exxos's Atari, Amiga & retro forum

I want to build a display output board (for my recased STe), and although I know there are several such thing already out there (Ubeswitch etc.) none of them allow for Atari monitors to be attached as well.
I also want to make it "idiot proof" so that in case multiple monitors are attached they won't interfere with each other.
Additionally I'm considering if I should add an S-video output and a composite one as well. Here's my idea so far:


The board will have a 3-way slide switch (3 position, 8 pole or something similar) for the following settings:
1) ATARI: a new 13-pin DIN connector with the exact same signals as before. Depending on being a colour or mono monitor the STe will boot into low/medium or hires mono
2) VGA MONO: same thing as a multisync switchbox set to "mono" for a VGA monitor accepting a hires mono signal.
3) VGA COLOUR: same thing as a multisync switchbox set to "colour" for a VGA monitor capable of low/medium resolution signals

Does the circuit look OK? I figured that the following lines needed to go through the switch to eliminate any conflicting problems with say an Atari SM-124 mono monitor and a VGA colour capable monitor attached at once.
For S-video I know there's an Atari-article on how to get this from the RF-modulator with some modifications, but IIRC it does away with the composite signal on pin 2 of the DIN-13 video output connector. I have an add-on circuit for an RGB-S-video/composite output, so I could use this instead if needed.

Anyone? :)

Looks OK at a glance, but really using physical switches for video isn't generally a good idea,

Would a mechanical switch controlling an analog multiplexer IC of some sort be a better solution?
Which signal lines are essential to switch? Or should I switch them all between the 3 modes?

Pacman wrote: 20 Jun 2021 11:04 Would a mechanical switch controlling an analog multiplexer IC of some sort be a better solution?

Yes.

Pacman wrote: 20 Jun 2021 11:04 Which signal lines are essential to switch? Or should I switch them all between the 3 modes?

Any video carrying signal.

So basically replace the (mechanical) switch locations in my schematic to go through the switch locations inside an analog multiplexer IC of some kind?
Switching the following signals to allow for switching between the 3 modes:

- Pin 2 (Composite sync)
- Pin 4 (Mono detect)
- Pin 6 (Green)
- Pin 7 (Red)
- Pin 9 (Hsync)
- Pin 10 (Blue)
- Pin 11 (Monochrome)
- Pin 12 (Vsync)
- Pin 13 (GND)

Yeah same as you got now but with a video switch ic of some kind.

Cool!
Apart from the number of analog switches it should have, I suppose the other requirement is that it can handle the following frequency range without (noticeable) degradation, right?:

50 - 35700 Hz
1V peak to peak video level

(the numbers above taken from ST colour mode (15.75 KHz Hsync/50-60 Hz Vsync) and ST mono mode (35.7 KHz Hsync/71.2 Hz Vsync).

Pacman wrote: 20 Jun 2021 14:25 So basically replace the (mechanical) switch locations in my schematic to go through the switch locations inside an analog multiplexer IC of some kind?

If you're going to use a mechanical switch I would make sure to debounce it. Just using a switch throws a butt load of switching transits out. Debouncing gives the switch nice edges.

https://circuitdigest.com/electronic-ci ... ce-circuit

Ah! That's useful info I didn't already know.
Out of the two (mechanical switch with a debouncer circuit or a solid state switch/multiplexer IC), what would be the preferred method?

I'm having trouble finding a 3-way multiplexer though. Maybe I'll have to cook up a more complex method with a switch controlling a switch etc.