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RGBI to SCART

The Commodore 128 features an 80-Column color display using an RGBI output via the 8563, known as the VDC. It offers higher resolution than the 40-Column display of the VIC-IIe (8564 NTSC, 8566 PAL-B often called simply VIC).  The VDC also supports video interlacing in hardware.

The VIC is easy to connect to common video displays (TVs and monitors) due to various widely-used outputs: Composite, Chroma/Luminance (S-Video), and RF (analog TV antenna).

Unfortunately, the RGBI output of the VDC is not compatible with modern equipment. This poses a problem for C128 owners due to the dwindling supply (and expensive shipping) of RGBI and CGA monitors (CGA is very much like RGBI). RGBI is similar to VGA, but uses different voltages and horizontal frequency which most VGA monitors do not like.

This page is about a SCART solution (other options are S-Video and VGA).  Although SCART has RGB lines, it doesn't accept seperated H-Sync and V-Sync and you may need to apply a voltage to the "Fast Switching Line" (pin 16).  More important, even if you can work around the Sync and Pin 16 voltage issues, you would only get 8 colors.  This is because SCART has no line for Intensity.

 
  All 16 Colors  
Recently (2014), Tero Säärelä emailed me to report he used my Pull Both circuit to successfuly connect his C128 to his TV using a SCART connection.  Based on his description, I drew up a circuit and sent it to him for verification.  I don't own any SCART monitor or TV, so I can't test this myself.  However I'm providing my circuit diagram along with some notes and photos by Tero.  So if you have a SCART display, you can try building one yourself, and enjoy all 16 colors from your C128!
 
Below is the circuit diagram.  On the left side is the male DB-9 connector that goes to the C128's RGBI port. In the center is the pull-both circuit that adds Intensity to the R,G,B lines. On the right is the SCART connector that goes to your monitor/TV.  This is all required.  On the left is another connector that might be needed, depending on your circumstances.  This is a male DIN-8 connector that plugs into the C128's "40-column video" port.  Now we don't need anything dealing with 40-column (VIC) video, but the connector does provide some useful lines that are not available from the RGBI port.
 
The main reason you may want the DIN-8 connector is to get 2 to 3 volts for the "Fast Switching" line on the SCART connector (pin 16).  This tells your monitor/TV to actually use the RGB lines, instead of the composite line.  However, if your monitor/TV allows you to force RGB mode then you don't need the "Fast Switching" line or the associated resistors.  Tero didn't them for his TV.
 
The other reason you may want the DIN-8 connector is to get audio from your C128 to your monitor/TV.  If you are using seperate speakers, or you don't care about audio, then you don't need to wire the audio lines from the DIN-8 to the SCART.  If you do want audio from your monitor/TV, then note the circuit below sends the standard SID output to both left and right audio lines of the SCART cable.  So if you have a second SID, then you will probably want to modify the circuit so each SID output line goes to a seperate audio line.
 
Schematic diagram of RGBI to SCART adapter for Commodore 128

 

 
Below are photos of the adapter Tero made.  On the left is the circuit board.  It doesn't have the resistors for "Fast Switching" voltage because his TV can be manually switched to RGB mode.  On the right is the completed adapter.  Note there is only a DB-9 and SCART connector.  No DIN-8 connector because he didn't need audio from his TV either.

Photo of RGBI to SCART circuit board. Photo of RGBI to SCART case and cables.

 
And last but not least, the results of this RGBI solution: a photo of his TV showing all 16 colors.  Some of the colors are hard to see; dark blue is one, and dark gray is worse, practically invisible (see notes below)!   This simple circuit provides true RGBI colors.  Most (all?) of the CBM monitors supporting RGBI actually had a special circuit inside them to convert dark yellow into brown.  I guess somebody thought it looked better that way (although several people have reported it looks more like orange).  Anyway this circuit does not have the "brown fix" (color 10).

Photo of TV showing 80-column text with 16 colors, using SCART

 
  Project Notes  
In this section are some comments by Tero (his photos are shown above), additional feedback by C128 forum member cthulhu (who also built Pull-Both to SCART circuit), and my own thoughts on this adapter.  So first, Tero didn't need to supply voltage to the "Fast Switching" line, he says that most TVs will need between 1 and 3 volts to enable RGB display.  The two resistors connected to the DIN-8 provide 2.5V so it should work, but I can't very this.  Also, you may be able to use higher resistors than 1K Ohm for them; doing so would reduce the power consumption of your C128.  I'm thinking 2.2K or even 3.3K could be used (they should both be the same value).  Cthulhu says he hooked +5V straight to his SCART cable without any resistors!  That might work for you, but personally I would use at least one resistor (omit the second one which is connected to ground / pin 3 of the DIN-8).
 
Tero also said the text wasn't as bright as he would like (without turning up the contrast on his TV).  Looking at that photo of his screen, you'd probably agree!  Cthulhu also reports the colors are a bit dark with his SCART adapter.  So it seems SCART does need smaller resistors in the pull-both circuit, if you don't want to fiddle with your TV's contrast settings.  I would try replacing the three 1K Ohm resistors with 820 Ohm resistors, and the one 1.2K Ohm resistor with a 1K resistor.  I wouldn't use anything less than 680 Ohms for fear of straining the line driver in the C128.  Sorry, I don't have SCART myself, so I can't tell you the optimal value(s).  Please contact me if you use different values and get a good display so I can let people know.
 
You can include a variable resistor (1K to 3K) at the top of the pull-both, for the Intensity, if you want simple manual adjustment.  However, this will only allow you to vary the relative brightness of all dark colors verses all bright colors (for example, light red verses dark red).  In other words, it won't allow you to adjust the over-all brightness.  If you wanted that abilitiy, it would be more complicated: you would need 1K variable resistors on all three of R, G, B lines (and you would have to tune them all to the same value, or else white/gray would be wrong).  So I don't recommend that unless you like experimenting.
 
If you look at the schematic, you'll see that the H-Sync and V-Sync lines from the RGBI port of the C128 are not being used.  SCART needs composite sync.  The monochrome line contains composite sync, so that is being used.  The monochrome line also contains video data that is not needed; some TVs might not like it.  However, it works for both Tero and Cthulhu so I would try that first.  If the screen "rolls" or "tears" or just has a lot of interference, then you might need to use a different method.  You can use a XOR gate (like a 7486) to combine the H-Sync and V-Sync into a "clean" composite sync.  In this case you would need the DIN-8 connector (or some other source of +5V) to power the XOR gate, and you might need a resistor on the gate's output to lower the voltage (see my RGBI to VGA adapter for an example of using a XOR gate).
 
Yeah, the XOR method is a bit more complicated, so hopefully the monochrome line will work for your TV.  Unfortunately, it seems that at least some of the C128-DCR models have a non-functional monochrome line (reported by Miro/Mirkosoft).  I'm not sure if this applies to all DCRs, or just a particular run of them, or maybe he just got a defective unit.  Miro says monochrome works fine on his C128-D and flat C128.

© H2Obsession, 2009, 2012, 2013, 2014
Photos of SCART board, case, and TV © Tero Säärelä, 2014.  Used with permission.
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