[sdiy] Multiplexing (Atari video out)

[Dave Manley]

Hallvard Tangeraas wrote:
>
> I've already managed to create a block diagram on my own, but I don't 
> know which IC(s) to use and how to connect them together to make it 
> work like I need them to.
> It's all about AND/OR/YES/NO type electronics based upon the status of 
> two physical switches and a logic high/low signal.
> I've drawn all of this as "logic boxes" in my otherwise schematic 
> diagram:
> http://img405.imageshack.us/img405/3559/vga19bv6.png
>
> It's the coloured boxes on the left hand side.
> Does any of this make any sense to anyone?
>
A couple of questions and comments:

1. What are the input voltages and frequencies into the multiplexer?
2. Are the various sync signals digital or analog?
3. Generally you don't want to have the outputs of two devices wire-or'd 
together (the blue and violet boxes)
4. Generally you don't want floating inputs (B input on mux, IN2 on red box)
5. Have you selected a multiplexer?  The one you show seems to complex 
for the application, you just need a 2-1 mux (ie you're not using the B 
outputs).

Make a truth table, for all the input conditions, define the outputs A 
and C:

Inputs                  Outputs
=====================   ========
MonoDetect    S1   S2   A  C
0             0    0
0             0    1
0             1    0
0             1    1
1             0    0
1             0    1
1             1    0
1             1    1


Once you have this table it is easy to determine the logic function 
necessary to drive the A and C inputs.

-Dave