Easy mod to add graphic capability to your SB600

[Thomas]

Looking around for existing hi-res boards, or other video mods, I couldn't find an upgrade I have implemented in the early 80's to my SB600. The original modification used 2 TTL ICs to add a graphic bitmap mode in parallel to the onboard Character ROM. Later this was replaced by a much simpler solution, when I was able to burn 2732 Eprom’s.

Lines.PNG (12.19 KiB) Viewed 11373 times

To get an idea how it looks like, here a screenshot of a 128x32 graphic section above the bottom text section.

It’s a “very light” modification and will provide different graphic options, without modifying the video circuit logic nor by adding additional memory.

You can get up to 128x128 pixel for an updated C1P with 64x32 characters or 128x64 pixel on a standard SB600 that can be individually set or cleared, like in an additional graphic mode. The graphic section will be on top of the screen (Half screen) or can be set to Full screen. For gaming, the best option is to use the half screen mode, so you can display text at the bottom and do all graphic visualizations on the top.

The idea was to replace the socketed Character ROM by an 4k (2732) Eprom with a bitmap scheme in the second half of the Memory. It is possible to enable or disable the bitmap section by Software, when connected to the RTS line of the ACIA6850. Two versions of Character ROMS an available (4x2 type and 2x4 type) with different characteristics concerning horizontal and vertical resolution.

Look to my google site for downloads:
https://storage.googleapis.com/constant ... ator2.html

Here a short guide of the modification:

If you like to see, how this looks like, you may check out my Emulator that supports this graphic mode.

Thomas

[bxdanny]

Mark has the character generator ROM for creating this kind of pseudo-bitmapped screen available as OSIHIRES.ROM in the ROMs section of his site (marks-lab.com/osi/software/roms.html).

Back in the day, I ordered an EPROM like this and installed it on my 540 board, piggybacked on top of the OSI character generator with ALL pins connected to the corresponding pins of existing OSI CG ROM. Since one of the enable pins required the opposite polarity from the existing masked ROM. that pin could now be used to select which ROM was enabled. I connected it to bit 2 of the output port for $DE00, which was unused on my monochrome-only 540 board.

Unfortunately, I was confused about which way, in terms of left to right, the character images were stored in the ROM. I overspecified things when I ordered the EPROM from Steve Hendrix of Hexdos fame (I didn't have an EPROM programmer of my own), sending him a listing of a BASIC program that in fact produced characters that were mirror images of what I really wanted. I suspect that this mistake on my part eventually gave rise to the spuriously-labeled checkbox in the WinOSI video configuration dialogue labeled "Reverse horizontal (C2/C4/C8 type)". In fact, the c1P (600 board) and the 540 board interpret the character generator ROMs the same way.

[bxdanny]

Actually, IIRC, I quickly moved it to bit 3, assigned for controlling the BSR Home Control modules, since no other software used that bit, while 65D 3.3 (and probably some other software) turned color on by default.

[Thomas]

Mark's OSIHIRES.ROM follows a different bit layout within a Hires character but basically the same concept.
Back during my first encounters with the Superboard, I couldn’t burn nor effort a 2732 Eprom at that time, maybe because the 2732 types just came to market in the year 79.
So, my first solution was as you describe a wiring nest piggybacked on top of the OSI character generator.
By using a 74LS157 decoder to remap the character byte, the bit layout was limited to a single 4x2 layout. Plus, an octal driver and you have been all set.
Which was awesome for me at the time, to plot curves and objects to the screen. How much fun was possible with just half a kilobyte of memory back then.

[Mark]

I remember plotting a few graphs and running a demo program or two using the OSI hires character generator ROM, but I have lost the old programs and never took the time to figure out the plotting routines again. When I found the ROM I decided to upload it in case anyone else wanted it.

You are correct in that the character generator layout is the same for OSI540 and Superboard. I was uncertain of this in the early days of the emulator when I found some layouts available on the internet were reversed, plus I had no superboards to compare to. What I eventually determined was that it was the UK101 which was reversed compared to the OSI chargen ROM,. The UK101 did match the layout in the CGA chargen ROM used in PC compatibles. Later versions of the emulator fixed the option's label. (I need to post an update one of these days.)

The RTS line on the 6850 was also used as a simple 1-bit sound output. I think there was support for that in one of the OS packages, perhaps is was DOS/65? I don't remember. Some UK101 games used it as well.

I like the split screen option and using the RTS line as an enable is a great idea too!

[Thomas]

Here a little demo to see what can be done with a half screen low res graphic mod.

Animation.gif (6.69 MiB) Viewed 11321 times

In case you are interested in the source code, see below