Here is how I see how the slide switch operates:
There are three positions the switch can be in:
Position A: pin 1 is connected to pin 2
Position B: pin 2 is connected to pin 3
Position C: pin 2 is connected to pin 4
Aha!!! I was completely mistaken about switch position C, thinking the slider always shorts the next two pins, hence pin 3 connects to 4.
I should have looked more closely at the switch part in the schematic, which indeed gives away the connections.
Thanks for clearing this up!
The switch works the same way as described in this post:
https://electronics.stackexchange.com/questions/524856/sp3t-slide-switch-positions
Useful info indeed!
And the comment in there about always measuring non-standard components before designing/ordering the PCB is something I'm going to follow. Actually I read somewhere that it's always a good idea to print out the PCB layout on paper in order to measure if all the components fit/have the correct footprint).
Does this help?
It certainly does!
I had to spend some time with this (I tend to understand things better visually, hence my question about a beginner-friendly logic simulator), but your pointing out the switch behaviour and the resulting voltages on those pins was just what I needed!
But one thing still puzzles me regarding the logic control signals.
I've read about
the basics of logic gates, and while the 74AHC86 (U7) is an XOR gate, couldn't I just use a NOT gate instead (to invert the "enable" signals between the Atari and VGA multiplexers)?
In the current schematic, the VGA side uses 74AHCT
126 buffers (which are the same as the Atari side's 74AHCT
125 buffers, except they get enabled with the opposite logic signal).
So with a NOT gate, why not just use 74AHCT
125 buffers there as well, having its "enable" pins connected to the VGA multiplexer's (U1) enable pin? Or is there something I've missed, and there's a specific reason the XOR gate is chosen as an inverter?