[Coco] Dragon32 with factory Daughterboard SAM chip

[Phill Harvey-Smith]

On 31/10/2013 21:04, Luis Antoniosi (CoCoDemus) wrote:
> this is for the PAL timings right ?
>
> since there is no PAL 6847 the extra lines are added by a secondary chip.

Well in the case of the Dragon a bunch of 74LS & 4000 series CMOS chips, 
basically what happens (I believe), is that for 25 or so lines at the 
start and end of each frame the 6847 clock is stopped and extra HS 
pulses are generated by an external circuit as this happens in the 
border region it in effect makes the border bigger but increases the 
number of lines close to the 625 for PAL, it also has the effect of 
slowing the frame rate down to 25fps too.

I believe that the addon board did clean up the output by cutting down 
interference between the CPU & video.

Cheers.

Phill.

-- 
Phill Harvey-Smith, Programmer, Hardware hacker, and general eccentric !

"You can twist perceptions, but reality won't budge" -- Rush.