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Author Topic: Help hooking up a GBS-8220 CGA to VGA converter to replace Ceronix 1492 monitor  (Read 10336 times)

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knagl

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Greetings-

I am trying to replace a broken Ceronix 1492 13" monitor with a VGA LCD monitor.

I have purchased one of the CGA/EGA/YUV to VGA HD-Converter PCB (model GBS-8220) cards from Jammaboards.com - pictured here:



The documentation that comes with it is poor, to say the least.  It is, however, supposed to be able to convert CGA to VGA.

As a simple test to make sure my board was working, I used the Y Pb Pr inputs on the card with video from my DVD player, and it did convert it nicely to my VGA monitor, so I know the thing is somewhat functional and I know that it's compatible with the VGA monitor I'm using.

I think I'm bumping into a sync issue.  The game I'm working on outputs R, G, B, Ground, Horizontal Sync, Vertical Sync.  The GBS-8220 does have an input section that, according to the printing on the board is: Ground, Ground, VS, HS, S, B, G, R.  I have reason to suspect that the "S" here for this input is actually "shield" and not composite sync, and I'm not 100% sure what that means.  Regardless...

I tried hooking up my RGB, Ground, and VS and HS and would only occasionally get a flickering of something on the monitor, usually in a single color (green, blue, or red mostly) for just a second or two here and there.  The rest of the time I'd get a "No Signal" message generated from the GBS-8220 card.  From the tiny bit I could see, however, it did in fact appear that it was trying to display the signal the game was sending out.

The documentation (again, limited) implies that for RGBHV input, it only accepts in the 31k range, which CGA is not.  It does say that it accepts CGA at its 15k, but it seems to imply that for CGA it requires RGBS (Red, Green, Blue, Composite Sync) which one of the other inputs on the board ("P3" for those of you playing at home) is (which also has a ground pin, in addition to RGBS).  As such, I'm guessing that I can't hook up my CGA output to the card as RGB H V.

So... my question is, has anyone used one of these boards to convert a CGA signal that used RGB H V into a VGA monitor, and if so, how do you have it connected?  Do I need to somehow combine the sync signals and then try it on the P3 connector as composite sync?

Thanks in advance for any assistance you folks can provide.

MonMotha

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I don't know the board, but I'm guessing that the little post RGB header and the HD15 RGB connector are just hooked together possibly modulo some voltage divider networks to handle the fact that VGA is usually 0.7Vpp while CGA is usually 3.3-5Vpp signal levels.  That's how most of these types of things are built, but I've not messed with one of these specific units.

Where is your CGA video coming from?  If it's made by a PC, then you can just have the PC scale everything to VGA and not use this board at all.  Many PC cards also have an option for composite sync output, and convention is to place it on the horizontal sync line.

If you do have separate sync and you want composite sync, it's pretty easy to make.  Negative composite sync (standard in the arcade world) is negative horizontal sync XOR positive vertical sync.  This can be done using two gates from a 74xx86: XOR the positive horizontal sync with 1 (tie the other input to Vcc) to invert it then XOR that (negative horizontal sync) with the incoming positive vertical sync.  There are also some tricks you can play using RC circuits to make the polarities autoadjusting.  I can get you a PDF schematic if you want.

Some people report that just connecting both sync lines together works.  This is likely to create something resembling composite sync (with the horizontal pulses missing from the vertical pulse), but may damage the sync outputs on your source.  I don't recommend it, but it seems commonplace.

knagl

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Where is your CGA video coming from?

This is from an IGT Player's Edge Plus (PE+) video poker machine.  Based on the inputs to the monitor chassis and the schematics for the poker machine, we're able to identify RGB Gnd VS and HS.  Beyond that, though...

MonMotha

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Wow, an amusement device using separate sync.  Kinda rare.

Yeah, I guess just make composite sync as described and give that a shot on the connector prescribed in the manual.  The circuit I described with a 74xx86 can easily be built on perfboard/veroboard/donutboard.

grantspain

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i have connected one of these,i use the 5 pin plug from the cga source and link the v/h sync together and then use the vga d-sub output

knagl

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i have connected one of these,i use the 5 pin plug from the cga source and link the v/h sync together and then use the vga d-sub output

Wait, you've connected a PE+ poker machine to the GBS-8220?  How specifically did you "link the v/h sync together"?  Thank you very much in advance!


MonMotha- Thank you for your suggestions as well.  It's probably going to be a couple of weeks before I get to tinker again, as I'm headed out of town on Friday...

MonMotha

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How specifically did you "link the v/h sync together"?  Thank you very much in advance!

You simply connect the wires together.  It seems to work a surprisingly large amount of the time, though it's not really "proper".  I'd never do it on a real design, though, for aforementioned reasons.