RGB video with 540B1?

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sonofmagni
Posts: 3
Joined: Sat Jul 16, 2016 10:59 pm

RGB video with 540B1?

Post by sonofmagni »

First hey, it great to find this group!

So I'm trying to get my old C8P DF running and having video problems. It seems that someone (me) modified the video board around 1978 or so to get the V and H sync and video to tap into a B/W tv so I could skip the modulator. But now I'm trying to use the composite video output and it's dead. I tried, based on the schematics to get back to the original circuit for the color board but as luck would have it after (hopefully) undoing my modifications now my o-scope just died!

So, while I try to get a new scope I was wondering whether anyone ever tried swapping the jumper at J1-2 to Red instead of the color adjustment pot, and ran RGB from the video board!!!

Thanks so much for any help and for being a resource for OSI computers. Btw, the main reason I'm even doing this is I thought it would be cool to try running Sargon again :)
- Karl
Steve Gray
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Joined: Mon Oct 06, 2008 1:54 pm
Location: Markham, Ontario, Canada
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Re: RGB video with 540B1?

Post by Steve Gray »

I havent tried but it should be possible to use an RGBI (digital) monitor... Take a look at my 540b recreated circuit in another thread. RGBI uses 5v signals and separate h and v sync lines, so everything is available. You could also use an RGBA (analog) monitor (or tv with scart) as well but you may need to lower the RGB voltages and probably need combined h/v sync.

Steve
C4P working, C1P working. 600D Replica working, C4P+D&N floppy not working. 505 board, 610 board, Mittendorf board, TOSIE hacker board need testing, PicoDOS disk untested.
sonofmagni
Posts: 3
Joined: Sat Jul 16, 2016 10:59 pm

Re: RGB video with 540B1?

Post by sonofmagni »

Thanks for the reply Steve. I've worked on this a little since the original post. Managed to pick up a TI 465 o-scope on craigslist :)

The issue is more fundamental than I had first thought. I backed out the changes I had made back in the '70's and still not getting proper front porch or sync on the video out. Tracing that back it looks like I'm not getting VAS/ at all. This is dependent on correct combination of A11, 12, 13, 14, 15 and VMA. Either need a pulse at U6H.8 if it's a write cycle or U6F.8 if just displaying, and getting neither. All A11-A15 are toggling but without an analyzer I can't see them all at once.

I'm guessing the code that executes in the CPU to create the horizontal timing is triggered by an interrupt on the CPU board? So maybe reseating the CPU board and any socketed chips might do something, just afraid I could make matters worse.

So that's where I'm at. Any suggestions are very welcome.
- Karl
Steve Gray
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Re: RGB video with 540B1?

Post by Steve Gray »

sonofmagni wrote:Thanks for the reply Steve.
I'm guessing the code that executes in the CPU to create the horizontal timing is triggered by an interrupt on the CPU board?
- Karl
OSI's do not use IRQ or NMI on the CPU. The video timing is all on the video card. However, the 540B clock can be sent to the CPU card "00 in" pin (OSI BUS pin 18, OSI502 jumper W1, not sure about OSI505 board) allow the card to access video ram on the opposite clock phase from the CPU, to avoid video snow. At least that is my understanding.

Steve
C4P working, C1P working. 600D Replica working, C4P+D&N floppy not working. 505 board, 610 board, Mittendorf board, TOSIE hacker board need testing, PicoDOS disk untested.
sonofmagni
Posts: 3
Joined: Sat Jul 16, 2016 10:59 pm

Re: RGB video with 540B1?

Post by sonofmagni »

Ok, soon I'll go look at this again but the way I remember what I saw:
Looking at the middle of sheet 4, VAS/ needs to go low to create the 3us pulse from U4B. I had thought that was what created the "front porch" part of the horiz sync. VAS/ won't be generated unless the address bus A11-15 hits the right combination. This in turn is Dot Inhibit which through B/W composite drives the base of Q1.

Edit: But maybe U4B doesn't need to trigger, I just saw the 3us and thought that was the front porch...
Steve Gray
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Re: RGB video with 540B1?

Post by Steve Gray »

sonofmagni wrote:Ok, soon I'll go look at this again but the way I remember what I saw:
Looking at the middle of sheet 4, VAS/ needs to go low to create the 3us pulse from U4B. I had thought that was what created the "front porch" part of the horiz sync. VAS/ won't be generated unless the address bus A11-15 hits the right combination. This in turn is Dot Inhibit which through B/W composite drives the base of Q1.

Edit: But maybe U4B doesn't need to trigger, I just saw the 3us and thought that was the front porch...
I admit I haven't studied the schematics enough, but I do know the video board will output to the screen even without a working cpu card. If you look at the schematic recreation I did you can see that the video counters are driven by the 502's master clock, which gets divided down to "clock3". I'm still learning Kicad and I know that I need to fix some of the global labels on my schematics so that they use the proper input/output/bidirectional symbol.

I divided the schematic up according to what I thought the functions were. but if you see something in the wrong place let me know. If you want a PDF version with better zoom-ability let me know and I can send it to you.
I was thinking I might be able to reduce the decode logic with a PLA or some custom logic, but I dont have any experience with that kind of stuff yet...

Steve
C4P working, C1P working. 600D Replica working, C4P+D&N floppy not working. 505 board, 610 board, Mittendorf board, TOSIE hacker board need testing, PicoDOS disk untested.
MK14HAK
Posts: 356
Joined: Wed Mar 16, 2011 1:49 am
Location: New Zealand

Re: RGB video with 540B1?

Post by MK14HAK »

Did you manage to get Sargon Chess running Karl ? :)
600RevB:16K,2MHz,64x32,470,CEGMON
SuperKit:502,540B,542B,CEGMON, 8" and 5" FDDs
Cards:PE IO,6522 D-A-D, AY3-8910,ProgramGraphics,Color,UK101
WIP:HexDOS,FDD Emulator
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