480z system disk on ebayu

Tony Duell ard at p850ug1.demon.co.uk
Tue Oct 4 18:05:17 CDT 2005

> > What RGB board? The Hi-Res board has a 8 bit output, that's encoded to 
> > RGB by 3 resistor DACs on the little board behind the BNC sockets. Isn't 
> > that the board we're talking about.
> Ahh, if that's just RGB between the Hi-res board and the BNC board, then 

Not really RGB...

There's a colour lookup table (a couple of TTL RAMs) on the hi-res board. 
It outputs an 8 bit value depending on its contents and the contents of 
the video memory for the current pixel.

The output of that lookup table goes to 2 places. The first is an 8 bit 
DAC on the hi-res board. That's used for the monochrome greyscale output.

The other is to the 26 pin connector. To that you can connect the RGB 
output board or a PAL encoder (which I have never seen). The RGB output 
board, which is mounted on the bracket for the BNC connectors, is 3 
simple DACs (3 bits each for R and G, 2 bits for B) and a circuit to 
produce sync-on-green if you need it.

> it likely is just a fault on the BNC board. I'd just wondered if there 
> was some sort of signal passing back to the hi-res board from the BNC 


> board such that the actual fault was somewhere in the hi-res board 
> itself but that it'd work fine without the BNC adapter board plugged in.

If the hi-res board works without the RGB board, then the problem is on 
the latter.


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