RGB Scart Lead

[MatAtari]

I bought an RGB scart lead on ebay for an Atari St 520stfm. It works but only displays composite mode, I can't get RGB to work, I tried on 3 different TV's which all support RGB.
I took the lead apart and the wiring is:

13-Pin DIN----------------Scart
1 (sound out)------------ 2+6 (Sound in left + right)
2 (Composite sync)------20 (Video in)
6 (Green)-----------------11 Green In + Ohm resistor
7 (Red)--------------------15 Red in + Ohm resistor
8 (+12V)-------------------16 Fast Blanking + Ohm resistor
10 (Blue)-------------------7 Blue in + Ohm resistor
13 (Ground)---------------17 Video Ground Ohm resistor



Also I don't know if anyone can tell from the picture above what rating the resistors are from their color? The three RGB resistors are the light brown color and the one on fast banking pin is light blue.

[techie_alison]

The problem is the +12v to the Fast Blanking line. That's what controls the RGB.

It needs to be connected to one of the V or H Syncs, can't remember which one. The 12v is used on pin-8 to automatically switch to Scart when the ST is plugged in.

The resistors on the RGB lines are correct. As the ST outputs 1v peak to peak, which is a bit much.

Using a resistor on pin-16 is very bad play, as you simply can't predict how much current the TV will draw. Resistors ARE NOT a means to achieve a set voltage. Well they are IF you know PRECISELY the Voltage and Current Draw. The ST's 12v is only a Pull-Up, and NOT a current source.

If I were you then I'd take apart the 13-pin DIN, and move the wire from 12v (pin-8) to VSYNC (pin-12). That'll get it working. You'll still have to manually switch to Scart though. And you may need to knock out that resistor too.

The cable is wrong. Well it's fine for TVs that can be manually forced to RGB, just that most modern ones can't.

This is what it should be;

Code: Select all

1. Introduction.
2. Atari ST 13-Pin DIN Pinouts.
3. Scart Euro Connector Pinouts.
4. Level Adjusting Resistors.
5. Atari ST to Scart Connector Wiring Assignments.
6. References.
 

1. Introduction.
================
This document discusses building a colour RGB Scart cable for the Atari ST.  It is relatively straightforward and no more complicated than connecting the right pins together.  The ST's RGB and SYNC signals are a little high being 1v peak-to-peak such that limiting resistors are recommended, they are not wholly necessary though to display a picture just that it may be slightly bright.
 

2. Atari ST 13-Pin DIN Pinouts.
===============================
The following is viewed from the back of the connector, ie. solder connections;
 
            *   *   *   *          4   3   2   1
 
            *   *   *   *          8   7   6   5
 
            *   *   *   *         12  11  10   9
                         
                  *                     13
 
       1 = Audio Output           2 = Composite Video
       3 = General Purpose Output 4 = Monochrome Detect
       5 = Audio In               6 = Green
       7 = Red                    8 = Plus 12-Volt Pullup
       9 = Horizontal Sync       10 = Blue
      11 = Monochrome            12 = Vertical Sync
      13 = GND
 

3. Scart Euro Connector Pinouts.
================================
The following is viewed from the back of the connector, ie. solder connections;
 
          20  18  16  14  12  10   8   6   4   2               
 
     +--------------------------------------------+
      \                                           |
       \   *   *   *   *   *   *   *   *   *   *  |
        \             Male From Behind            |
        |  *   *   *   *   *   *   *   *   *   *  |
        |                                         |
        +-----------------------------------------+
 
          19  17  15  13  11   9   7   5   3   1
 
       1 = Audio Output Right     2 = Audio Input Right
       3 = Audio Output Left      4 = Audio GND
       5 = Blue GND               6 = Audio Input Left
       7 = Blue                   8 = Function Switching
       9 = Green GND             10 = Comm Data 2
      11 = Green                 12 = Comm Data 1
      13 = RED/Chroma GND        14 = Comm Data GND
      15 = RED/Chroma            16 = Blanking
      17 = VIDEO/Sync GND        18 = Blanking GND
      19 = VIDEO/Sync Ouput      20 = VIDEO/Sync Input
      21 = GND
 

4. Level Adjusting Resistors.
=============================
Resistors of 150ohms to be placed on the Red, Green, and Blue lines.  75ohm resistor to be placed on the Vertical Sync to Fast Blanking line.
 

5. Atari ST to Scart Connector Wiring Assignments.
==================================================
ST Signal Name       13-Pin DIN        Scart        Scart Signal Name
--------------       ----------        -----        -----------------
Audio Output             1              2+6         Audio Input Right+Left
Composite Video          2               20         VIDEO/Sync Input
Green                    6               11         Green
Red                      7               15         Red
Plus 12-Volt Pullup      8                8         Function Switching
Blue                    10                7         Blue
Vertical Sync           12               16         Fast Blanking
GND                     13               21         GND
 

6. References.
==============
http://www.howell1964.freeserve.co.uk/Atari/ST/Atari_ST_Video.htm
 
http://www.algonet.se/~guld1/hardware/atari-scart.html
 
http://www.volny.cz/milarb/Atari/specable.txt
 
 

[MatAtari]

thank for info. If I understand from what you said I don't need to use Atari pin 8(+12) for anything, it just auto switches to scart channel. I can just use a tv remove instead to switch to AV.
I'll connect pin 12 (V-sync) to scart pin 16( but should I leave the resistor, because on the pinouts you posted it says "75ohm resistor to be placed on the Vertical Sync to Fast Blanking line.")

By the way this is the lead I bought,
http://cgi.ebay.co.uk/Atari-ST-520ST-10 ... m153.l1262
I thought I wouldn't have to mess around with it but looks like I may as well have built my own. This was the only place in the UK I could find the lead.

[techie_alison]

Yep, no 12v.

Just start by desoldering that wire on the 13-pin DIN plug, and connecting it to VSYNC. They're right next to each other, no it should be straight forward.

Then try it out. Worry about the resistor later if it doesn't work, then take that out.

The 75-ohm in my listing isn't strictly necessary.

In some respects ALL Scart cables for the ST are a hack, even the ones that I make, as the ST wasn't designed for Scart, it was designed for an RGB monitor with Vertical and Horizontal Syncs. It just happens to work with a Scart socket, with some fudging of the Sync signals to make it work. That usage of the 12v line isn't particularly clean, given what I've already said about it being a pull up and resistive loads. It may work on their television, but it doesn't mean that it'll work on everyone's.

Swap the wire over, and report back. You may get lucky, and will be a happy bunny.

Oh, and one last thing. Not every Scart socket on the back of TVs is RGB implemented... It's usually only one of them, often the first Scart socket.

[Anemos]

here is main diagram,working perfectly with pal European,s TV,s CRT-LCD.