Displaying CGA from a PC using GBS-8220 v3

C

chinnyhill10

Member
Joined
Jan 2, 2014
Posts
101
Country
UK
Region
Hampshire
Ahoy-hoy!

Acquired an Amstrad PPC 640 for very little the other week. Built in MDA screen but CGA output on the back. The small size compared to desktop machines and the fact it can be packed away seemed to make it an ideal candidate to add PC reviews to my Youtube channel ( https://www.youtube.com/channel/UCifcRlP9KST8T0irCHyykrA ) where I review games from lots of different formats on original hardware.

However I have completely failed to get any usable output from the machine.

Firstly I purchased a CGA to VGA cable but it turned out that the entire thing was a waste of time. All the bottom pins of the 9 pin end were electrically connected! So it was never going to work.

Undeterred I purchased a GBS 8220 (v3) board as it seemed cheap and specifically promised CGA to VGA. Not quite what I needed to capture with but enough to get a display on a screen. And the board looked interesting for other used to boot.

I made up a lead, plugged it in and nothing. Just an occasional rolling picture.

Anyone have any experience of these boards? Are they in fact only suitable for CGA from an arcade machine and not CGA from a PC?

What I really need is some way to get CGA to RGB SCART so I can capture it but in the short term just getting the GBS working would be a start.

Anyone have a GBS board working with CGA output from a PC?

Thanks.
 
Hi,

I have plugged the Amstrad PPC 640 with the GBS. The original firmware can sync only with RGBS and not with RGBGV at 15 Khz, and the sync should not be a TTL sync but an "arcade" sync (so it has to be lowered from the 5V to 1V).

If you don't care about getting luminance (so being able to use 8 colours), you can solve both the issue building a XOR circuit between horizontal and vertical sync with a TTL IC (like the 74LS86, a capacitator to stabilize the voltage and 3 resistors to lower the voltage). If you need a schematics, I'll search or make one from the one I have done.

Alternatively, you could use a Raspberry Pi to reprogram the GBS with a custom firmware to sync also at 15Khz but I haven't tried it (since the hardware way is cheaper for one who didn't have a Raspberry :) )

If you want to include luminance, the circuit is more complex since you have to mix the luminance with the colors. There are some schematics on C64 to SCART sites.
 
What's needed is an RGBi to analog RGB conversion board.

I use one with my C128 and it works quite well. It works with the GBS-8220 but also with the Ambery AV-1.

This simple circuit will do it but there are higher quality solutions.
http://www.frank-buss.de/c128/vdc/index.html

This project is specifically designed for the GBS converter:
http://www.insentricity.com/a.cl/219/cgargbi-to-analog-for-the-commodore-128

Schematics:
http://d81.de/shared/C=128-VGA/C=128-VGA-IBM-Colorfix-sch.png

This schematic does not include the XORing of the sync signals required by the GBS and Ambery. However, it will work well with any monitor compatible with Amiga analog RGB 15KHz signal.
 
Last edited by a moderator:
Thanks for the replies. I was hoping the GBS was going to be an off the shelf solution (I'd read of people using it with TTL but it seems its not going to work. I had a look at those circuits and they seem a bit beyond me. Probably best forget about the GBS board.

Anyone selling a CGA to SCART solution? In an ideal world that's what I need in order to capture footage.
 
I use a Video DAC converter for my C128 that works along with the GBS. It should also work for your situation.

http://he-insanity.blogspot.com/2014/03/commodore-128-cga-video-dac-board.html

So far as I know, he doesn't make them any more but I have an extra assembled unit that I'd be willing to sell for the price I paid. I'll also include the connector to attach to the cable that comes with the GBS.

Heather
 
This sounds very interesting thank you.

I know Amibay has very specific rules about trading and naming prices and I don't want to cause problems in the community. What's the best way to proceed? Is a PM permissible?

Thanks.
 
Doesn't the PPC have at least one ISA slot? Get an ISA VGA card and you should be all set.
 
I'll send you a PM. If we agree to anything then we'll make a sale thread. :)

Heather
 
SkydivinGirl said:
I use a Video DAC converter for my C128 that works along with the GBS. It should also work for your situation.

http://he-insanity.blogspot.com/2014/03/commodore-128-cga-video-dac-board.html
Click to expand...
I purchased a board exactly like that a few years ago from a "SirRichard".

For reasons I don't understand the output that comes from it will work well with an Ambery AV1 or GBS 8220 but not directly with an Amiga 15KHz RGB monitor.

Yet, the signal from an Amiga will work directly on the Ambery. The only difference I could notice is with the sync signals - the DAC converter performs a logic NOR on the syncs.

Also, SirRichard indicated in his notes the colors look best with the Gonbes GBS-8220 but this converter does not support 50Hz refresh rates.

I will try and alter my board for compatibility with 15KHz RGB monitors.
 
Last edited by a moderator:
I think chinnyhill10 is looking to connect to VGA so it shouldn't be an issue. Here's the output of my C128 using the Video DAC converter with GBS 8220 on an LCD monitor.



The quality is quite sharp and clear.

Heather
 
You must log in or register to reply here.

Similar threads

Bastich
  • Locked
Sold GBS 8220 v3
Replies
3
Views
879
Gravislord
G
Phipscube
SCART RetroBox using GBS 8220
Replies
10
Views
12K
Phipscube
Phipscube
Back
Top Bottom