged into the TV.

And I think most (all??) AGP video cards will have +5v on pin 9. Definately many PCI cards do not...

[john]

Many thanks for the explanation. As you've probably guessed, I'm no expert when it comes to electronics. When I measured the voltages of the console SCART leads (between pin 16 and the SCART shroud) I didn't have them plugged into the TV, so maybe that explains the odd values I was seeing.

I'll use a 180 Ohm resistor then.

Thanks also for the info re: video cards. I assume most PCI-e cards will have the +5V on pin 9 too.