[sdiy] Multiplexing (Atari video out)

[Hallvard Tangeraas]

I'm slowly working on modifying a standard Mega STe computer into a 
"MIDI specialized computer" by placing it in a 19" rack enclosure with 
all sorts of additional MIDI and other hardware connected internally.

I need some help with the video output board as my electronics knowledge 
is pretty basic (mostly practical knowledge, not much in the theoretical 
department).

Here's my latest schematic: 
http://img405.imageshack.us/img405/3559/vga19bv6.png

And here's a block diagram of the "logic" section which I haven't 
figured out how to apply in real life:
http://img405.imageshack.us/img405/5424/vgalogicblockdiagram19co6.png


Here's a quick explanation for how the Atari ST video works:

-The Atari ST has two video output modes. Games work mostly in low/mid 
res colour mode while "serious" applications (MIDI software etc.) works 
mostly in hires mono mode. Some software works in both modes:

  - Low/medium mode (colour):  15.75 KHz horizontal/50-60 Hz vertical
  - High mode (monochrome):    31.5 KHz horizonal/72 Hz vertical

-The Atari ST "knows" which mode to be set to according to which type of 
monitor is inserted in its 13-pin DIN plug.
If no monitor is inserted it defaults to "colour mode" and also outputs 
an RF output (suitable for TVs) from its separate "TV out" plug.
If an Atari hires monochrome monitor is inserted the computer switches 
over to "high" mode because the monitor's connector has its "mono 
detect" pin connected to GND (which the colour monitor doesn't).


What I want to do:
- The computer should be able to detect Atari mono or colour monitors as 
before, and set the video mode accordingly
- I also want to add extra connectors for different output devices:
     -composite video (colour mode only)
     -SCART for TVs (colour mode only)
     -VGA (mono mode only)
     -possible also S-video
- When in a certain video mode, those video signals presently used 
should only be output on their appropriately compatible connectors (e.g. 
if the computer is set to "colour" mode then no video should be present 
on the VGA connectors and hence won't damage the VGA monitor by applying 
sync signals outside of its range. likewise not sending "mono" signals 
to the SCART and composite video outputs).

How to make this work:
- there will be two ways of changing the video mode:
                                                      -a physical switch 
(S1)
                                                      -a "logic" 
electronic switching circuit
- A micro-switch (S2) placed inside the 13-pin DIN connector which 
detects if an Atari monitor is present.
                           -if an Atari monitor is found to be present 
then the position of switch S1 is ignored and the logic circuitry takes 
over to determine if an Atari colour or mono monitor is used
                           -if an Atari monitor is NOT found, then the 
position of switch S1 is used to decide the video mode

The block diagram 
(http://img405.imageshack.us/img405/5424/vgalogicblockdiagram19co6.png) 
explains the idea behind the "Logic" circuitry.
Have a look at the schematic again.
U1 is an electronic switch which changes between the two video modes. 
This sort of thing is a little beyond my knowledge in electronics, but I 
believe what I'm looking for is a "multiplexer" IC of some sort. Correct 
me if I'm wrong.
U1's task is to set the computer to mono or colour mode (by connecting 
"mono detect" to gnd or not) and routing the sync lines only to where 
they belong depending on the mode the computer is in.

S2 is the micro-switch inside the 13 pin DIN connector which detects if 
an Atari monitor is connected or not.

As you can see, both U3 and U4 control U1's switch positions. But 
they're not supposed to interfere with each other.
Let's say U3 has set U1 to switch position "A"(Atari mono monitor) and 
the manual switch (S1) is set to "colour" mode.
In this case S1 should have no effect. But if the Atari monitor is 
removed then S1's switch should be check, and in this case, since it's 
set to "colour" the computer should switch over to colour.

I think I have the schematic working in theory, but I need some help 
with the actual design concerning the "logic" section of it. I've tried 
to draw the block diagram into the schematic to get an idea.
Any suggestions?


Hallvard