hi,

12. For cables carrying signals from one board to the other It's a good idea, to use a flat cable for this purpose, where each second, or at least third line is a ground line. All these ground lines MUST be connected to ground at each board!

I disagree completely. Ground planes of two boards must not be connected by wires of ribbon flat cable because:

Case 1. There are alot of applications where two boards have separate power. This way they are isolated each from other with optocouplers and no ground should be connected together.

Case 2. Wires of flat cable are not suitable for power conducting. They are useful for signal buses but not for power where high current may occur. Connecting grounds of two boards via signal bus cable you create good conditions for malfunction due the power influence on signal wires (here: with current through these "common ground" lines).

As for me so we use ribbon flat cables often. And we always avoid any power connections via flat cable if at least one signal line presents over it. For example, next is from one of projects:


Regards,
Oleg