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Getting a Megascreen 3 card going...


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@wymtb Yes, I did measure the voltage across 4.7K ohm resistor I inserted between the video signal and ground and I got 1.12 volts.  Keep in mind this is when displaying the 50% gray desktop background, so I multiplied this by 3 to get 3.36 volts, then I used that to calculate the source impedance I've mentioned previously, approx 2.3K ohms.  Since VGA terminates with 75 ohms, this forms a voltage divider that results in a voltage of 0.16 volts, well within the VGA standard.

 

Another good point in hand, 75 ohm terminated VGA lines require a lot of power to drive at a voltage at 5 volts, so that's another practical limit that prevents that condition from occurring.

 

Related to double-checking if this is sane, I found this great article for describing how voltage works with terminated transmission lines in play.

 

https://resources.altium.com/p/using-terminations-control-reflections

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Very very nice! Might I suggest adding a straight row of headers duplicating the connector on the board. Female to female jumper wires could then be color coding matched row to row for those without proper cables. The PCB would then be useful for any proprietary iteration of DE9 or even the CoAx connection on Radius Cards.

 

There's a bare Radius FPD card on eBay ATM that could use such a board.

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Yeah, that sounds like a great idea.  I found out that many PCB manufacturers don't want to do circuit boards smaller than 51x51mm and asking for such ones can cost a premium, so I was looking for a legitimate excuse to expand the circuit board to that size.  My main concern on widening the circuit board was potential mechanical fit problems but looks like there should still be enough clearance.

 

Worthy of note here is that if you have no idea what signal you're working with and want to just test permutations with a VGA monitor, using 1K ohm resistors in lieu of jumper wires should be a safe bet with 5 volt cards.

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Okay, so regarding resistors on the adapter, I'm in a bit of a dilemma.  I'm not concerned about the card putting out too much voltage and damaging the monitor on the other side.  But, I am concerned about a naive passive wiring approach requiring too much current sourcing from the MegaScreen driver chips.  Datasheet limits for most chips on the board are 1 mA high logic level, 20 mA low logic level, with the exception of one driver chip that has 5 mA high logic level.  My experimental measurement seems to point to the video data line coming from the 1 mA source, i.e. 5K-ohm output impedance, so this means the resistance is coming from the chip itself?  Doesn't sound good long term... though maybe it was historically used just that way in practice.

 

But addressing this concern would entail using 4.7K-ohm resistors and with these, the VGA 75-ohm line termination voltage drop would be a rather low intensity/weak VGA signal.

 

If I compromised I would just end up picking 1K-ohm, or go back to my explicit line drivers design.  The problem with the line drivers design is routing a power source.

 

Anyways... making a breakout board form factor sidesteps this issue entirely.  The remainder of wiring is then "use at your own risk."

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Posted (edited)

I don't know what to believe... I need to figure out how to remove the MegaScreen from the MLB PDS connector so I can photograph the underside of my board.

 

Inferred on-board configuration of series resistors with outputs:

  1. Black, GND: 160 ohm
  2. White, VID shield: no resistance?  Could possibly have switched access to the 430 ohm resistor.
  3. Red, VID data: 200 ohm
  4. Green, HSYNC: 1K ohm
  5. Orange, composite video: 1K ohm?
  6. Blue, VSYNC: 1K ohm?

 

If the series resistor with GND is correct... that's really weird for me to understand the exact effect.  At first thought, it prevents proper line termination, but on second thought, the line can still be properly terminated, this just functions as further limiting the current to the line.  I'll have to think about that more.

Edited by quorten
Understanding GND resistor
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The PDS connection should pry apart easily using just your fingers on the SE. Doing it gently/firmly at both ends alternately where the board mounting flanges are located should do the trick. You'll mostly touching the circuit boards. Some connectors have shallow grooves for twisting a screwdriver on the sides.

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  • 2 weeks later...

I just purchased a rather dusty Mac SE and when I popped it open I found one of these, a "MegaScreen for Macintosh SE".  I'm so glad to see your work here, and that the drivers are available!

 

Hopefully I can put together a little adapter like you've done sometime soon.

megascreen for macintosh se.jpg

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Ooo, that's interesting, looks like MegaScreen introduced a later revision of the MegaScreen SE that nixed the solder-in coprocessor option.  Of course, because if you wanted to add the coprocessor by then, you'd just buy the SE*M and plug in the expansion board.  Exact same video controller chip and still doesn't have any ROM on the board by the looks of it, so the drivers should be compatible.

 

I've been meaning to send a design to PCB manufacturing to get one of my own adapter boards, and of course that would mean I'd get three boards and wonder what to do with the two others.  So that would be up for grabs if you're interested @cy384.  In any case, the board design is up for grabs, see "ms2vga" in here.

 

https://github.com/quorten/megascreen/

 

My main holdup in placing the purchase was that I was just wondering whether it would make sense to go for a hard-wired board or do a "breakout board" in case we can't deal out enough dedicated boards for MegaScreen users.

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Just now seeing this thread and wishing I hadn't given up my MegaScreen 3 - nice job working this all out, documenting it up, and making it available for the future!

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Does anyone know the connector used on the board? Looks like a Molex SL to me.

 

I think my ideal solution would be just an internal cable with VGA at the panel, assuming I'm correct in that you get a nice image without any other components at all?

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Yep it's definitely compatible with Molex SL, you could even just do 1/10 inch pin headers if you didn't care for the polarizing latch.  When taking into consideration my experimental measurements thus far, hypothetically that is correct that you don't need any additional components to get a nice image.

 

The thing is that in the original connection setup, there was a full-blown video breakout board that could plug into the DE-9 connector.  I'd prefer not to encourage people to do away with the vintage panel connectors in case the original video breakout board ever turns up.  Unless, of course, you've turned up a MegaScreen without the panel mount connector.  The pinout of both ends of the cable should be the same on all revisions of the MegaScreen.

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