exxos's Atari, Amiga & retro forum

Hi All,

I have a noob question for electronics

Each Atari ST and TT have possibility to use external synchronization V & H, to synchronize with the Genlock. And I'd like to check that.
I'd like to connect two ST to one LCD. The first one would output RGB or MONO signal to my LCD and the second one would output VSYNC & HSYNC to both: LCD and the first ST (that one will be switched to external synchronization).
Fortunately my LCD supports 15kHz and 31kHz through VGA port.

My question is: is it safe a such connection (wired VSYNC & HSYNC) for both STs and LCD?

Below you can see my schematic.

Be sure to check connectivity (e.g., with a multimeter) from the GLUE to the connector for the sync signals. Afaik, some revisions of the ST pass these signals through a (unidirectional) buffer -- as does the TT. In that case, you cannot use the monitor connector as input for sync. Trying to do so could then theoretically damage something.

May I ask what the ultimate goal of this experiment is?

@czietz We were talking about the feasibility of creating some kind of TT VGA 'cleanup' adapter, which could convert the timings to standard VGA. As we know with an OSSC and the right setting you can dial in a perfect TT picture, but this costs a lot of money. We're wondering if something lower cost could be made.

It would have to be something similar to the OSSC but with only one input and fixed settings. The audio input is useful so that you can send stereo to the display.

If you want to use VGA output you'd have to record the scan line and then replay it at a slower rate out of the other connector.

None of this is simple.

On the TT it's not possible to input external HSYNC / VSYNC via the monitor connector because of the buffers that I mentioned in the previous post. Besides, the main difference is the pixel clock, anyway, not the timing of the sync signals.

czietz wrote: 13 Feb 2021 19:24 On the TT it's not possible to input external HSYNC / VSYNC via the monitor connector because of the buffers that I mentioned in the previous post. Besides, the main difference is the pixel clock, anyway, not the timing of the sync signals.

So would it be possible to create a tiny device that just converts the pixel clock to standard VGA? Like the way the TT ECL adapters connect up.

Just been at work all day and the forums have gone crazy! :roll:

Anyway, I asked this same question a while back and got a fairly detailed response from Ijor on AF:

https://www.atari-forum.com/viewtopic.p ... 96#p363896

Basically, changing the pixel clock is not worth it.

Better to go down the OSCC route, or better still, cut out the whole problem at the root and build a TT overscan unit! ;)

Steve wrote: 13 Feb 2021 22:13 So would it be possible to create a tiny device that just converts the pixel clock to standard VGA? Like the way the TT ECL adapters connect up.

No. The TT high ECL->VGA adapters do not change the pixel clock, either.
If you want to change the pixel clock in the TT to standard VGA, you have two options: a) design a new TT Shifter chip to decouple input clock from output clock, or b) use an external device such as the OSSC that captures video data and replays it at a different clock.

But since the original question was about the ST: On some revisions of the ST (those without buffers) you can use the sync signals as input. But very little can be achieved this way. If you input sync signals with different timing than the ST would produce itself, it does not work.

czietz wrote: 13 Feb 2021 16:36 Be sure to check connectivity (e.g., with a multimeter) from the GLUE to the connector for the sync signals. Afaik, some revisions of the ST pass these signals through a (unidirectional) buffer -- as does the TT. In that case, you cannot use the monitor connector as input for sync. Trying to do so could then theoretically damage something.

@czietz those buffers are not good news.

czietz wrote: 13 Feb 2021 19:24 On the TT it's not possible to input external HSYNC / VSYNC via the monitor connector because of the buffers that I mentioned in the previous post. Besides, the main difference is the pixel clock, anyway, not the timing of the sync signals.

Below is VESA standard for similar pixel clock 31.5 MHz but with different horizontal and vertical timings.
http://tinyvga.com/vga-timing/640x480@75Hz
http://tinyvga.com/vga-timing/640x480@73Hz
We would try to adopt those horizontal and vertical timings, if we were able to switch to external synchronization.
I wonder why TT documentation shows that external sync can be used.


Back to my question and my schematic, whether is it safe from electronics point of view?

Cyprian wrote: 16 Feb 2021 18:20 @czietz those buffers are not good news.

Just to illustrate that I'm not imagining things, here are the buffers for HSYNC from an ST schematic from dev-docs:

Cyprian wrote: 16 Feb 2021 18:20 Below is VESA standard for similar pixel clock 31.5 MHz but with different horizontal and vertical timings.
http://tinyvga.com/vga-timing/640x480@75Hz
http://tinyvga.com/vga-timing/640x480@73Hz
We would try to adopt those horizontal and vertical timings, if we were able to switch to external synchronization.

I'm afraid it does not work that way. As I've said before (also applies to the TT): "If you input sync signals with different timing than the ST would produce itself, it does not work."

Cyprian wrote: 16 Feb 2021 18:20 Back to my question and my schematic, whether is it safe from electronics point of view?

Yes, assuming you use an ST without buffers and you switch the sync pins on the GLUE to input before connecting the cable. I still wonder, though, what this is supposed to achieve.