RE: VDC bitmap with 8x2 cells

From: Richard Atkinson (
Date: 2002-02-07 06:28:36

On Mon, 4 Feb 2002 wrote:

> > than I expected; any idea what the node pulled high by R266
> > (2K2) is for?
> I don't know what IC251 is but connecting two outputs means it is a
> OC-uotput IMHO. The resistor makes sure of a defined level when none of
> those outputs are active.

My brain must have been on strike when I first thought about that circuit.

The node is always low unless RGBI is 1100. In that case, the outputs of
the invertors on the R and G paths will be high, and the diodes on the B
and I paths will be connected to invertor outputs which are also high.
This is then inverted by IC252/6C and pulls the main G node low through a
1K resistor (R271).

I think it's basically a little modification that makes dark yellow look
more like brown. To be true to Commodore's intentions for the C128 VDC, I
think you have to incorporate this into VDC SCART leads. It's a matter of
personal taste, of course. I would be interested to hear if there are any
Commodore 80 column RGBI monitors which don't have this circuit.


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