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Getting a Megascreen 3 card going...


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Alright, so I'm back on this adventure thanks some software I was able to get from @philiplj.  As I've previously noted, this class of Macintosh SE video cards are unusable without any software drivers, and of course the original Boeing military-contracting owner of my vintage Macintosh SE was conscientious enough to zero every sector of the internal 40 MB hard drive.  But philiplj got his Macintosh SE from an alternate world that was kinder about selling a computer with a hard drive preloaded with useful software, and hence I have a copy of at least some of the original software.

 

Anyways, as a good start, I've put together a GitHub repo/website with the info I know so far.

 

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

 

I have the Megascreen control panel extension installed, but it appears I'm missing additional software required to interface with different types of external monitors.

 

Past 68kMLA forum threads:

 

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@Trash80toHP_Mini Regarding the earlier question about how this fits in with MultiFinder and the Radius... from what I'm seeing so far playing around with the little software I have, there isn't any MegaScreen software to specially handle multitasking, this would all be covered by MultiFinder.  From philiplj's system, the software is apparently known to work on System 6.0.5, I'm running 6.0.7.

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Wow, that must be a Megascreen 1 from the looks of it.  The video interface has the same pinout.

 

Thanks for the software!  That will definitely be handy to at least have a complete image of the software for a Megascreen 1.  I believe it should actually "just work:" for Megascreen 3 since the card itself is rather simple, and Megascreen 1 might just use an earlier revision of the same graphics chip.  Looks like the main difference between my card and yours is that mine has a bunch of VRAM onboard.

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I see four VRAM packages on that card. What do the specs on the ICs say, I'm curious about capacity?

 

What's really interesting to me is the expansion header on the left side. We've seen similar connectors at the rear of accelerators for VidCard installation. Wondering about ThinNet on the Co-Ax connector? Buzz the center connection to see if it might wander over to a pin on that the expansion header without interacting with the VidCard maybe? It's most likely an alternate video signal over that connection to a different monitor interface. Radius TPD uses a Micro Co-Axial connector on the board., but a ThinNet NIC daughtercard would be a neat trick. What's the width of the PCB, comparing it to SE card max. width spec. will be interesting.

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Posted (edited)

These are the specs I've found for the VRAM chip on my card:

TMS4461-12NL: Fast Page Dual-Port Video DRAM, 64Kx4, 120ns, NMOS, PDIP24
 https://4donline.ihs.com/images/VipMasterIC/IC/TXII/TXIID099/TXIID099-4-27.pdf?hkey=FB3F1F3F2A09A989A6BF9D772C3B8264

So my card, with 4 such chips, has a total of 128K bytes VRAM, plenty of space for a Megapixel monochrome display.

Edited by quorten
Clarification
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Yep, side view required. PCB real estate conservation and straight line bus trace implementation would be the reasons for that IC packaging.

 

Zip_chip_socket.jpg

 

https://en.wikipedia.org/wiki/Zig-zag_in-line_package

 

Not all that important or readily apparent in single bit graphic card implementations, but for 24bit color in the NuBus era it was a big advantage. Corn row upon corn row of those VRAM packages can be seen spaced tightly with straight line trace connections on the likes of  TRUEVISION NuVista cards. Staggering pins on the ICs instead of traces on the PCB as required for wider spaced DIP packages reduce board real estate requirements remarkably for high density, high speed memory implementations.

 

 

 

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  • 68kMLA Supporter

Interesting. I think I have one of these for a Mac Plus with a Killy clip over the 68000. Never really knew what it was or how to get it to work. Perhaps you guys know...

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Well, I'll be.  Now I'm using the phrase "unreasonably excited" to describe having the full drivers disk image from @wymtb.  I was feeling old skool so I transferred it to a physical 800K disk with Apple Disk Transfer ProDOS even though nowadays I have the means to do a more direct file transfer.  And yes, I see, there's a configuration utility on the full disk image.

 

And... booting right off of the Megascreen drivers disk, the Megascreen appears to be working!!!  But... the crux is that the menu bar is configured by default to appear on the big screen, and I have nothing connected at the moment.  So I literally can't see anything but a blank desktop.  How do you take a BNC video output and connect it to a composite video input?  I think that's the first thing I want to try out.

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Found an interesting pic for comparison in your other thread:

 

pds_card.jpg

 

IMG_20210318_085721.jpg

 

The cutout in the foam looks to be the size of the standard spec. SE card above. The vacant cubic looks remarkably like a compact NIC footprint? Could be for a VRAM expansion card for a huge virtual desktop setup as well.

 

Curiouser and curiouser  .  .  .  :?:

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The BNC connector is a composite signal. Was able to get a visible picture using a GBS 8200. Not a sharp one but there was a picture.

Today I found some time to play araound with the 9 pin connector:

HSYNC Pin 3
VSYNC Pin 4
Video Shield 2
Video Pin 1
Ground Pin 6

I started with building a

(hard to find all the needed ICs) some time ago. So I had a VGA adapter form this project lying around. Using this pcb I am able to get the MagaScreen connected to my LCD with a 1024x768 resolution.
At the the end this is more or less SYNC directly routed to the VGA connector and inside the video line two resistors 130 and 150 Ohm. RGB connected while soldering the VGA connector. Not a perfect picture but I am happy to be able to get the MegaScreen working at all.

 

 

IMG_20210319_201408.jpg

IMG_20210319_211219.jpg

IMG_20210319_214655.jpg

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@wymtb Glad to know what the pinout is!  As for the electrical interface matching on the DE-9 connector, that's a far less cautious design than I was sketching up.

 

ttl_vga_mock.png.dc99705209fc0cd1faa2bc9bd9546cac.png

 

When the monitor end terminates with a 75 ohm connection, do you have any idea what the output voltage of your circuit is?  I'm guessing the minor video quality problems might be due to not terminating at the characteristic impedance.  150 ohms would probably be correct for differential signalling line termination, but I believe single-ended VGA signals should terminate with 75 ohms.

 

It's worth studying how the resistors on the Megascreen board itself are connected with the video outputs.  There are, after all, not many resistors on the Megascreen board.

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Looking at the two board variations carefully, looks like @wymtb's board is a newer design iteration than my board.

  • Rather than soldering in the FPU coprocessor option (64-pin socket), there is a 60-pin header for you to just plug it in.  The board is correspondingly shorter to allow for sufficient space.
  • This is why the VRAM chips need to stand vertically.
  • Some design cleverness must have been able to reduce the number of standard glue logic chips required to support it, this probably entailed redesigning the two PALs, and U16 stuffs in a few more circuits into a single chip by virtue of having more pins.
  • Transistor Q1 was added onto the board, and there are two electrolytic decoupling capacitors instead of one.

I'm guessing the video controller chip and the logical software interface to it is functionally identical between our boards.

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Yeah, the main difference is my card has an iron filtering ring, I'm guessing they found out that was unnecessary when designing the newer card.

 

Where did your Megascreen card come from?  Is that the original boxing it came in, and what about the drivers/tools floppy disk?

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Posted (edited)

Looking through the documentation, this is the name of the game of the newer card.  Official name is "MegaScreen SE*M Video System".  The name of the 60-pin connector interface is MegaModule.  Mega Graphics actually released a MegaModule Developer Kit with full specifications and support software.  I'm guessing that my older Megascreen 3 card can be jury-rigged to support MegaModules, the main issue being that there wouldn't be the conventional space to place the additional board.

 

It's possible that there may have been a MegaModule developed for expanding with networking capabilities, but we're getting into the territory of exceedingly harder to find software and hardware.

 

Also, I'm guessing Megascreen 3 was the internal code name of what was actually marketed as MegaScreen SE.  I don't see "MegaScreen 3" listed in the product catalog included on the floppy disk.  MegaScreen 2 is the processor clip-based format.

Edited by quorten
Grammar, more Megascreen models
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@LaPorta Sounds like that's a MegaScreen II, and using it should be straightforward with the info I've found so far.  Just use the same software posted here, the same VGA adaptation, and it will just work.  And, of course, there is always the NTSC composite video route you can go, but you probably want to get hassle free high resolution.

 

Well, when we have a super-slick adapter board designed and available, that is.

 

Interestingly enough, the MegaScreen purportedly had a huge multi-video interface adapter board that was available, but that's going to be pretty rough to hunt down in the modern world.

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On 3/19/2021 at 7:46 PM, quorten said:

Where did your Megascreen card come from?  Is that the original boxing it came in, and what about the drivers/tools floppy disk?

I bought it back in 2003. As I can see in my old mails I probably owned a Mac Plus with a build in MegaScreen card and a matching CRT. As I missed the drivers I was not able to test the monitor. So I probably bought the SE card for 15 Euro. It is the original box but taped and with address label etc. as the box was directly used for shipment. Does not look nice. I can not say if the floppy disk is original, but the handwriting does not look to be original.  

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Posted (edited)

Okay, so I was looking at the electrical output of the MegaScreen and I figured out that the video data pin in the DE-9 connector must have a source impedance somewhere around 2.3 kilo-ohms.  This source impedance is plausible based off of the resistors on-board and likely they are used to set it.  Also I found out termination on the transmitter end is not necessary in point-to-point one-way data transfer.  Only the receiver uses 75 ohm termination.  Together this means there should be no need to add additional resistors to the adapter board, so I came up with this simple PCB design.

 

ms2vga_front.jpg.c2c5cd4afaa5c4db47b3fa2bd861487c.jpgms2vga_back.jpg.5b44b2c239153d39e1900a2cd9e40cad.jpg

Edited by quorten
Formatting somehow got lost
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