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This adapter (developed by me / H2O), gives all 16 colors with a minimal amount of chips (2 or 3).  This adapter provides ALL 16 colors:
  • the fussy "dark grey"
  • the notorious "brown"
  • and all other colors with maximum saturation
I hope you like it (details follow)!

  How It Works  
It works mostly like my prior published solutions, giving RGBI -> RGBa video, but this time there are dedicated circuits for "Dark Gray" and "Brown".  By a minor miracle, both "Dark Gray" and "Brown" colors are provided by a single chip.  Honestly, "Dark Gray" needs help from another chip (to reverse the polarity), but that other chip is also (mainly) required for correct Synchronization, so there is no "extra" chip...  See the electronic schematic below for technical details...

Schematic of RGBI-to-RGBa Adaptor with all/true 16 color output

To summarize, the 7486 XOR chip, the 74LS138 decoder chip, and about a dozen diodes and resistors are required for the "ultimate" RGBI conversion!  Note my circuit only creates an RGBa output (analog RGB), so you still need a VGA adapter to connect to a typical computer monitor.  The Ambery VGA adapter is a widely available, moderately priced, adapter which does a good job (once you figure out its various settings).  That adapter needs an HD-15 input (hence the output of my adapter), and it also requires +12V to power it.  To make things easier for me, and hopefully you, my circuit includes a 7805 to get the +5 volts it needs from the required +12V.  If you happen to have a stable +5V power source, then you can use it directly, and omit the 7805 and its associated capacitors.

Note that some VGA adapters will work directly with V-Sync and H-Sync (if so, you might eliminate two XOR gates [I said "might" because of polarity issues]).  Some VGA adapters work with either positive or negative composite sync (if so, you can eliminate one XOR gate).  The Ambery VGA adapter has inputs for H-Sync and V-Sync, but for some reason they do not work at the frequency provided by the C128/VDC (approximately 15KHz horizontal).  Unlike some other adapters, the Ambery VGA adapter only works with negative composite sync, so my circuit uses two XOR gates to generate a composite, negative "Sync" (see bottom middle of the schematic above).  In short, if you have a VGA adapter which is more flexible than the Ambery model, you can eliminate one or two XOR gates.

  Circuit Board  
Enough theory!  Below is a photo of the circuit board I built.  I labeled many of the components in the photo.  Just to clarify:
  • "5V Regulator" is the 7805 chip (adapts the Ambery-required 12V to my circuit's needed 5V)
  • "3-to-8 Decoder" is the 74LS138 chip (it generates a special signal for "Dark Gray" and "Brown" colors)
  • "XOR gates" is the 7486 chip (4 XOR gates = 1 for "Dark Brown" + 2 for Negative Composite Sync + 1 unused)
Unlike many of my previous RGBI adapter circuits, this one has no variable resistor ("pot") for Intensity (the I of RGBI).  That is because this circuit gives pure/crisp colors for both "standard" and "light" colors (e.g., "Red" and "Light Red").  In other words, except for my very first "Pull-Down" circuit (which lacked "Dark Gray"), all others "polluted" (desaturated) the "light" colors.  This circuit doesn't use the "pollution" (desaturation) method, so there is no need for an "I-pot", and is another reason I call this the "Ultimate".

In the photo you can see five (5) variable resistors ("pots").  Three of them are for the basic Red, Green, and Blue (RGB).  I used 2K pots to give the "best" (this is an opinion) option between "usable range" and "sensitivity".  You could use 1K pots, but then you might have a "range" problem (for example, if the colors are too bright with maximum resistance, there is nothing you can do).  Or you could use 3K (or 5K) pots, but then you could see a relatively large change in brightness with a relatively small twist of the pot (a "sensitivity" problem).  So I think 2K is ideal, but I have built similar ones with 3K and 5K pots with success.

Labeled circuit board of RGBI-to-RGBa Adaptor with all/true 16 color output

Besides the expected "Red", "Green", and "Blue" variable resistors ("pots"), there is also one for "Dark Gray" and another for "Brown".  The "Dark Gray" pot allows you to change its color from approximately "Black" to "Light Gray".  The "Brown" pot allows you to change its color from "Dark Red" to "Dark Yellow".  Also note that I took this photo before I added strain-relief to the input and output cables.  You really need to add strain relief to cables going into/out-of a box... otherwise anybody who "pulls" on the cables (by accident or malevolence) will probably rip the wires out of the circuit board... then either the board will silently fail, or perhaps (Murphy's Law!) destroy your computer, monitor, VGA adapter, or power supply!!  Finally note I built the case out of aluminum, per my client's request.  Not only does it give a shiny appearance and a very solid feel, but it should also shield the circuit from outside interference (and minimize "broadcast" interference).  However, I have built similar circuits using a plastic case with good results (plastic is cheaper and slightly easier to sculpt).
  Screen Shot  
"The proof is in the pudding" is the saying which comes to my mind at this point.  Below is a photo of a VGA monitor using my "Ultimate" RGBI converter with the Ambery VGA adapter.  The photo shows all 16 colors to be very distinct.  The colors are all "crisp" (no desaturation).  In particular, "Brown" is really brown (and not "dark yellow"), and "Dark Gray" is labeled as "L[ight] Black" (which is correct from "a certain point of view" to quote Obi-Wan Kenobi).  This photo is from the C128 operating in standard (non-interlace) mode.  I also tested it with interlaced-text mode and got excellent results (I'll see if I can find a photo of that for you).  The only thing I didn't test is all the non-standard video modes that the VDC of the C128 can generate.... because the Ambery VGA converter works differently with different horizontal frequencies (no official documentation on limits), my solution (or rather the Ambery converter) might fail with custom graphic modes of the C128.  In other words, I haven't tested everything (could anyone?), but what I have tested works superb... which is yet another reason I call this circuit the Ultimate.

Photo of a VGA monitor using my Ultimate RGBI-to-RGBa Adapter (and Ambery VGA adapter) with all/true 16 colors!

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