CGA/EGA to VGA adapter

[bugman2112]

Looking to adapt an CGA or EGA PC XT card for use on a modern vga monitor (CRT). I've seen several plug adapters for about $16. I think there may be some issues with frequencies if this type of setup is used for an LCD screen without additional circuitry. But can anyone confirm that this type of adapter will work with a standard CRT VGA monitor?

 

[Jorg]

Not sure but as far as I know CGA/EGA are digital signals and VGA is analog?
It doesn't sound easy to me. If that is possible, people might as well build their own IDE card.
Well, I guess someone more knowledgable will show up very soon now.

 

[krebizfan]

I have seen one adapter that did all the necessary conversions but it was priced at about $100. I don't remember the model so I can't recommend one.

It might be easier and cheaper to track down an 8-bit VGA card.

 

[Jorg]

I have seen one adapter that did all the necessary conversions but it was priced at about $100. I don't remember the model so I can't recommend one.

It might be easier and cheaper to track down an 8-bit VGA card.

A lot of 16 bit vga cards work in an 8 bit slot AFAIK.

 

[Marrr]

I suspect those cheap plug adapters are designed for early "multisync" monitors, capable of working with different frequencies, including less than 31 kHz.

Modern monitors don't support frequencies below 31 kHz, and this is the main problem about connecting them to CGA/MDA/HGC/EGA cards (TTL->analog conversion is very easy). So, a pretty complicated converter is necessary. Yes, I've heard such converters do exist, but haven't seen one myself, and I'm definitely interested, too...

 

[bugman2112]

Ok, exploring the 8-bit vga adapters. I have seen them and they are pretty affordable. They use the 15 pin connector and should work with modern screens. However, I have seen reports that VGA is very slow in an XT. I understand why this would be the case, with limited processing capability. But my question is, are people referring to the fact that applications that use VGA would be slow? Or would any type of graphic application be slow, even if they are calling on the older CGA/EGA support that a VGA card does contain? Keep in mind, my only goal here is to use a modern VGA monitor to display old CGA/EGA applications. I have no intent on using VGA applications.
Thanks for your input..

 

[Marrr]

There should be no slowdown in CGA or EGA modes.

 

[Fallo]

Or would any type of graphic application be slow, even if they are calling on the older CGA/EGA support that a VGA card does contain? Keep in mind, my only goal here is to use a modern VGA monitor to display old CGA/EGA applications. I have no intent on using VGA applications.
Thanks for your input..

VGA graphics modes are slow on XTs because they use 64k+ of video memory, which is a lot of stuff for an 8088 to move around (even turbo XTs with 8086s, NEC V20s, or 286 accelerators won't be that fast).

But if you just want to run CGA or EGA, there won't be as much of a problem. However, programs with CGA graphics frequently have performance issues and incompatibilities with VGA. It varies considerably, though. Often you'll just get an incorrect color palette, but one or two games (such as Jungle Hunt) will simply not run on VGA at all. EGA software by contrast rarely ever has compatibility problems. As for performance, EGA modes use up to 128k of video memory, so they can be slow, especially on scripted games like Sierra adventures.

 

[carlsson]

Hm. A friend of mine will swap me an 8-bit VGA card to be used on my 8088 Victor PC. The computer currently only has 512K RAM and built-in CGA/MDA graphics. Do you think it would be unwise of me to install the VGA card? I would mostly use it in text mode, but perhaps also try to install a few simple games, those who are compatible.

Does it make any difference if I source four more RAM chips to upgrade the internal memory to 640K? I suppose not, but on the other hand the Victor can run at 7.16 MHz instead of the default 4.77 MHz so perhaps the performance loss is not as bad.

 

[Chuck(G)]

The widgets to convert from CGA/MDA/EGA outputs to VGA are known as "scan converters" or, less accurately, "scan doublers". They're mostly in the $100 range, as they essentially accumulate a frame's worth of information and re-serialize it at a higher frequency.

There are a few reviews on some of the vintage gaming forums.

There also exist LCDs wtih expanded range scanning capabilities, mostly intended for refurbishing old retail game consoles, but they're pretty expensive, and I believe they usually come as a "naked" caseless unit.

I have an LCD monitor here that claims to be able to get down to 24KHz horizontal and I have an enhanced EGA card that claims to be able to get up to 24KHz, but I've never tried the pair together to see if the combination works.

 

[tezza]

I tried an 8-bit VGA card in my IBM AT during my recent rebuild. Mostly worked in text mode except for reverse video. That didn't work at all. Graphics mode was also screwed up somewhat.

I'm not sure why, but then there were dip switches on the card I had no clue about as I couldn't find a manual (various combinations made no difference). I didn't purse the problem as I eventually I got an EGA card, which is the card I actually wanted in the machine.

Tez

 

[bugman2112]

Well, I'm starting to think that an EGA card is the way I should go. I am beginning to remember those incompatabilities and wrong color pallettes with VGA. I was trying to avoid having another monitor type (EGA). I'm running out of room as I'm sure many of you can relate to. It's funny to hear my wife's friends and relatives ask her "why does he need so many TV's?" Maybe I'll check out one of those scan converters.

Just curious, is there any way to incorporate an apple II color composite monitor into this mix? I'm probably really crossing lines with this one....

 

[Fallo]

There also exist LCDs wtih expanded range scanning capabilities, mostly intended for refurbishing old retail game consoles, but they're pretty expensive, and I believe they usually come as a "naked" caseless unit.

I've seen sites with naked CRTs for arcade machines, but never LCDs. I did once see a fully cased LCD that claimed to accept TTL RGB, but I can't remember the site it was on. A lot of them are multifunction, but this monitor was listed as being CGA-only.

 

[Chuck(G)]

I've seen sites with naked CRTs for arcade machines, but never LCDs. I did once see a fully cased LCD that claimed to accept TTL RGB, but I can't remember the site it was on. A lot of them are multifunction, but this monitor was listed as being CGA-only.

They exist. The other market served is the CNC community, where the iron costs a lot more than the associated controller.

Many think an LCD on a CGA-output arcade game is way too sharp in terms of focus. And it doesn't flicker. Hardly gives a retro look.

 

[per]

Well, I'm starting to think that an EGA card is the way I should go. I am beginning to remember those incompatabilities and wrong color pallettes with VGA. I was trying to avoid having another monitor type (EGA). I'm running out of room as I'm sure many of you can relate to. It's funny to hear my wife's friends and relatives ask her "why does he need so many TV's?" Maybe I'll check out one of those scan converters.

Just curious, is there any way to incorporate an apple II color composite monitor into this mix? I'm probably really crossing lines with this one....

I think you are missunderstanding a little.

EGA has most of the incompabilities with CGA programs that VGA cards has. However, there is almost no compability problems running EGA graphics WITH a VGA card.

Here is a small table I made:

Hardware the software expects > Actual Hardware = Ressult
MDA > MDA = Fine
MDA > HGA = Fine
MDA > CGA = Not possible on most cards, however some can handle it (read: 'ATI GS').
MDA > EGA = Not possible on most cards, however some can handle it (read: 'ATI EGA-Wonder').
MDA > VGA = Not possible on most cards, however some can handle it (read: 'ATI VGA-Wonder').
HGA > MDA = Won't work.
HGA > HGA = Fine
HGA > CGA = Not possible on almost all cards, however some can handle it (read: 'ATI GS').
HGA > EGA = Not possible on almost all cards, however some can handle it (read: 'ATI EGA-Wonder').
HGA > VGA = Not possible on almost all cards, however some can handle it (read: 'ATI VGA-Wonder').
CGA > MDA = Avoid by all means!!!
CGA > HGA = Won't work, avoid this too!
CGA > CGA = Fine
CGA > EGA = Some compability problems (wrong colors, flickering graphics, etc.)
CGA > VGA = Some compability problems (wrong colors, flickering graphics, etc.)
EGA > MDA = Avoid by all means!!!
EGA > HGA = Won't work, avoid this too!
EGA > CGA = Won't work.
EGA > EGA = Fine
EGA > VGA = Fine
VGA > MDA = Avoid by all means!!!
VGA > HGA = Won't work, avoid this too!
VGA > CGA = Won't work.
VGA > EGA = Won't work.
VGA > VGA = Fine

 

[Fallo]

EGA has most of the incompabilities with CGA programs that VGA cards has. However, there is almost no compability problems running EGA graphics WITH a VGA card.

EGA's register set is very similar to VGA, except that many of the registers are write-only, whereas on VGA they're read/write. However, EGA software almost always uses the BIOS instead of writing directly to the registers.

The performance problems that some CGA games have appear to be related to the 70Hz refresh used by VGA. I've put them into three categories: those that slow down on VGA (eg. Alley Cat, Snack Attack II), those that speed up (eg. Shamus, Defender, Frogger), and those that exhibit no difference (eg. Ms. Pac-Man, Dig Dug, Donkey Kong). EGA has a 60Hz refresh, so it should not have these problems. Games that write directly to the CGA registers are still incompatible with EGA, though.

 

[Marrr]

Hardware the software expects > Actual Hardware = Ressult
MDA > VGA = Not possible on most cards, however some can handle it (read: 'ATI VGA-Wonder').

I would say vast majority of VGA cards can do text mode 7, so there's no problem, unless you insist on register-level compatibility, but what MDA software needs that?

HGA > MDA = Fine

No, you definitely can't do 720x348 graphics on a MDA.

CGA > HGA = Won't work, avoid this too!

There's plenty of CGA simulation software, so it's possible to run most CGA games on a HGC. It doesn't look good, but it usually works.

 

[per]

I would say vast majority of VGA cards can do text mode 7, so there's no problem, unless you insist on register-level compatibility, but what MDA software needs that?

I do insist on register-level compability because there ARE programs that requires it. The VGA version of mode 7 uses another character-set, by the way. In addition, it doesn't allways support the (exact) same text-attriblutes than the original MDA.

No, you definitely can't do 720x348 graphics on a MDA.

My bad, I was on a train trip last nigth, and I couldn't manage to sleep because of all the noise :sleepy:.

There's plenty of CGA simulation software, so it's possible to run most CGA games on a HGC. It doesn't look good, but it usually works.

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.

 

[Marrr]

I do insist on register-level compability because there ARE programs that requires it. The VGA version of mode 7 uses another character-set, by the way. In addition, it doesn't allways support the (exact) same text-attriblutes than the original MDA.

OK, I admit, I never paid much attention to details like text attributes or character set (the latter you can easily change on a VGA anyway).
But what's that thing about MDA software requiring register compatibility? What programs need it, and why?

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.

If a program doesn't use BIOS to switch to graphics mode, you can force the simulator to switch the mode itself, before launching the main program. This is usually enough for the program to work.
I also used to use a CGA simulator which had the option to initialize bootstrap from A:, but I don't remember actually using that option, all the booter games I had were already ripped to .com files...

 

[per]

But what's that thing about MDA software requiring register compatibility? What programs need it, and why?

The BIOS calls where slow, and many programmers simply wrote their own and faster routines instead.

But there is not much software that require MDA to be used, so there is not too many examples.

If a program doesn't use BIOS to switch to graphics mode, you can force the simulator to switch the mode itself, before launching the main program. This is usually enough for the program to work.
I also used to use a CGA simulator which had the option to initialize bootstrap from A:, but I don't remember actually using that option, all the booter games I had were already ripped to .com files...

Ok, but anyways, what I wrote above mostly goes for CGA too; the BIOS calls where slow so many wrote their own routines. In addition, lots of programs was based on low-level glitches in the CGA (wasn't it something called 160*100 pixel graphics mode?), and 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.