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Old 08 June 2008, 16:52   #1
Boot_WB
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Just managed to get my homebrew RGB - SCART cable working finally!

I treated myself to a new LCD TV for my desktop last week, which got delivered on Friday. Workbench via the Voodoo3 (on Prometheus) looks stunning compared to my old CRT, but I also wanted to display native modes so I can play games again. This TV has a SCART input capable of receiving both S-video and RGB signals - perfect (I thought)!

I used to have a couple of Amiga > SCART leads, but I seem to have got rid of them in a recent clearout, so I decided to created my own from a standard SCART-SCART cable, following the SCART_2_AMIGA hack from Aminet, but the first test resulted in a black screen - no input detected by the LCD.

Problem was, although I'd realised that Audio-in and audio-out were crossed over from one end of the scart to the other (pins 1 and 3 at one end connect to pins 2 and 6 respectively), I had not realised that the composite sync was also crossed (pin 19 at one end connects with pin 20 at the other) - hence no sync signal was being received by the LCD, ergo no signal detected.

Sorted that out, and was presented with a white screen. Hmmm... had to sleep on that one. This morning, did a bit of searching and thanks to EAB (AlexH's previous posts in particular, thankyou matey) and Wikipedia realised that for a SCART connection capable of receiving composite AND RGB signals, pin16 of the scart must have an input between +1.4V and +3V. Hmmm, Amiga RGB only has +12V, -5V, and +5V...

I measured the resistance across my LCD's pin16 - ground as 76 Ohm, therefore reasoned that inserting a ~76 Ohm resistor between +5V on the RGB port and pin 16 of the SCART should (fingers crossed) create a +2.5V input into pin16 of the LCD SCART socket (if I'd remembered my first year Mech Eng electronics module correctly). Found a suitable resistor, soldered it in and hey presto - steady as a rock signal through the SCART lead.

As a note, I'd advise anyone following this to measure resistance between pin16 and ground on their own TV scart input. For my TV (Samsung Pebble SM932MW) this was 76 Ohm, this may vary between different TVs though, and the higher the resistance on the TV scart socket, the greater proportion of the full +5V will be passed on pin 16.

The resistor you solder in between +5V out on the RGB port, and pin16 of the SCART should be NO LESS THAN 0.7 x measured resistance measured between pin16 - ground, and no greater than 2.5 x measured resistance to achieve the 1.4V - 3V required by the RGB sense circuit on pin 16.

I'm happy now, as I have a 19" LCD screen displaying my Voodoo3 output on the VGA port, and native Amiga modes on the EXT (SCART) port, and I can play Frontier again! First time since I got rid of my Phillips CM8833 monitor years ago!

EDIT: Corrected bad maths. Doh!
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Old 08 June 2008, 17:35   #2
alexh
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How do you switch between VGA and SCART? LCD menu?
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Old 08 June 2008, 17:38   #3
Boot_WB
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Quote:
Originally Posted by alexh View Post
How do you switch between VGA and SCART? LCD menu?
Yes - it's not automatic, so I use the remote control. There are buttons on the side of the screen to de exactly the same job, but the remote is a bit friendlier.

Regards



Rich
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Old 08 June 2008, 19:12   #4
Zetr0
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@Boot_WB

A Big thanks for the write up

heres a schematic that I made from Pinouts RU



However it produces this result



It needs a revision a little later today, as i think this is down to grounding.

I shall keep all posted.
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Old 08 June 2008, 20:10   #5
Boot_WB
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I think it may be because you do not have CSYNC GROUND (Pin 17 SCART) connected.

I have updated your artwork to show the layout I used*

* with the exception that I used pin 20 for grounding the CSYNC, however I now realise I should have used pin 19. As there seems to be no complaints from the TV or Amiga so far I'll only change it over if I get around to it. The layout drawing shows the corrected version.

Hope this helps - as per my previous post, the value of the resistor may need tweaking for your particular TV.

EDIT: Updated minor errors/inconsistencies on my version of the layout.

Regards



Rich
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Old 08 June 2008, 20:35   #6
Zetr0
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Awesome stuff, I appreciate that,

I will hack on tonight
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Old 08 June 2008, 20:47   #7
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Quote:
Originally Posted by Zetr0 View Post
Awesome stuff, I appreciate that,

I will hack on tonight

Hi Zetro,

I agree, On the scart side remove from pin 20 & solder to pin 17 instead.
Resistance values are cool, Thats pretty much how I wire mine...
It's probable that all the grounds are commoned either in the TV or Miggy anyway

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Old 09 June 2008, 22:34   #8
Boot_WB
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Quote:
Originally Posted by Zetr0 View Post
Awesome stuff, I appreciate that,

I will hack on tonight
Zetro, did you manage to get your SCART connection up and running?
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Old 09 June 2008, 22:38   #9
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@ Alexh

Have you considered 12v (if I remember correctly) to pin 8 of the scart, for auto switching of the TV over to the input?
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Old 09 June 2008, 22:41   #10
Boot_WB
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@Merlin

I have the 12v connected, however it does not autoswitch between the VGA (Amiga RTG) input and the SCART (Amiga native modes) input. I suspect it would switch between the TV input and SCART, but just not between VGA and SCART.

Regards



Rich

PS - I've got some stuff to put in the recycle bin on Amibay - any chance of granting me access?
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Old 09 June 2008, 22:44   #11
Merlin
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Ahh, I see, it's just that pin 8 wasn't connected on your diagram. Oh yes it is....

Last edited by Merlin; 09 June 2008 at 22:52. Reason: Being a plonker and not reading the diagram properly
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Old 09 June 2008, 22:47   #12
Boot_WB
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Hmm, you seem to be mis-reading the diagram. Referring to the diagram in post #5, Pin 8 of the SCART is connected by the purple line to the +12v output.

It must have been a long day!

Regards


Rich
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Old 09 June 2008, 22:53   #13
Merlin
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It's a fair cop, guv. I is a plonker.....
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