How hard would it be to make a CGA card?

Discussion about old graphics cards, monitors and video related things.

Re: How hard would it be to make a CGA card?

Postby reenigne » 2018-7-23 @ 13:14

anetanel wrote:
keropi wrote:that card looks like the myriad cga/mda/hercules cheap clone cards , no way color means EGA ... not a bad card especially for mda/herc

Man.. I thought I had an excuse to buy a scope :)


Well... you could still make your own CGA card from scratch, just for fun, even if you don't actually need to! You'd surely need a scope to debug it... (Though I'd have said that about cloning an Adlib card as well - impressive that you did that with no scope!)
User avatar
reenigne
Oldbie
 
Posts: 506
Joined: 2006-11-30 @ 05:13
Location: Cornwall, UK

Re: How hard would it be to make a CGA card?

Postby anetanel » 2018-7-23 @ 14:27

reenigne wrote:Well... you could still make your own CGA card from scratch, just for fun, even if you don't actually need to! You'd surely need a scope to debug it... (Though I'd have said that about cloning an Adlib card as well - impressive that you did that with no scope!)

Oh, I did not design the board. I used the same plans as everyone did:
http://tubetime.us/index.php/2016/07/22 ... ound-card/

My card indeed looks almost exactly as the one in root42's link: http://vgamuseum.ru/gpu/ram/ram-mcg2502/
Any idea how can I find the right transistor (?) for Q1?
And how did you came to the conclusion that It will enable composite?
anetanel
Member
 
Posts: 103
Joined: 2017-1-01 @ 23:10

Re: How hard would it be to make a CGA card?

Postby reenigne » 2018-7-23 @ 14:43

anetanel wrote:
reenigne wrote:Well... you could still make your own CGA card from scratch, just for fun, even if you don't actually need to! You'd surely need a scope to debug it... (Though I'd have said that about cloning an Adlib card as well - impressive that you did that with no scope!)

Oh, I did not design the board. I used the same plans as everyone did:
http://tubetime.us/index.php/2016/07/22 ... ound-card/


Even so it's rare to have a soldering project work first time and debugging it when it doesn't is much easier with a scope!

anetanel wrote:My card indeed looks almost exactly as the one in root42's link: http://vgamuseum.ru/gpu/ram/ram-mcg2502/
Any idea how can I find the right transistor (?) for Q1?


2N3904 should work - that's what the CGA card has in the same spot.

anetanel wrote:And how did you came to the conclusion that It will enable composite?


It's the resistor values. For digital logic the exact values don't matter too much, so you'll need to see 10K for a pull-up and not much else. When you've got values like 2.7K here and 5.1K there you know something analog has got to be going on. Plus, a few resistors like that and a transistor is basically all you need for a CGA composite DAC output stage. Many buyers of the card wouldn't have cared about composite output so it makes economic sense to design a single card that can be built both with and without composite, and to just leave the composite components off the cheaper model.

If you want to double-check before buying components, you can use a multimeter to trace out how those pads are connected up, and compare it to IBM's schematic for the composite output stage. The emitter of Q1 is probably connected to pin 7 of the RGBI connector (that's the obvious one to use for composite, since it's not used by CGA). I'm guessing the P1 jumper will switch pin 7 between being the CGA composite output and the MDA video signal output.
User avatar
reenigne
Oldbie
 
Posts: 506
Joined: 2006-11-30 @ 05:13
Location: Cornwall, UK

Re: How hard would it be to make a CGA card?

Postby anetanel » 2018-7-23 @ 14:53

I just found that you had a somewhat similar discussion on another board 5 years ago :)
I pulled out my multimeter and started following the traces. So far I was able to confirm that the pins behind the RGBI connector are connected to the RGBI output..
anetanel
Member
 
Posts: 103
Joined: 2017-1-01 @ 23:10

Re: How hard would it be to make a CGA card?

Postby anetanel » 2018-7-23 @ 15:23

And yeah, the emitter is connected to the rightmost pin of J1, which when closed with the middle pin, connects to pin 7 of the RGBI connector...
I'm getting excited :)
Say it works, and pin 7 starts outputting a video signal, is it a simple matter of connecting an RCA header to it?
anetanel
Member
 
Posts: 103
Joined: 2017-1-01 @ 23:10

Re: How hard would it be to make a CGA card?

Postby reenigne » 2018-7-23 @ 15:29

anetanel wrote:I just found that you had a somewhat similar discussion on another board 5 years ago


I forgot all about that (I seem to remember sending Scali some resistors to add composite output to a card he had, but I didn't think that was a public discussion). Can you post a link to the thread here for posterity?

anetanel wrote:And yeah, the emitter is connected to the rightmost pin of J1, which when closed with the middle pin, connects to pin 7 of the RGBI connector...
I'm getting excited :)
Say it works, and pin 7 starts outputting a video signal, is it a simple matter of connecting an RCA header to it?


Yes, I think so.
User avatar
reenigne
Oldbie
 
Posts: 506
Joined: 2006-11-30 @ 05:13
Location: Cornwall, UK

Re: How hard would it be to make a CGA card?

Postby anetanel » 2018-7-23 @ 15:30

I hope it's OK to bring links to (competing?) forums...
http://www.vcfed.org/forum/showthread.p ... components
anetanel
Member
 
Posts: 103
Joined: 2017-1-01 @ 23:10

Re: How hard would it be to make a CGA card?

Postby reenigne » 2018-7-23 @ 16:46

anetanel wrote:I hope it's OK to bring links to (competing?) forums...
http://www.vcfed.org/forum/showthread.p ... components


I'm sure it's fine - there's lots of cross-pollination between VCF and here. I hope you have more success than they did.
User avatar
reenigne
Oldbie
 
Posts: 506
Joined: 2006-11-30 @ 05:13
Location: Cornwall, UK

Re: How hard would it be to make a CGA card?

Postby keropi » 2018-7-23 @ 16:52

-or-
find an ATI Small Wonder and if cvbs is missing follow the instructions here: viewtopic.php?f=63&t=61330 LOL LOL LOL
wrong cvbs colors included ofcourse , just to remind you that you don't use the real deal :dead:
User avatar
keropi
l33t++
 
Posts: 7196
Joined: 2003-9-08 @ 06:45
Location: Greece

Re: How hard would it be to make a CGA card?

Postby Stiletto » 2018-7-23 @ 21:17

anetanel wrote:I hope it's OK to bring links to (competing?) forums...
http://www.vcfed.org/forum/showthread.p ... components


No worries with linking VCF. :happy:
"I see a little silhouette-o of a man, Scaramouche, Scaramouche, will you
do the Fandango!" - Queen

Stiletto
User avatar
Stiletto
l33t
 
Posts: 4350
Joined: 2002-7-01 @ 21:57

Re: How hard would it be to make a CGA card?

Postby anetanel » 2018-7-25 @ 19:19

Well, some progress has been made!
I bought the needed resistors and transistor and soldered them along with pins to P1, and made a female rca cable with sockets that would go directly to pin 7 and 2 (ground).
Turned on the computer and it didn't blow up!! First goal achieved!
Furthermore, the composite output actually outputs the video signal!

Sadly I don't seem to be able to a good sync. I played with all the knobs but the picture isn't steady.
I tried to connect to my Plasma flat screen but it does not display anything, which makes me think that the signal is not a "valid" component signal.

could it be that I need to add some extra components to make the picture stable?

Interestingly, it doesn't seem to matter whether the P1 pins are short or not, a picture is showing either way...

Here's a video that shows what I mean:
https://youtu.be/c7iwUJenUNI
Attachments
2018-07-25 22.05.48.jpg
anetanel
Member
 
Posts: 103
Joined: 2017-1-01 @ 23:10

Re: How hard would it be to make a CGA card?

Postby root42 » 2018-7-25 @ 19:28

Hm. Are you trying to run Hercules and CGA at the same time? It looks like the right monitor shows Hercules. Have you tried running only CGA and setting the jumper to Color? Right now it's on Mono in your picture.
Soldering, retro game reviews and more on YouTube and Bonus videos
80386DX@25 MHz, 8 MiB RAM, Tseng ET4000 1 MiB, Jazz16, PC MIDI Card + SC55MkII + MT32, XT CF Lite, OSSC 1.6
User avatar
root42
Oldbie
 
Posts: 1199
Joined: 2018-1-27 @ 13:23

Re: How hard would it be to make a CGA card?

Postby keropi » 2018-7-25 @ 19:33

hercules won't work via composite port like root42 says try some cga game
User avatar
keropi
l33t++
 
Posts: 7196
Joined: 2003-9-08 @ 06:45
Location: Greece

Re: How hard would it be to make a CGA card?

Postby anetanel » 2018-7-25 @ 19:40

root42 wrote:Hm. Are you trying to run Hercules and CGA at the same time? It looks like the right monitor shows Hercules. Have you tried running only CGA and setting the jumper to Color? Right now it's on Mono in your picture.

I did, but I double checked now again.
When the jumper is on COLOR, nothing is displayed out of the composite. whether the MDA monitor is connected or not, and whether the P1 jumper is shorted or not...
With the jumper on COLOR, the output on the MDA monitor does look different. More stretched vertically.
anetanel
Member
 
Posts: 103
Joined: 2017-1-01 @ 23:10

Re: How hard would it be to make a CGA card?

Postby anetanel » 2018-7-25 @ 19:58

keropi wrote:hercules won't work via composite port like root42 says try some cga game

I tried running Prince of Persia in CGA mode, but it does not allow it when the card is on MONO (the game says "Graphics mode not available") .
and when the card in on COLOR, it allows CGA mode, but the composite output does not work at all.
anetanel
Member
 
Posts: 103
Joined: 2017-1-01 @ 23:10

Re: How hard would it be to make a CGA card?

Postby reenigne » 2018-7-25 @ 20:10

anetanel wrote:and when the card in on COLOR, it allows CGA mode, but the composite output does not work at all.


Interesting... So it seems like the outputs that the resistors are connected to only enabled in MONO mode, so that composite output seems to have been designed for a monitor with MONO frequencies but composite input (which I didn't know was a thing).

I wonder if populating some of the other unpopulated resistors would enable the composite output in CGA mode. This is the point where an oscilloscope would really come in handy, so that you can see which lines carry what signals in COLOR mode.
User avatar
reenigne
Oldbie
 
Posts: 506
Joined: 2006-11-30 @ 05:13
Location: Cornwall, UK

Re: How hard would it be to make a CGA card?

Postby anetanel » 2018-7-25 @ 20:13

reenigne wrote:This is the point where an oscilloscope would really come in handy, so that you can see which lines carry what signals in COLOR mode.

Literally on another tab I'm looking for oscilloscopes :)
I have no idea what I need though. (and how much does one costs)
anetanel
Member
 
Posts: 103
Joined: 2017-1-01 @ 23:10

Re: How hard would it be to make a CGA card?

Postby reenigne » 2018-7-25 @ 20:22

anetanel wrote:
reenigne wrote:This is the point where an oscilloscope would really come in handy, so that you can see which lines carry what signals in COLOR mode.

Literally on another tab I'm looking for oscilloscopes :)
I have no idea what I need though. (and how much does one costs)


At the barest minimum you want a 2-channel, 100MHz-bandwidth cathode-ray oscilloscope. Depending on where you live, you may be able to find a second-hand one for less than US $100 or equivalent. For somewhat more you can get a brand new digital oscilloscope with lots more features (and the more features you want the more you should expect to pay).
User avatar
reenigne
Oldbie
 
Posts: 506
Joined: 2006-11-30 @ 05:13
Location: Cornwall, UK

Re: How hard would it be to make a CGA card?

Postby anetanel » 2018-7-26 @ 09:19

I found this card that uses the same chip and has CGA composite output:
Image
(Source: http://oldcomputer.info/pc/board_gfx1/index.htm)

Can it be used to reverse engineer my card to output CGA composite?

(And how do you resize img tags?)
anetanel
Member
 
Posts: 103
Joined: 2017-1-01 @ 23:10

Re: How hard would it be to make a CGA card?

Postby anetanel » 2018-7-26 @ 09:24

Also found the front and back of this card on Ebay :P
Attachments
back.jpg
front.jpg
anetanel
Member
 
Posts: 103
Joined: 2017-1-01 @ 23:10

PreviousNext

Return to Video

Who is online

Users browsing this forum: No registered users and 3 guests