I've spent the weekend absorbing video lectures from Eric Bogatin and reading his book "Signal and Power Integrity - Simplified"
He states that the the return path for the PCB may be any DC plane which could be a VCC rail underneath the signal path.
Consider the following simple circuit
If U1 and U2 are placed on the top layer and TX and RX are routed only the top layer, then the return path for the signal (TX to RX) would be Vcc. I'm ok with that.
My question is, when the return current reaches just under the TX pin, where does the current go ? At this point does it find its way to Gnd or does it go back into the TX and through the die, back to ground ?
** Added Text from book **