[Coco] CoCoFEST! Challenge: CoCo DMA

Gene Heskett gheskett at shentel.net
Tue Mar 3 00:17:37 EST 2020

On Tuesday 03 March 2020 00:05:01 Gene Heskett wrote:

> On Monday 02 March 2020 20:30:40 RETRO Innovations wrote:
> > On 3/2/2020 7:29 PM, RETRO Innovations wrote:
> > > http://www.go4retro.com/2020/02/26/direct-memory-access-possibilit
> > >ie s-on-the-tandy-color-computer/
> >
> > http://www.go4retro.com/2020/02/28/coco-dma-early-efforts/
> >
> > Jim
> Jim, congrates and all that! My first experience at writing a program
> for a tv station used the dma capability of an  RCA 1892
I could swear I fixed that >>>>>>>>>>>>>>>>>>>>>>>>>>>1802
> microcontroller, circa 1978, for several things, the most impressive
> being the creation of a 2 digit plus decimal point character
> generator, needed to replace the dirty, noisey reproductions of a 10
> second video count down with a modern dead frame accurate digitally
> generated "academy leader" which was in fact horribly inaccurate due
> to the video tape editing capability available to even the high priced
> left coast production houses of the day.  The need was brought about
> by our possession of a "station break sequencer" which could take a
> list of machines, 3/4" u-matics of the day, load the commercials tapes
> by hand and tell it what order they played in, which would then search
> the tapes for the queue tones on the 2nd audio channel and park the
> machine in pause at the detection of the tones.  When it was time to
> do the break, a one button start ran it thru what you had set it to
> do.
> As the only way to lay those tones was to copy a master tape that by
> then had several thousand play passes on it, then "dub" the commercial
> over the top of that copy, the on air results were horrible at best.
> Observing how it was done, I saw that I could improve the timing AND
> get rid of the image damaging dub by laying the tones down on the
> finished commercial tape. But like topsey the project grew to need to
> replace the crappy academy leader with modern digitally generated
> countdown. I first wrote in a digital clock display but it was too
> small to see on a 5" from the switchers operating position up to 16
> feet away.
> So I came up with a system cpu clock driven counter to drive the 4
> address lines of a 74150 and a gang of diodes and logic that let me
> make characters 103 lines high and 2/3rds of a screen wide, that would
> decode a single byte of dma, triggered by the scan counter to start 50
> lines down from the top of the screen with a byte of dma that enabled
> the top line of the character, then counted to 53 and wrote by dma,
> the data that would be the legs of the character down to the 150th
> line, at which point a 3rd dma cycle wrote the center horizontal line,
> a 4th dma cycle at line 153 then wrote the bottom legs of the
> character, a 5th cycle at 200 lines wrote the bottom of the character
> until a 6th dma cleared the rest of the screen. Update those 5 bytes
> if needed for the next 1/10 second during the vertical interval, wash,
> rinse and repeat until 2 seconds before first video, when the video
> insert was turned back off.
> I did all that while the chief was recovering from a heart attack and
> I was the acting chief, but I went on down the road in search of
> taller grass a couple months after he came back to work. I was back on
> the left coast in 1997 to see before it was too late, some kin in
> Salem OR at the time and wasted a phone call to Redding to catch up on
> old times. I put that little cosmac super elf to work doing that in
> early 79, and in 97 they were still using it many times a day.
> Equipment changes much faster than that in any tv station control
> room, so learning that it was still in use 18 years later impressed
> me, a lot.
> I should have bought the copyright back (I wrote it on their time so
> it was their's) and went into production making it. I was good to them
> as I left instructions as to how to change the software to update its
> ballistics to handle faster machines so it wasn't married to a Sony
> 2850 for its last frame of accuracy. I would have had to make a lot
> more video hardware to do that w/o dma.
> So I want to see how this works out, Jim, you have my attention.
> This could easily be this decades "forgotten chip" as it will
> certainly enhance the coco's abilities. One thing I think is missing
> though, the 1802, with its 16, 16 bit registers, dedicated one to act
> as the dma source address with auto-inc, so the cpld will need at
> least one such register to supply the read address (with an auto-inc,)
> and possibly another to supply the write address (also with auto-inc).
> And possibly a 3rd with auto-dec to control how many bytes were to be
> moved if a burst was setup.
> And finally, this would be an excellent time to overhaul the top page
> decoding to make room for all the i/o that would open up on a coco3.
> Something I've been ranting about since the cc3 came out.  There are 7
> 4 byte wide locations that could easily be used for those registers
> hiding between the pia's, and 7 more 4 byte wide locations between the
> 2nd pia and the floppy controller. Allow those to be memory locations
> in main memory and you've got your dma registers.
> > RETRO Innovations, Contemporary Gear for Classic Systems
> > www.go4retro.com
> > store.go4retro.com
> Cheers, Gene Heskett
> --
> "There are four boxes to be used in defense of liberty:
>  soap, ballot, jury, and ammo. Please use in that order."
> -Ed Howdershelt (Author)
> If we desire respect for the law, we must first make the law
> respectable. - Louis D. Brandeis
> Genes Web page <http://geneslinuxbox.net:6309/gene>

Cheers, Gene Heskett
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
If we desire respect for the law, we must first make the law respectable.
 - Louis D. Brandeis
Genes Web page <http://geneslinuxbox.net:6309/gene>

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