CGA/EGA to VGA adapter

[bugman2112]

Thanks for all the great input. Tell me if you follow my logic. I'm thinking of sticking with the CGA adapter and then use a scan adapter to go to VGA. Reason being my goal is to experience these early games and applications in native form (CGA). I dont want to introduce incompatabilities to lessen the expereince. Furhtermore, many later games designed to use better graphics adapters still offered a CGA mode. So I wouldnt be automatically alienating myself from all later games. Make sense? My goal never was to have a fully decked out XT. I have emulators to cover that gap. I just want a functional early setup.

 

[per]

Thanks for all the great input. Tell me if you follow my logic. I'm thinking of sticking with the CGA adapter and then use a scan adapter to go to VGA. Reason being my goal is to experience these early games and applications in native form (CGA). I dont want to introduce incompatabilities to lessen the expereince. Furhtermore, many later games designed to use better graphics adapters still offered a CGA mode. So I wouldnt be automatically alienating myself from all later games. Make sense? My goal never was to have a fully decked out XT. I have emulators to cover that gap. I just want a functional early setup.

Ok.

Did you know that the CGA has a NTSC composite output (for connecting it to the TV)?

 

[Chuck(G)]

Lots of CGA games was booters, and you simply hadn't a chance to load such a program before starting a booter (unless you are invoking Int19h manually). In addition, the software only worked at BIOS-level while lots of CGA games alter the registers directly. Those games will simply not work rigth even with one of therse simulators installed.

I think I know something about this--I wrote SIMCGA. The simulator support software could install itself at the top of RAM and then decrease the "available RAM" indication by 1K or so and then warm-boot the PC with the game. It worked in the overwhelming number of cases.

 

[bugman2112]

Yes, I was first going to see if the CGA out to TV met my needs first. However, I'm not optimistic because some of the games I was looking to use rely on 80 column text.

 

[Marrr]

you also got programs that writes directly to video-RAM. Those may be a problem with Hercules cards since they decode video-RAM in another way than the CGA do.

ALMOST ALL games write directly to video RAM.
So, if they usually work with CGA simulation on an HGC, then there must be a way to set the HGC to decode its RAM CGA-like.
BTW: many CGA games which work pretty good on a HGC, display garbage when run on a VGA. So I still insist HGC's CGA compatibility is good enough to be mentioned in that "expected hardware/actual hardware" matrix.

Back on the main topic: if you want maximum compatibility with CGA games, keep in mind that games look different on the RGBI output and on the Composite output. So, you can connect a VGA monitor to RGBI via scan converter, but also connect a TV via Composite.

Less compatible, but easy option is to get one of the following:
- 8-bit VGA card
- 16-bit VGA card capable of running in 8-bit slot (very easy to get and very cheap)
In any case, look for a card with CGA register-level compatiblity option, eg. Trident TVGA8900 is pretty good in this area, and easy to get. But I don't think you can experience Composite look this way.

 

[per]

In any case, look for a card with CGA register-level compatiblity option, eg. Trident TVGA8900 is pretty good in this area, and easy to get. But I don't think you can experience Composite look this way.

I got the TVGA8800CS, and you can actually output MDA/CGA/EGA to an analog VGA monitor, BUT it doesn't doublesync or patch the sync signals in any way. However, if you got a flat-screen monitor, you may be able to get a proper picture anyways.

 

[Fallo]

Yes, I was first going to see if the CGA out to TV met my needs first. However, I'm not optimistic because some of the games I was looking to use rely on 80 column text.

I have my 5150 connected to a TV with an RF modulator. With that setup, you'll go blind looking at 80-column text. I used the computer like that for over three months before I finally got a Tandy CM-11 on Ebay. Hopefully your TV has composite inputs. 80-column text will be a little better with a straight composite connection, but you really need a RGB monitor for it. Some TVs unfortunately have an off-centered picture (like this one I'm using) due to bad adjustment at the factory. DOS provides a variant of the MODE command that loads a TSR to keep the picture centered, but you can't use it on booter games.

The first thing you'll notice about the composite output is the color bleed, which causes everything to be filled with artifact colors. Some games (Archon, Ms. Pac-Man, most Sierra games) use this to give a 16-color display that would be impossible on a RGB monitor. Games also sometimes use the cyan-red-white palette, which appears in grayscale on composite monitors. In summary, you need both a composite and a RGB display to fully utilize the CGA card's capabilities.

It was quite common for games to write directly to the CGA registers. Even many games that had EGA and VGA support would do such things in CGA mode. The various tricks that mess up EGA/VGA cards include:

* Changing the color palette by writing directly to port 3D9h instead of using BIOS function 10h 0Bh

* Use of the cyan-red-white palette (just appears as cyan-magenta-white on EGA/VGA)

* Tweaking the horizontal and vertical position registers to shake the screen

* Raster tricks (eg. Frogger)

* Use of the 160x100 pseudographics mode (but can be fixed by simply tweaking the maximum scan line register)

* Sierra's favorite trick of setting composite mode by switching to 320x200 graphics and then writing a 1Ah to port 3D8h to set 640x200 graphics with the color burst enabled (to dupe the BIOS into printing 40-column text)

* Setting the normal 320x200 graphics by writing directly to the registers (Digger does this)

Still others like Troll's Tale and Microsoft Decathlon produce a split-screen effect on VGA. I'm not sure why this is. One or two like Jungle Hunt and Moon Patrol simply lock up on VGA and aren't playable at all.

As far as Hercules is concerned, it is 6845-based just like CGA. The video memory in its graphics mode is interlaced in the same manner as CGA, but with four rather than two banks of scan lines. Programs like SIMCGA just rearrange the CGA graphics data to fit in the Hercules' memory properly.

 

[bugman2112]

Back to another question I had earlier in the post. Is it possible to use an apple ii color composite monitor with the IBM cga card? Not the RGB apple //gs monitors, but the color composite with a RCA type connector used in conjunction with the RCA plug out of the CGA card. There are two types of video out from this style of plug, right? One which can go directly into the composite IN on a TV as well as into an apple ii color composite monitor. The other type needs to go through an RF converter. I believe the IBM CGA composite out is such, and therefore won't work directly with the apple ii composite monitor?

 

[tezza]

I think I know something about this--I wrote SIMCGA. The simulator support software could install itself at the top of RAM and then decrease the "available RAM" indication by 1K or so and then warm-boot the PC with the game. It worked in the overwhelming number of cases.

You wrote that highly useful SIMCGA freeware program of the mid-80s Chuck(G)? The one that let hercules owners such as myself enjoy all those CGA games? If so, thank you. That program was one of the essential utilities on my hard drive during my mono (Hercules) years (about 1985-1988 ).

Tez

 

[Chuck(G)]

You wrote that highly useful SIMCGA freeware program of the mid-80s Chuck(G)? The one that let hercules owners such as myself enjoy all those CGA games? If so, thank you. That program was one of the essential utilities on my hard drive during my mono (Hercules) years (about 1985-1988 ).

Yup, that was me--I wrote it to take advantage of my wife's "Spartan II" XT clone which had a Hercules clone card in it. My 5150 had an MDA, so the issue never came up. Andrew Lynch now has the monitor that I used.

 

[Fallo]

Back to another question I had earlier in the post. Is it possible to use an apple ii color composite monitor with the IBM cga card? Not the RGB apple //gs monitors, but the color composite with a RCA type connector used in conjunction with the RCA plug out of the CGA card. There are two types of video out from this style of plug, right? One which can go directly into the composite IN on a TV as well as into an apple ii color composite monitor. The other type needs to go through an RF converter. I believe the IBM CGA composite out is such, and therefore won't work directly with the apple ii composite monitor?

There's no such thing as "separate" types of composite output. You either connect it directly to the input on the monitor/TV, or to an RF modulator if the TV doesn't have composite inputs.

Composite video just contains the chroma, luminance, and sync signals without the channel information needed by the TV's tuner. RF modulators add this component (and also combine it with the sound signal if present) so that it can be connected to the antenna in on the TV. In the process of decoding this, some sound and picture quality is lost and you also get interference, which is absent in a straight composite connection. IBM did not include an RF modulator with the CGA card because of FCC regulations (same reason the Apple II didn't have one).

 

[MikeS]

Yup, that was me--I wrote it to take advantage of my wife's "Spartan II" XT clone which had a Hercules clone card in it. My 5150 had an MDA, so the issue never came up. Andrew Lynch now has the monitor that I used.

That was *also* YOURS, along with your other less-known but much-used contributions ???

Well, let me add my thanks; I installed that on every client's system, even if only to run the "Hit any Key" hammer-swinging duck screen saver; guess I owe you some retroactive royalties... ;-)

 

[bugman2112]

Sorry, I always get confused with these signals. I'm thinking of the composite out that combines audio and video, like the atari 2600 and colecovision. These do not work with the composite input on TVs. The CGA card outputs video only so I realize it will likely work with my apple color composite monitor.

 

[bugman2112]

The display looks great on the apple color composite EXCEPT the far left edge of the screen cuts off the first character. I can see its there beyond the front bezel of the monitor, but not visibile enough to be usable. None of the adjustments (external) can correct this. I'm comfortable opening a CRT in order to correct the H position slightly. Would anyone agree that this is workable, or does this slight misalignment indicate incompatabilty and I shouldnt mess with it?

 

[NobodyIsHere]

Yup, that was me--I wrote it to take advantage of my wife's "Spartan II" XT clone which had a Hercules clone card in it. My 5150 had an MDA, so the issue never came up. Andrew Lynch now has the monitor that I used.

Hi Chuck! I recall using SIMCGA back in the 1980's when I was in going to college in North Dakota. What a great program! Thanks!

I think there is some interest in a video_mode to VGA converter circuit project. Probably a fast enough uC circuit could do this, I think like an ATmega128, fast PIC, and/or Propeller. Basically video_mode in on one port and VGA output on another much like the AT2XTKBD project.

If you are up for some uC programming I'll do the PCB. Heck if we can get Hargle on board we can get the jazz band back together for a return engagement.

Thanks and have a nice day!

Andrew Lynch

 

[hargle]

If you are up for some uC programming I'll do the PCB. Heck if we can get Hargle on board we can get the jazz band back together for a return engagement.

I'm up for anything! I wish we could go 1 step back and make a "perfect video card" for our machines. Something that outputted to a VGA monitor, but was either switchable between CGA/EGA/VGA on the fly, or was capable of being VGA, but accepting all the tricks that CGA used with proper compatibility and speed.

I'm really quite video dumb though, so I don't know if anything like this would even be possible, and it would probably require a serious amount of FPGA work to accomplish. Anyway, sorry for the topic drift.

--
I bet a majority of the users here were SIMCGA users. I used it to play Thexder 1 on my mono 5150 back in the early 90's! The ghosting on it was really quite trippy.

 

[Chuck(G)]

I think there is some interest in a video_mode to VGA converter circuit project. Probably a fast enough uC circuit could do this, I think like an ATmega128, fast PIC, and/or Propeller. Basically video_mode in on one port and VGA output on another much like the AT2XTKBD project.

If you are up for some uC programming I'll do the PCB. Heck if we can get Hargle on board we can get the jazz band back together for a return engagement.

I haven't given it much thought, although going from the digital-only (MDA/CGA/EGA) interfaces to DVI VGA would remove the tough aspect of AD and DA conversion. It might be possible without FPGA (well, maybe CPLD).

Back in the 80's I was partner in an operation that attempted to salvage the investment companies made in their mini- and mainframe operations. We designed a PCB with a Z80 on it that looked to a PC just like an MDA, but that interfaced to a VT220-style terminal. It was pretty cool--you fed your normal serial interface through the card (it also had a DART and some other stuff) and it tracked what was going on over the serial line as well as in the MDA memory and allowed you to split-screen or swap views of both the mainframe and the PC displays on the same terminal.

We sold a couple hundred of the things, but our timing was exceedingly bad--minis sunsetted pretty fast and the demand for higher-resolution graphics displays such as EGA pretty much ended the venture. I still have one of the cards and am a little surprised that no others have turned up in the "What the heck is this?" department.

 

[NobodyIsHere]

I haven't given it much thought, although going from the digital-only (MDA/CGA/EGA) interfaces to DVI VGA would remove the tough aspect of AD and DA conversion. It might be possible without FPGA (well, maybe CPLD).

Back in the 80's I was partner in an operation that attempted to salvage the investment companies made in their mini- and mainframe operations. We designed a PCB with a Z80 on it that looked to a PC just like an MDA, but that interfaced to a VT220-style terminal. It was pretty cool--you fed your normal serial interface through the card (it also had a DART and some other stuff) and it tracked what was going on over the serial line as well as in the MDA memory and allowed you to split-screen or swap views of both the mainframe and the PC displays on the same terminal.

We sold a couple hundred of the things, but our timing was exceedingly bad--minis sunsetted pretty fast and the demand for higher-resolution graphics displays such as EGA pretty much ended the venture. I still have one of the cards and am a little surprised that no others have turned up in the "What the heck is this?" department.

Hi Chuck, you know way more about the video conversion issues that I ever will. I've done some simple stuff with VDUs but not much. I am getting ready to order PCBs for the N8VEM VDU which is almost the lowest form of video display possible. It's pure character mode composite video much like the KayPro 10.

I am thinking similar to what you said; CGA/MDA/EGA would be basically a fast sampler in input port 5 or 6 bit (R, G, B, HS, VS, I?) repeating a scan doubled equivalent out of a VGA port. That's probably within the realm of possible. I think a Propeller or equivalent uC could do it. Sync on H & V and basically repeat as scan doubled output.

Doing analog video is much more difficult problem since you'd need a sampler capable of reading at what 12 MHz or so @ 8bits? Although there are AD converters able to do such a thing they are not cheap and the uC would be going like crazy trying to keep up. I don't think a Propeller could do it.

It would also require more intensive processing since you wouldn't have obviously front porch/back porch and sync signals like the digital video modes do. Possibly there are video sampling chips that can process a composite signal and produce separate HS and VS signals. I don't know.

As for designing a completely new video card that's even somewhat compatible with the PC video standards, I don't think that's feasible. A simple scan converter is probably doable. An analog to VGA converter would be much more difficult but a full blown video card is beyond my skill level for sure.

I'd like to keep it DIP/PLCC if possible. SMT, especially fine pitch, is not very appealing to me as a home builder.

Thanks and have a nice day!

Andrew Lynch

 

[Chuck(G)]

Hi Andrew,

At this point, I'm not entirely convinced that any microprocessor is required at all--or that one would make the job any easier.

I think we could keep the design to PLCC and DIP, however.

Although the Propeller is a very cool chip, for the ultimate in doing the most with the least, there's a Russian fellow who's made a terminal that drives a VGA display (it looks to be limited by the internal RAM of the uC) using nothing more than an ATMega8!

 

[Marrr]

Doing analog video is much more difficult problem since you'd need a sampler capable of reading at what 12 MHz or so @ 8bits? Although there are AD converters able to do such a thing they are not cheap and the uC would be going like crazy trying to keep up. I don't think a Propeller could do it.

Wait a min...
What exactly you need A/D conversion for? CGA/MDA/HGC/EGA all output TTL, so no need for A/D here.
Perhaps for CGA's Composite out? If so, it's not that necessary: standalone TV tuners with VGA-out and eg. RF-in, Composite-in, S-Video-in are easy to get and cheap.