So a bit more chaos today..
I had on the board 2x220R (normal wire ended types ) resistors on the buffer input and this is what was working fine. So I change this for a normal 100R 1206, and it crashed after about 30 seconds
obviously the system should not be that critical over just a few ohms.. So I had a little bit more investigation to do..
Cut a long story short, I think just a tiny bit of noise on the 40MHz line is enough to cause problems.. The input value resistor to the buffer seemed overcritical in all my previous tests and of course this was proven again today. I tried a lot of things, there was seems to work is simply placing a 22pF capacitor on the buffer input. Now I am going via a 33R resistor and it all seems fine again.
I'm using a schmitt buffer, so it should filter out all sorts of noise and glitches. Obviously there is something else going on which it is not filtering out. It's not relating to minimum or maximum voltages as I did a lot of experimenting with that this morning. Even altering DC offset levels etc..
In any case, I will update the PCB design to include this capacitor.. I will leave the board on testing throughout today again to make sure's works.. I'm not too worried about investigating this issue.. If I investigate every tiny little thing the project will never move forward..
In relation to the dualtos boards, where currently I am soldering on the ROM directly onto the PCB.. The problem is that I wanted to keep stock of the ROM's separately in order to keep costs down a lot. It is not really practical to build up 20 of each language.. It's why I put a lot of the current boards (with sockets) built up, so I can keep a large stock of them, I just program low quantity of ROMs as needed. But obviously this would be a huge problem if I went to solder all the ROM's onto the new PCBs...
So I'm going to order a few SMT sockets and see if these are better than the larger through-hole sockets.. I don't have any spare boards, so I will have to order some more, so it could be some time before updating relation to those...