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Model 3 video output.

P

prime

Experienced Member
Joined
Sep 20, 2009
Messages
153
Location
Coventry, UK
Hi all,

I have a working TRS80 model 3 motherboard, but that's all I have :(
I'm using a PS2->Crosspoint switch to implement the keyboard, and have a circuit using a 74LS02 and some discretes to combine the syncs and video into a composite sync like signal.

However when I feed this into one of my LCD TVs I get lthe right hand columns going off the side of the screen.

Is there a well known solution for this that is known to work? e.g. a scan converter or something.

Cheers.

Phill.
 
I don't think the frequencies are compatible with NTSC.

There's a guy on one of the TRS-80 discords who made a contraption that replaces the CRT analog board and outputs VGA. It sounds to me like this is what you need. But I can't remember who it was that made it. Hopefully someone smarter than I will mosey on along and hit us with the clue stick, though.
 
I'm surprised that syncs at all. The sync timings are not the same as composite video. I vaguely recall doing something similar a long time ago using a mono CRT TV but I think I may have had to modify the TV to make that work.

I'd also be interested in a solution as I also have a motherboard that needs some work, but no CRT or case.
 
There's an adapter available that should work, although the notes say it may not work with all monitors.

https://blue-print.be/#products

I picked one up a few weeks ago, but haven't put my 4 back together enough to test it.
 
prime said:
However when I feed this into one of my LCD TVs I get lthe right hand columns going off the side of the screen.
Click to expand...

The Model III's basic scan frequencies are roughly NTSC compatible, but the machine uses a really slow dot clock for its 64 column (512 dot wide) display. It's only about 10mhz, and if you do the math that means there's almost *no* "clearance" around the horizontal sync porches. (Or to put it another way, the Model III's monitor is tweaked so there's no overscan.) They sold simple "sync combiners" like what you've built back in the day but you usually needed a monitor with horizontal sizing and positioning controls to make it fit.

Finding an LCD monitor or "cheap" scaler that will let you go "all the way to the edges" might be a challenge.

(Trivia: The Model I has an almost identical 10mhz-ish dot clock. It only fits its 64 columns on a much-less-tweaked TV because it's only doing 384 dots across with its 6-bit wide font.)
 
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That looks nice. For those of us without Eagle, would you mind possibly generating a PDF of the schematic? While a set of gerbers would be really nice, just the schematic in PDF would be sufficient.
 
lowen said:
That looks nice. For those of us without Eagle, would you mind possibly generating a PDF of the schematic? While a set of gerbers would be really nice, just the schematic in PDF would be sufficient.
Click to expand...

I've updated the Hardware/Generic folder with both.

Though since I use Eagle V6, you should be able to use V6 or above to open them. Recent versions of KiCad can also import Eagle V6 and above files too.

I've bought the crosspoints from here : https://www.futurlec.com/Zarlink/MT8816AEpr.shtml Though I believe Mouser have something compatible too.
For the AVR any chip that is compatible with the Mega16 pinout should work, current board I'm using has a Mega324a init.

Cheers.

Phill.
 
prime said:
I've updated the Hardware/Generic folder with both.
Click to expand...

Fantastic, and many thanks!

Though since I use Eagle V6, you should be able to use V6 or above to open them. Recent versions of KiCad can also import Eagle V6 and above files too.
Click to expand...

Ah, yes. Didn't know which version of Eagle was in play.


I've bought the crosspoints from here : https://www.futurlec.com/Zarlink/MT8816AEpr.shtml Though I believe Mouser have something compatible too.
Click to expand...

What a creative use of an analog crossbar as used in telephone switches...... with multiple sources, too. Nice!
 
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