In the process of recasing and modifying my Atari Mega STe I've come to the video output section which I'm building a new output board for. Since I don't have much design experience I need some help in getting this right.
Here's my schematic, drawn in Eagle:
First some information about how the video modes work on an unmodified Atari ST. There are two modes. Each mode only works with its own dedicated Atari monitor:
- High resolution monochrome mode: 35.70 KHz horizontal, 71.2 Hz vertical
- Low/mid resolution colour: 15.75 KHz horizontal, 50-60 Hz vertical
The computer defaults to colour mode if a colour monitor or no monitor at all is attached to the computer. If a monochrome monitor is attached however then computer "senses" this because the monitor's 13 pin male DIN connector has its pin 4 (mono detect) connected to pin 13 (GND), which sets the computer to mono mode once such a monitor is attached.
My new video output board is meant to take the existing signal and add on it so that I can:
- use standard PC (VGA/SVGA) monitors (mono mode)
- use a standard TV (colour mode)
- use Atari colour and mono monitors as before
I've designed the circuit to work like this:
- a manual switch allows me to choose between colour/mono modes
- in order to prevent damage to monitors/TVs because they receive sync signals outside of their range (especially older monitors) I'm routing only the appropriate sync signals belonging to its dedicated connector.
So far so good, but I need some help with what I've so far figured out in theory, but I don't know which components I need to get this done or how to connect them together. Here's my idea (which I've drawn into the schematic as block-diagrams):
A (4 pole?) "electronic switch" (U1) of some sort (is this what's called a "multiplexer" IC?) works as a multiple 3 way switch. Those 3 are: A: when an Atari mono monitor (SM-124 etc.) is present B: when no Atari monitor is present C: when an Atari colour monitor (SC-1224 etc.) is present
In positions A and C the manual colour/mono switch (S1) is bypassed so changing the modes will have no effect. In other words, whenever an Atari monitor is used it will work just like with an unmodified Atari ST. In position B however the manual switch (S1) will be in control.
Control of the electronic switch (U1) for positions A and C is dependant on some logic (AND/OR gate ICs?), and to determine this a micro-switch is to be placed inside the 13 pin DIN (female) connector. This switch will sense whenever an Atari monitor's (male) connector is inserted. So now we have a system which "knows" when an Atari monitor (mono or colour) is present or not.
Next we need to determine if that Atari monitor is a colour or mono monitor. A mono monitor as we know has pin 4 (mono detect) connected to GND (pin 13) internally, which the Atari colour monitor doesn't. I assume this will generate a logic high/low (correct me if I'm wrong) and can further be used to control the U1 logic switch circuit and set it in position A or C. Positions A and C will as mentioned above shut off the manual switch, and also give sync signals only to those connectors (TV or VGA) which correspond to the mode which the computer is currently in.
Finally, there's the LEDs. If the pin 4/GND issue above works I would think that yet another logic circuit can be used, only this time for controlling two LEDs. One LED indication "colour mode" and another LED for indicating "mono mode".
How do I design this with my existing circuit? If necessary I can upload my Eagle schematic file.
Finally, there's the matter of optimizing the video signals. I've been told that my schematic has several shortcomings where I should terminate the signals properly, that my R-G-B level trimpot (RV1, RV2 and RV3) solution isn't a good one and so on. All that stuff is unknown territory for me, so any suggestions are welcome.