[Coco] How does the VDG work exactly?

[Robert Gault]

Fedor Steeman wrote:
> Hello fellow CoCo-enthousiasts,
> 
> I have been trying to figure out how exactly the CoCo's VDG works.
> However, I am uncertain about getting the math right and understand
> what precisely happens when an analog video signal is produced by
> processing binary data. Can anyone guide me?
> 
> This is what I understand so far:
> 
> The VDG is allowed access by the SAM to a specific portion of RAM that
> it cyclically iterates. The VDG accesses one byte per bus cycle. Since
> the frequency is (normally) at 0.98 Mhz, say 1 Mhz, one byte is read
> every 0,001 milliseconds, so 1000 bytes are processed per millisecond.
> 
> On the basis of the contents of this particular portion of RAM, and
> the video mode, an analog signal is generated. This analog signal has
> a frequency of 50 Hz meaning that all the pixels constituting the
> screen must be processed at least every 1/50 seconds, or every 20 ms.
> 
> Depending on the video mode, the following amounts of bytes would have
> to be processed:
> 
> TEXT MODE: 	32*16 = 256 (8bpp)
> PMODE3: 	128*192/4 = 6144 (2bpp)
> PMODE4:		256*192/8 = 6144 (1bpp)
> 
> According to my understanding, the VDG can get to process about 20.000
> bytes for each 20ms. So even at the largest screen resolution, it can
> process the screen around 3 times for each screen pulse sent to the
> display.
> 
> So what is happening? Does the VDG slow down the binary signal when
> converting it to an analog signal? Does it buffer the information in
> the video RAM to burst it out at the right time? Or does it just send
> whatever is coming from VideoRAM directly on to the display? What is
> going on precisely?
> 
> Thanks for any direct help and a Happy New year already to everyone
> reading this.
> 
> Fedor Steeman
> 

If you want to follow the operation of the VDG, you should try to find 
the data sheet for the MC6847. It probably is on the Internet somewhere.

The data sheet contains summaries, timing charts, block diagrams, and 
symbolic circuit diagrams.