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FarmerPotato

What would you do with dual screens on a 99/4A?

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Question: what video connectors would you care about?
  • DE-15HD (VGA) + jumper for RGB-Csync or VGA-Hsync-Vsync
  • SCART with jumper to choose RGB-S or composite
  • S-Video (basically free with CXA2075 encoder)
  • RCA composite (basically free with CXA2075 encoder)
  • 99/4A Din-5 composite
  • Commodore 1084S 6-pin DIN
  • Apple DB-15 RGB-S port (Apple IIgs, Mac era)
I consider SCART because some of us recommend Samsung 910MP with SCART.
I'm not sure what DE-15HD to SCART cable is available unless you wire your own.
I think Apple passive adaptors are generally available.
I think there should be a separate PCB for connectors, with a ribbon cable.
Pros: relieves stress, allows connectors to change without major rework.
Cons: impact on impedance, crosstalk

 

 

doesn't matter long as i can build a cable.. scart is preference for rgb/15

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SCART connectors are prohibitively large on boards. I also tire of building cables. I do prefer RGBc signals usually. I use an XRGBMini and a big SCART switch box.

 

There are suppliers of retro DIN to SCART cables. It'd be nice to be able to grab a pre-made Sega mega drive to SCART cable or something like that.

 

Then there are plenty of SCART/RGB to HDMI upscalers available.

 

I don't feel that 15khz VGA is very US common.

 

[email protected]

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SCART connectors are prohibitively large on boards. I also tire of building cables. I do prefer RGBc signals usually. I use an XRGBMini and a big SCART switch box.

 

There are suppliers of retro DIN to SCART cables. It'd be nice to be able to grab a pre-made Sega mega drive to SCART cable or something like that.

 

Then there are plenty of SCART/RGB to HDMI upscalers available.

 

I don't feel that 15khz VGA is very US common.

 

[email protected]

 

I looked up the Sega mega drive cable. I like that idea.

 

What I came up with was: put all the signals on a 2x6 header. Two of them for twin VDPs. Connect by ribbon cable to a small PCB with the port. So that could be a choice of two SCART, two VGA, or two Sega mini-DIN-8, all equally inexpensive from DirtyPCBs. For the EXT video source, I'm not sure what makes sense.

 

I'm not sure what RGBs sources you would be likely to have today? I imagine adding 99/4A graphics over your YouTube video production would be the typical use of an EXT source.

 

And how to record the combined result?

 

 

The more I look into it, the more I want to just have twin VDP with the ability to superimpose VDP2 over VDP1.

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This is a very interesting thread.

 

Guess cross-sharing ideas can't hurt; in the Colecovision section there is some VDP stuff being discussed and worked on as well:

 

 

https://atariage.com/forums/topic/291602-yet-another-80-column-expansion-idea/

 
 

 

 

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I've worked out most of the BOM and schematic for the twin 9958 option. No PCB layout yet. Parts ordered.

Features it does include:

  • Superimpose VDP1 over VDP2, or VDP2 over VDP1 on connector 1 only (simply set the Superimpose bit on one VDP, but never both.)
  • Mirror mode at startup
  • Independent VDPs after the first write to VDP2 only
  • 128K VRAM each.
  • RGBs, CV, SV out (Sony CXA2075 composite/SV encoder)
  • NTSC/PAL jumper
  • Choice of any 1 or 2 monitor jacks (twin 6x2 ribbon cables from board to jack PCB)
  • Single 9958 is an option by removing half the circuit.

Does NOT include:

  • External video input 
  • Extra 64K expansion VRAM
  • Fitting inside the console... maybe, maybe not. Uses 2x20 footprint to the 9918A socket plus 2 jumpers. I'm planning ribbon cable to external.
  • Convenient kit form.. uses surface mount but no smaller than 1206, 1.27mm, 1.78mm. I decided against AD1675-3 in 0.6mm TSSOP. The 1.27mm costs more $.

 

BUT this went down a priority level when my FORTI-2 PCB arrived. Too many projects!

 

I am also a big fan of the F18A and mk2. I anticipate making only about 10 of these Twin 9958.

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This is the superimpose switch:

         ________
YS1* ---|Set     |
Vcc-----|D      Q|________
YS2* ---|Reset   |        !
        |________|        !
       JK flip flop       !
                     _____!__
YS1* ---------------|A   sw  |
                    |       Y|__
!YS2*---------------|B       |  !
                    |________|  !
                  analog switch !
                                !
                           _____!__
RGB1 --------------------1|\   sw  |
                          | > --RGB|------ RGB
RGB2 --------------------0|/       |
                          |________|
                        fast RGB switch

VDP1 outputs YS1*, RGB1 (R,G,B)
VDP2 outputs YS2*, RGB2 (R,G,B)
VDP1 and VDP2 are synchronized (not shown)

 

When a superimpose bit is active, YS1* or YS2* emit 0s for transparent pixels. The first one to emit 0, flips Q to its side.


If YS1* flips Q to 1 (the default on powerup), the analog switch passes YS1*. If YS2* flips Q to 0, the analog switch passes !YS2*.

 

The fast RGB switch normally passes RGB1. When Q=1 and YS1*=0, it passes RGB2. That is, transparent pixels on VDP1 result in VDP2 showing through. If Q=0, the situation is reversed. The switch passes RGB2, but transparent pixels on VDP2 cause the switch to pass RGB1.

 

This circuit minimizes the parts cost: there is only one expensive (350MHz) RGB switch (AD1675-1 x3). It minimizes skew: VDP1 over VDP2 is the direct path. But VDP2 over VDP1 may have a slight skew from the NOT function of !YS2* which may blur the pixel transition a bit.

 

It's undefined what happens if both VDP1 and VDP2 have the superimpose bit set. They would constantly fight for Q, which is not a fast switcher; neither is Y. The output could be deeply weird.

 

What use cases does this support?

 

1. VDP2 over VDP1 would let you put titles onto VDP2, then run a normal 99/4A program (that is unaware of the Twin 9958) on VDP1 under the titles.

2. VDP1 over VDP2 would naturally be a way to get another scrolling plane, extra colors or deep background sprites.

 

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